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This paper presents a survey of the cell masses in the brainstem of the Australian lungfish Neoceratodus forsteri, based ontransversely cut Bodian-stained serial sections, supplemented by immunohistochemical data from the recent literature. This study is intended to serve a double purpose. First it concludes and completes a series of publications on the structure of the brainstem in representative species of all groups of anamniote vertebrates. Within the framework of this comparative program the cell masses in the brainstem and their positional relations are analyzed in the light of the Herrick-Johnston concept, according to which the brainstem nuclei are arranged in four longitudinal, functional zones or columns, the boundaries of which are marked by ventricular sulci. The procedure employed in this analysis essentially involves two steps: first, the cell masses and large individual cells are projected upon the ventricular surface, and next, the ventricular surface is flattened out, that is, subjected to a one-to-one continuous topological transformation [J Comp Neurol. 1974;156:255–267]. The second purpose of the present paper is to complement our mapping of the longitudinal zonal arrangement of the cell masses in the brainstem of Neoceratoduswith a subdivision in transversely oriented neural segments. Five longitudinal rhombencephalic sulci – the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis and the sulcus medianus superior – and four longitudinal mesencephalic sulci – the sulcus tegmentalis medialis, the sulcus tegmentalis lateralis, the sulcus subtectalis and the sulcus lateralis mesencephali – could be distinguished. Two obliquely oriented grooves, present in the isthmic region – the sulcus isthmi dorsalis and ventralis – deviate from the overall longitudinal pattern of the other sulci. Although in Neoceratodus most neuronal perikarya are situated within a diffuse periventricular gray, 45 cell masses could be delineated. Ten of these are primary efferent or motor nuclei, eight are primary afferent or sensory centers, six are considered to be components of the reticular formation and the remaining 21 may be interpreted as “relay” nuclei. The topological analysis showed that in most of the rhombencephalon the gray matter is arranged in four longitudinal zones or areas, termed area ventralis, area intermedioventralis, area intermediodorsalis and area dorsalis. The sulcus intermedius ventralis, the sulcus limitans, and the sulcus intermedius dorsalis mark the boundaries between these morphological entities. These longitudinal zones coincide largely, but not entirely, with the functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the viscerosensory nucleus of the solitary tract, several general somatosensory and special somatosensory centers. The isthmus region does not exhibit a clear morphological zonal pattern. The mesencephalon is divisible into a ventral, primarily motor zone and a dorsal somatosensory zone. The boundary between these zones is marked by the sulcus tegmentalis lateralis, which may be considered as an isolated rostral extremity of the sulcus limitans. The results of this study are summarized in a “classical” topological map, as well as in a “modernized” version of this map, in which neuromere borders are indicated.

Causal analysis of molecular patterning at neural plate and early neural tube stages has shown that the central nervous system (CNS) of vertebrates is essentially organized into transverse neural segments or neuromeres and longitudinal zones which follow the curved axis of the brain. The intersection of the longitudinal and transverse patterning processes in the embryonic brain leads to the formation of a checkerboard pattern of distinct progenitor domains called “fundamental morphological units” (FMUs). The topologically invariant pattern formed by the ventricular surfaces of the FMUs of a given taxon represents the “Bauplan” or “blueprint” of the brain of that taxon. The FMUs initially represent thin epithelial fields; during further development they are transformed into three-dimensional radial units, extending from the ventricular surface to the meningeal surface. It is of note that the boundaries of the neuromeres, longitudinal zones, and radial units all strictly adhere to a non-Cartesian coordinate system inherent to the CNS of all vertebrates. The major neural histogenetic processes, including cellular proliferation, radial migration, and differentiation, as well as the formation of grisea (cell masses, nuclei, and cortices), occur principally within the confines of the FMUs, although tangential migration may also take cells to distant sites. Hence, recognition and delimitation of these units is essential for the identification and interpretation of grisea. An outline of the procedure to be followed in these processes of identification and interpretation is presented, and a list of the pertinent homology criteria is provided.

The Actinopterygii or ray-finned fishes comprise, in addition to the large superorder of teleosts, four other superorders, namely the cladistians, the chondrosteans, the ginglymodes, and the halecomorphs, each with a limited number of species. The telencephalon of actinopterygian fishes differs from that in all other vertebrates in that it consists of a pair of solid lobes. Lateral ventricles surrounded by nervous tissue are entirely lacking. At the end of the nineteenth century, the theory was advanced that the unusual configuration of the forebrain in actinopterygians results from an outward bending or eversion of its lateral walls. This theory was accepted by some authors, rejected or neglected by others, and modified by some other authors. The present paper is based on the data derived from the literature, complemented by new observations on a large collection of histological material comprising specimens of all five actinopterygian superorders. The paper consists of three parts. In the first, a survey of the development of the telencephalon in actinopterygian fishes is presented. The data collected show clearly that an outward bending or eversion of the pallial parts of the solid hemispheres is the principal morphogenetic event in all five actinopterygian superorders. In all of these superorders, except for the cladistians, eversion is coupled with a marked thickening of the pallial walls. In the second part, some aspects of the general morphology of the telencephalon in mature actinopterygians are highlighted. It is pointed out that (1) the degree of eversion varies considerably among the various actinopterygian groups; (2) eversion leads to the transformation of the telencephalic roof plate into a wide membrane or tela choroidea, which is bilaterally attached to the lateral or ventrolateral aspect of the solid hemispheres; (3) the lines of attachment or taeniae of the tela choroidea form the most important landmarks in the telencephalon of actinopterygians, indicating the sites where the greatly enlarged ventricular surface of the hemispheres ends and its reduced meningeal surface begins; (4) the meningeal surface of the telencephalon shows in most actinopterygians bilaterally a longitudinally oriented sulcus externus, the depth of which is generally positively correlated with the degree of eversion; (5) a distinct lateral olfactory tract, occupying a constant topological position close to the taenia, is present in all actinopterygians studied; and (6) this tract is not homologous to the tract of the same name in the evaginated and inverted forebrains of other groups of vertebrates. In the third and final section, the concept that the structural organization of the pallium in actinopterygians can be fully explained by a simple eversion of its walls, and the various theories, according to which the eversion is complicated by extensive shifts of its constituent cell groups, are discussed and evaluated. It is concluded that there are no reasons to doubt that the pallium of actinopterygian fishes is the product of a simple and complete eversion.

The forebrain of actinopterygian fishes differs from that of other vertebrates in that it consists of a pair of solid lobes. Lateral ventricles surrounded by nervous tissue are entirely lacking. This peculiar configuration of the actinopterygian forebrain results from an outward bending or eversion of its lateral walls during ontogenesis. Due to this eversion, the telencephalic roof plate is transformed into a wide, membranous structure that surrounds the dorsal and lateral parts of the solid lobes and is attached to their lateral or ventrolateral aspects. Another effect of the eversion is that the ventricular surface of the telencephalic lobes is very extensive, whereas their meningeal surface is small. In many recent publications on the forebrain of actinopterygian fishes, these structures are presented as solid lobes, without any reference to the fact that they are the product of an eversion process, and without any indication concerning the location and extent of their ventricular and meningeal surfaces. It is explained here that, in light of current concepts concerning the histogenesis of the brain, these omissions are intolerable. It is also strongly recommended that the location and extent of these surfaces should always be clearly indicated in brain sections in general, because the simple notion that in the brain of vertebrates the ventricular surface is on the inside and the meningeal surface on the outside has numerous and notable exceptions.

During the 2008 Karger Workshop considerable progress was made towards defining a model or structural plan, valid for the prosencephalon of all vertebrates. The presentations demonstrated that the following features, which are valid for tetrapods, also hold true for most groups of fish: (1) The diencephalon proper is clearly composed of three neuromeres, p1–p3; (2) in the pallium four, rather than three, fundamental longitudinal zones can be distinguished; (3) during ontogenesis, numerous GABA-ergic elements migrate tangentially from the subpallium to the pallium.

Let us begin our discussion of the neural tube and its regional differentiation by looking at some historic pictures, one from His (1893b), and three from Karl Von Kupffer (1906).

Plate 17 presents dorsal views of early frog and human embryos (Fig. A, B). In the frog embryo, the neural plate is surrounded by a distinct ridge (neural fold). In the human embryo, such a demarcation is lacking and the neural plate can only be arbitrarily delimited morphologically from the remainder of the external germ layer, though a precise neural versus non-neural boundary is visible with molecular markers. The apparent lack of a border can also be observed in the transverse section through the human embryonic disk, shown in Plate 17, Fig. C.

The living deuterostomes comprise six monophyletic groups: (1) echinoderms + hemichordates, (2) tunicates, (3) cephalochordates, (4) myxinoids, (5) petromyzontoids, and (6) gnathostomes. The morphotype of the craniote (myxinoids + petromyzontoids + gnathostomes) central nervous system (CNS) comprises a fixed number of histogenetic units, formed by the intersection of transversely oriented neuromeres and longitudinally arranged zones. A well-developed built-in, natural coordinate system adds the third dimension to this morphotype. The classical subdivisions of the craniote CNS: prosencephalon (P), mesencephalon (M), rhombencephalon (R), and spinal cord (S) are each composed of a number of neuromeres. Chordates (larval tunicates + cephalochordates + craniotes) share a highly characteristic axial complex, encompassing a dorsal tubular CNS, a notochord and bilateral series of segmental muscles. In all chordates the CNS can be divided into a rostral (P-like + M-like), an intermediate (R-like) and a caudal (S-like) sector, and sets of homologous developmental genes play a role in this tripartitioning. There are no indications for the presence of olfactory or other telencephalic regions in the brain of non-craniote chordates. Convincing evidence that parts of the chordate CNS are homologous to parts of the larval or adult CNS of non-chordate deuterostomes (echinoderms + hemichordates) is lacking. The dorsal tubular CNS is most probably a chordate autapomorphy.

The human insular cortex forms a distinct, but entirely hidden lobe, situated in the depth of the Sylvian fissure. Here, we first review the recent literature on the connectivity and the functions of this structure. It appears that this small lobe, taking up less than 2% of the total cortical surface area, receives afferents from some sensory thalamic nuclei, is (mostly reciprocally) connected with the amygdala and with many limbic and association cortical areas, and is implicated in an astonishingly large number of widely different functions, ranging from pain perception and speech production to the processing of social emotions. Next, we embark on a long, adventurous journey through the voluminous literature on the structural organization of the insular cortex. This journey yielded the following take-home messages: (1) The meticulous, but mostly neglected publications of Rose (1928) and Brockhaus (1940) are still invaluable for our understanding of the architecture of the mammalian insular cortex. (2) The relation of the insular cortex to the adjacent claustrum is neither ontogenetical nor functional, but purely topographical. (3) The insular cortex has passed through a spectacular progressive differentiation during hominoid evolution, but the assumption of Craig (2009) that the human anterior insula has no homologue in the rhesus monkey is untenable. (4) The concept of Mesulam and Mufson (1985), that the primate insula is essentially composed of three concentrically arranged zones, agranular, dysgranular, and granular, is presumably correct, but there is at present much confusion concerning the more detailed architecture of the anterior insular cortex. (5) The large spindle-shaped cells in the fifth layer of the insular cortex, currently known as von Economo neurons (VENs), are not only confined to large-brained mammals, such as whales, elephants, apes, and humans, but also occur in monkeys and prosimians, as well as in the pygmy hippopotamus, the Atlantic walrus, and Florida manatee. Finally, we point out that the human insula presents a unique opportunity for performing an in-depth comparative analysis of the relations between structure and function in a typical sensory and a typical cognitive cortical domain.

According to Wilhelm His (1891, 1893) the brainstem consists of two longitudinal zones, the dorsal alar plate (sensory in nature) and the ventral basal plate (motor in nature). Johnston and Herrick indicated that both plates can be subdivided into separate somatic and visceral zones, distinguishing somatosensory and viscerosensory zones within the alar plate, and visceromotor and somatomotor zones within the basal plate. To test the validity of this ‘four-functional-zones’ concept, I developed a topological procedure, surveying the spatial relationships of the various cell masses in the brainstem in a single figure. Brainstems of 16 different anamniote species were analyzed, and revealed that the brainstems are clearly divisible into four morphological zones, which correspond largely with the functional zones of Johnston and Herrick. Exceptions include (1) the magnocellular vestibular nucleus situated in the viscerosensory zone; (2) the basal plate containing a number of evidently non-motor centres (superior and inferior olives). Nevertheless the ‘functional zonal model’ has explanatory value. Thus, it is possible to interpret certain brain specializations related to particular behavioural profiles, as ‘local hypertrophies’ of one or two functional columns. Recent developmental molecular studies on brains of birds and mammals confirmed the presence of longitudinal zones, and also showed molecularly defined transverse bands or neuromeres throughout development. The intersecting boundaries of the longitudinal zones and the transverse bands appeared to delimit radially arranged histogenetic domains. Because neuromeres have been observed in embryonic and larval stages of numerous anamniote species, it may be hypothesized that the brainstems of all vertebrates share a basic organizational plan, in which intersecting longitudinal and transverse zones form fundamental histogenetic and genoarchitectonic units.

The neurons of the mesencephalic periaqueductal grey substance (PAG) in the rat are small and medium sized. The cells are frequently located in small clusters, without interdigitating glial elements and may be connected by direct membrane appositions or by gap junctions. The inner zone of the PAG is cell poor. In many cases, the cytoplasm of the cells is filled with extensive rough endoplasmic reticulum, free ribosomes, Golgi apparatus, and large lysosome-like granules. The nuclei show large indentations. The cells have a high nucleus-cytoplasm ratio. The neuropil is very extensive and particularly rich in large numbers of small unmyelinated axons, dendrites, axonal varicosities, and synaptic connections. Myelinated fibres are relatively scarce. The orientation of the fibres was studied in transverse and horizontal sections, in combination with HRP track tracing experiments. It appeared that throughout the PAG most of the fibres were orientated longitudinally. Quantitation showed that most fibres were present in the inner zones of the PAG. Moreover, the diameter of the fibres adjacent to the aqueduct was smaller than that of the fibres in the peripheral parts of the PAG. The thin unmyelinated fibres made extensive synaptic connections within the PAG. Many synaptic varicosities were found in the neuropil of the PAG. There were four types of synaptic varicosities, characterized by different populations of clear and dense-core secretory granules and by the different morphology of the synaptic specializations. In general, the different types of varicosity were homogeneously distributed in the different parts of the PAG. Electron dense secretory granules, when present, were located at some distance from the synaptic junction. Serial sections revealed varicosities which contained only dense-core secretory granules, without synaptic specializations. The dendrites of PAG neurons generally lacked synaptic spines. Many dendrites, particularly those of neurons located in the peripheral parts of the PAG, were directed toward the aqueduct. The present study shows that the PAG is a very complex brain area. The crisscrossing of axons and dendrites with synaptic connections at considerable distances from the cell bodies render it very difficult to unravel the relationships between the possible sources and destinations of ongoing information. This structure complicates the search for relationships between the functional organization and the cytoarchitectural borders in the PAG area.

The forebrain of actinopterygian fishes differs from that of other vertebrates in that it consists of a pair of solid lobes. Lateral ventricles surrounded by nervous tissue are entirely lacking. Comparative anatomical and embryological studies have shown that the unusual configuration of the forebrain in actinopterygians results from an outward bending or eversion of the dorsal portions of its lateral walls. Due to this eversion, the telencephalic roof plate is transformed into a wide, membranous structure which surrounds the dorsal and lateral parts of the solid lobes and is attached to their lateral or ventrolateral aspects. The taeniae, i.e. the lines of attachment of the widened roof plate, represent important landmarks in actinopterygian forebrains. In the present paper, the process of eversion is specified and quantified. It is pointed out that recent suggestions to modify the original eversion concept lack an empirical basis. Eversion is the antithesis of the inward bending or inversion that occurs in the forebrains of most other vertebrates. The forebrain lobes in actinopterygians, like those in other vertebrates, comprise a pallium and a subpallium, both of which include a number of distinct cell masses. The morphological interpretations of these cell masses over the past 130 years are reviewed and evaluated in light of a set of carefully selected criteria for homologous relationships. Special attention is paid to the interpretation of a cell mass known as Dp, situated in the caudolateral portion of the pallium in teleosts (by far the largest clade of living actinopterygians). Based on its position close to the taenia, and given the everted condition of the pallium in teleosts, this cell mass clearly corresponds with the medial pallium in inverted forebrains; however, Dp receives a dense olfactory input, and it shares this salient feature with the lateral pallium, rather than with the medial pallium of inverted forebrains. There is presently no consensus regarding the homology of Dp. Several recent authors [Wullimann and Mueller, 2004; Yamamoto et al., 2007] consider the lateral pallium in inverted forebrains and Dp in teleosts to be homologous because they believe that these cell masses originate from the same germinative zones, but that Dp attains its ultimate position only through migration. On the other hand, the present author believes that Dp is situated in the immediate vicinity of its germinative zone and that it represents a specialized part of the lateral pallial zone in teleosts, a zone that can be homologized topologically with the medial pallium in inverted forebrains. Further, it is proposed that the lateral olfactory tract in teleosts, which supplies most of the olfactory fibers to Dp, is not homologous to the same-named tract in the inverted forebrains of most other vertebrates.

Orientation. Development. Gross anatomy. Blood supply, meninges and liquor system. Brain slices and microscopical sections. Topography of spinal cord, brain stem and cerebellum. Diencephalon: introduction and epithalamus. Diencephalon: dorsal thalamus. Diencephalon: ventral thalamus. Diencephalon: hypothalamus. Telencephalon: introduction and olfactory system. Telencephalon: hippocampus and related structures. Telencephalon: amygdala and claustrum. Telencephalon: basal ganglia. Telencephalon: neocortex. Functional systems.- General sensory systems and taste. Vestibular system. Auditory system. Visual system. Cerebellum. Motor systems. Limbic system

The boundaries of the medial forebrain bundle (MFB) of the rat have been presented in previous work on the structure of this fiber system (Nieuwenhuys et al.: J. Comp. Neurol. 206:49-81, '82). Neuronal cell bodies within these outlines constitute the bed nucleus of the MFB. Many fiber components of the MFB appeared to be spatially arranged within the bundle and featured an orderly topography (Veening et al.: J. Comp. Neurol. 206:82-108, '82). As the fibers of the MFB are thought to be a major source of afferents to the bed nucleus (Millhouse: In P. J. Morgane and J. Panksepp (eds): Anatomy of the Hypothalamus, Vol. 1. New York: Marcel Dekker, pp. 221–265, '79), the latter has been subjected in this and the companion study (Geeraedts et al.:J. Comp. Neurol. 294:537-568, '90) to a detailed cytoarchitectonic analysis. This analysis is based on continuous series sectioned in the three conventional planes. On the basis of cytoarchitectonic characteristics, including size and shape, staining intensity, packing density, and spatial orientation of the cell bodies, it was found that the bed nucleus of the MFB as described in the literature is by no means a cytoarchitectonic unit per se. Rather, the neuronal cell population located within the telencephalic stream of the MFB can be parcellated into a number of cellular groups, which partly or entirely belong to more-or-less known basal telencephalic structres. These structures are designated here as the MFB-related areas. They correspond largely to the subcommissural substantia innominata (SIC), the sublenticular substantia innominata (SIL), the nucleus of the diagonal band of Broca, the olfactory tubercle, the magnocellular preoptic nucleus (POMA), the lateral preoptic area (LPOA), and the interstitial nucleus of the stria medullaris (ISM). The complex of the MFB-related areas is surrounded by the following cellular entities: the nucleus accumbens (ACB), the caudatus-putamen region (CPU), the globus pallidus (GP), the bed nucleus of the stria terminalis (BST), the anterior amygdaloid area (AAA), the amygdaloid nuclear complex (A), the medial preoptic area (MPOA)and the anterior hypothalamic area (AHA). Both MFB-related areas and their surroundings have been identified and delimited in this study. This resulted in a new cytoarchitectonic atlas of the rat's basal telencephalon. Our atlas does not only show the relative positions of the above mentioned cellular groups, but also those of their subdivisions. The SIC, the SIL, the POMA and the LPOA have been parcellated into four, three, three, and six subareas, respectively; the GP, the BST, the MPOA, and the AHA into four, seven, fourteen, and six subareas, respectively. Several of these subdivisions have not been identified previously. The amygdaloid nuclear complex was not further parcellated cytoarchitectonically, as this was felt to be beyond the scope of our study. In preparing the atlas, it was kept in mind that the basal telencephalon should be parcellated in cellular groups and subgroups easily recognizable even by investigators not well trained in cytoarchitectonics. When the outlines of the MFB on the levels 1 to 4 of the atlas of the bun- dle (Nieuwenhuys et al.: J. Comp. Neurol. 206:49-81, '82) are superimposed upon the corresponding tracings of our cytoarchitectonic atlas, the following cellular groups appear to constitute the rostral or telencephalic part of the bed nucleus of the MFR: the deep layer of the olfactory tubercle, the magnocellu- lar preoptic nucleus, the horizontal limb of the diagonal band of Broca, the lateral preoptic area and the sublenticular substantia innominata. After superimposing the areas of labeling of 21 autoradiographically identified fiber components of the MFB (Veening et al.: J. Comp. Neurol. 206:82-108, '82) upon our tracings of the rostral bed nucleus, a remarkable correspondence appeared with the borders of the cytoarchitectonically defined cellular entities and those of the traversing fiber components to and from specific origins and destinations. These findings can be summarized as follows: the ventrolateral or POMA-section of the MFB is occupied by olfactory related fibers; the ven- tromedial or LPOA-section of the MFB contains septal, preoptic, and medial hypothalamic fibers; and the dorsal or SIL-section of the MFB contains mesolimbic fibers, including fibers arising from the central amygdaloid nucleus, the bed nucleus of the stria terminalis, the ventral tegmental area, and the parabrachial nuclei of the brainstem region. Further morphological analysis of the connectivity of the neurons of the cellular entities of the rostral bed nucleus and their proposed dendritic rela- tionships with collaterals of the fibers of the MFB-components will provide more profound information on the intrinsic organization of the MFB and its so-called bed nucleus.

In the preceding study (Geeraedts et al.: J. Comp. Neurol. 294:507-536, '90), the rostral or telencephalic portion of the rat's bed nucleus of the medial forebrain bundle (MFB) has been parcellated into several cytoarchitectonically distinct cellular groups and subgroups. The purpose of the present investigation is to subject the caudal or lateral hypothalamic (LH) portion of the MFB bed nucleus to a detailed cytoarchitectonic analysis. This analysis is based on the same materials, methods, and cytoarchitectonic criteria that were also employed in the preceding study. In contrast to descriptions in the literature, it was found that the LH-region constitutes a very heterogeneous population of neurons with an evident arrangement into groups, several of which have not been identified previously. Many of these cellular groups are partly or entirely located within the boundary of the LH-trajectory of the MFB as previously established by Nieuwenhuys et al. (J. Comp. Neurol. 206:49-81, '82). These groups are designated here as the MFB-related cellular groups. They appear to be arranged into two longitudinal zones. Both zones are caudally replaced by the ventral tegmental area (VTA) and a part of the mesencephalic tegmentum (TEGM1). The lateral zone lies in close proximity to the internal capsule/cerebral peduncle and comprises the following cellular groups: the ventrolateral subarea of the lateral hypothalamic area (LHVL), the anterolateral subarea of the lateral hypothalamic area (LHAL), the lateral tuberal nucleus (TUL), the pre-subthalamic nucleus (PSUT), the retro-subthalamic nucleus (RSUT), the anterodorsal subarea of the lateral hypothalamic area (LHAD), and the lateral hypothalamic nucleus (LHN). The medial zone consists of the following cellular groups: the intermediate hypothalamic area (IHA), the medial tuberal nucleus (TUM), the perifornical nucleus (PFX), the lateral supramammillary nucleus (SUL), the submammillothalamic nucleus (SMT), and the nucleus geminus posterior (GEP). The cellular groups of the medial zone together with the tuberomammillary nucleus (TUMM) are positioned at the interface between the lateral and the medial hypothalamus, and form an array of cellular groups indicated in our study as the intermediate division of the hypothalamus. The MFB-related cellular groups are dorsally, medially, ventrally, and laterally surrounded by rather well-known brain structures. Both the MFB-related cellular groups and the surrounding structures have been identified and delimited. This resulted in a new, elaborate cytoarchitectonic atlas of the rat's lateral hypothalamic region. When the outlines of the MFB on the levels 5-9 of the atlas of the bundle (Nieuwenhuys et al.: J. Comp. Neurol. 206:49-81, '82) are superimposed upon the corresponding tracings of our cytoarchitectonic atlas, the following cellular groups appear to constitute the caudal or lateral hypothalamic portion of the bed nucleus of the MFR: the LHVL, the LHAL, the LHAD, the TUL, the PSUT, the RSUT, the LHN. The GEP, the TEGM1, and, moreover, the lateral portions of the IHA, the TUM, the PFX, the SUL, and the SMT. After super- imposing the areas of labeling of 21 autoradiographically identified fiber com- ponents of the MFB (Veening et al.: J. Camp. Neurol. 206:82-108, '82) upon the corresponding tracings of our atlas, we noted a certain correspondence with the borders of the cytoarchitectonically defined cellular groups and those of the fiber components. These data can be summarized as follows: the inter- mediate division of the hypothalamus contains septal, medial preoptic, and medial hypothalamic fibers; the ventrolateral or TUL/LHVL-part of the MFB is occupied by olfactory related fibers; the dorsolateral part of the MFB, occupying the dorsolateral portion of the LHN-region, is traversed by meso- limbic fibers; the central and dorsomedial portions of the LHN-region are occupied by fibers arising from the lateral hypothalamus, the bed nucleus of the stria terminalis, the ventral tegmental area, and presumably also by fibers from (until now) unknown origin or destination.

Marked neuron loss in the nucleus basalis of Meynert complex (NBMC) in Alzheimer's disease has repeatedly been reported in the literature. However, most of these studies quantitated only magnocellular, hyperchromatic (putative cholinergic) neurons of just a small part of the NBMC, and counts were expressed as numerical density. Applying a 3-dimensional-sampling design throughout the entire rostrocaudal extent of the NBMC and sampling neurons regardless of their size and staining characteristics, an overall neuron loss of only 15.5% was demonstrated for the whole NBMC. Neuron loss varied from 0% rostrally in the NBMC up to 36% in the most caudal part of the nucleus basalis of Meynert. Moreover, a significant increase in the number of small-sized neurons and a significant decrease in the number of large, putative cholinergic neurons could be detected, suggesting that apart from neuron loss neuron shrinkage appears to be another characteristic neuropathological feature of this degenerating cholinergic NBMC system. Preservation of these magnocellular cholinergic neurons in shrunken form renders it likely that cholinergic dysfunction, characteristic of Alzheimer's disease, may be responsive to neurotrophic influences.

Publisher Summary In this chapter, the morphological evidence for the occurrence of volume transmission in vertebrates and invertebrates are discussed. The assembled data is also surveyed and placed in a comparative and a functional perspective. While studying the morphological evidence in invertebrates, the relevant data on the occurrence of other forms of intemeuronal communication are included and special attention will be paid to the structural relations in the first three phyla of the animal kingdom— that is, the Placozoa, the Porifera, and the Cnidaria. From a survey of the ultra structural data discussed, it is concluded that volume transmission is most probably very common in the greater limbic system of mammals, and that this mode of inter neuronal communications also occurs in the neostriatum, the hippocampus, and the neocortex of this group. From the studies in invertebrates, it is concluded that volume transmission is dominant in the neuropils of many invertebrates.

The preceding chapters of this work have presented a survey of our present knowledge of the structural organisation of the CNS of the various groups of vertebrates. This last chapter will highlight some of the major features revealed by our survey and offer some final comments.

Crossopterygians are a distinctive and once widespread group of fishes, first known from rocks of Devonian age. They are characterised by the possession of two dorsal fins, the cosmoid structure of their scales and dermal bones, and well-formed fleshy lobed paired fins. The internal skeleton of the latter is much concentrated so that only a single bone (humerus, femur) articulates proximally with the girdles and distally with two bones (radius and ulna, tibia and fibula). The latter are followed by the more variable distal fin supports. The most important distinguishing feature of these fishes is the division of the neurocranium into two parts, an anterior, ethmo-sphenoid portion and a posterior, otico-occipital portion with an intracranial joint between them. The large, persistent notochord inserts upon the posteroventral margin of the ethmo-sphenoid portion of the skull (Thomson 1969; Moy-Thomas and Miles 1971; Andrews 1973).

The preceding chapter dealt with the structure and function of the cellular elements in the central nervous system. In the present chapter the same features of assemblies of neurons will be discussed. The approach will be purely morphological, passing from simple to complex and from generalized to specialized, although some theories about the phylogenetic changes in the vertebrate neuraxis will be considered. Following some notes on grey and white matter, the periventricular grey and the neuropil and fibre zone which covers this grey peripherally in the brain of many different anamniotes will be covered. The neuropil and its differentiation into various zones or sheets will next be considered, and then the organisation of the diffuse or reticular grey will be discussed. Having dealt with the structure and function of simple and generalised grisea, we will then survey the organisation of more complex neuronal assemblies. Attention will be paid to the position, cytoarchitecture and organisation of cell masses or nuclei as well as to their dendritic and axonal ramifications and extrinsic connections. Many grisea in the central nervous system of vertebrates show a laminar organisation, and this interesting feature will be treated in some detail. The last section is devoted to the chemoarchitecture of grisea.

By way of introduction, an outline is presented of the origin and evolutionary development of the neocortex. A cortical formation is lacking in amphibians, but a simple three-layered cortex is present throughout the pallium of reptiles. In mammals, two three-layered cortical structures, i.e. the prepiriform cortex and the hippocampus, are separated from each other by a six-layered neocortex. Still small in marsupials and insectivores, this "new" structure attains amazing dimensions in anthropoids and cetaceans. Neocortical neurons can be allocated to one of two basic categories: pyramidal and nonpyramidal cells. The pyramidal neurons form the principal elements in neocortical circuitry, accounting for at least 70% of the total neocortical population. The evolutionary development of the pyramidal neurons can be traced from simple, "extraverted" neurons in the amphibian pallium, via pyramid-like neurons in the reptilian cortex to the fully developed neocortical elements designated by Cajal as "psychic cells". Typical mammalian pyramidal neurons have the following eight features in common: (1) spiny dendrites, (2) a stout radially oriented apical dendrite, forming (3) a terminal bouquet in the most superficial cortical layer, (4) a set of basal dendrites, (5) an axon descending to the subcortical white matter, (6) a number of intracortical axon collaterals, (7) terminals establishing synaptic contacts of the round vesicle/asymmetric variety, and (8) the use of the excitatory aminoacids glutamate and/or aspartate as their neurotransmitter. The pyramidal neurons constitute the sole output and the largest input system of the neocortex. They form the principal targets of the axon collaterals of other pyramidal neurons, as well as of the endings of the main axons of cortico-cortical neurons. Indeed, the pyramidal neurons constitute together a continuous network extending over the entire neocortex, justifying the generalization: the neocortex communicates first and foremost within itself. The typical pyramidal neurons represent the end stage of a progressive evolutionary process. During further development many of these elements have become transformed by reduction into various kinds of atypical or aberrant pyramidal neurons. Interestingly, none of the six morphological characteristics, mentioned above under 1-6, has appeared to be unassailable; pyramidal neurons lacking spines, apical dendrites, long axons and intracortical axon collaterals etc. have all been described. From an evolutionary point of view the typical pyramidal neurons represent not only the principal neocortical elements, but also the source of various excitatory local circuit neurons. The spiny stellate cells, which are abundant in highly specialized primary sensory areas, form a remarkable case in point.(ABSTRACT TRUNCATED AT 400 WORDS)

The efferent connections of the hypothalamic area of the rat, where attack behaviour can be elicited by electrical stimulation, were studied using iontophoretic injections of Phaseolus vulgaris-leucoagglutinin. Specificity for the hypothalamic "attack area" was investigated by comparison with efferents of hypothalamic sites outside the attack area. The hypothalamic attack area consists of the intermediate hypothalamic area and the ventrolateral pole of the ventromedial hypothalamic nucleus. Fibres from the hypothalamic attack area, as well as fibres from several other hypothalamic sites, form diffuse fibre "streams" running rostrally or caudally. Many varicosities that are found on the fibres suggest, that these fibres are capable of influencing many brain sites along their way. Projection sites were found throughout the brain. In the comparison between attack area efferents and controls, many overlapping brain sites were found. Hypothalamic efferents preferentially originating in the largest part of the attack area, i.e. the intermediate hypothalamic area, were found in the mediodorsal and parataenial thalamic nuclei. Within the septum, a spatial organization of hypothalamic innervation was found. Fibres from the attack area formed specialized "pericellular baskets" in the dorsolateral aspect of the intermediate part of the lateral septal nucleus. Fibres from other hypothalamic sites were found in other septal areas and did not form these septal baskets. Within the mesencephalic central gray, fibres from the attack area were found specifically in the dorsal part and dorsal aspect of the lateral part of the central gray. Physiological and pharmacological studies have shown that several brain sites are involved in different aspects of aggressive behaviour. Some of these areas, as for instance the dorsomedial thalamic nucleus, septum and central gray, are innervated by efferents from the hypothalamic attack area, whereas other sites, like ventral premammillary nucleus and ventral tegmental area, are not. It is concluded from the present findings, that a number of brain sites, that are known to be involved in agonistic behaviour, receive hypothalamic information preferentially from the hypothalamic attack area through diffusely arranged varicose fibres. The function of each connection in the regulation of specific behaviours remains to be further investigated.

This paper presents a survey of the cell masses in the brainstem of the generalized actinopterygian fish Amia calva, based on transversely cut Nissl-, Kluver-Barrera-, and Bodian-stained serial sections. This study is intended to serve a double purpose. First it forms part of a now almost complete series of publications on the structure of the brainstem in representative species of all groups of vertebrates. Within the framework of this comparative program the cell masses in the brainstem and their positional relations are analyzed in the light of the Herrick–Johnston concept; according to this the brainstem nuclei are arranged in four longitudinal, functional zones or columns, the boundaries of which are marked by ventricular sulci. The procedure employed in this analysis essentially involves two steps: first, the cell masses and large individual cells are projected upon the ventricular surface, and next, the ventricular surface is flattened out, that is, subjected to a one-to-one continuous topological transformation (Nieuwenhuys [1974] J. Comp. Neurol. 156:255-267). The second purpose of the present paper is to provide a cytoarchitectonic basis for experimental analysis of the fiber connectivity in the brainstem of Amia. Five longitudinal sulci–the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis, and the sulcus lateralis mesencephali–could be distinguished. Some shorter grooves, present in the isthmal region, clearly deviate from the overall longitudinal pattern of the other sulci. Although in Amia most neuronal perikarya are contained within a diffuse periventricular gray, 40 cell masses could be delineated: Eight of these are primary efferent or motor nuclei, 10 are primary afferent or sensory centers, seven are considered to be components of the reticular formation, and the remaining 15 may be interpreted as “relay” nuclei. The topological analysis yielded the following results. In the rhombencephalon the gray matter is arranged in four longitudinal columns or areas, termed area ventralis, area intermedioventralis, are intermediodorsalis, and area dorsalis. The sulcus intermedius ventralis, the sulcus limitans, and the sulcus intermedius dorsalis mark the bounderies between these morphological entities. These longitudinal areas coincide largely, but not entirely, with the functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the viscerosensory nucleus of the solitary tract, several general somatosensory and special somatosensory nuclei. The four longitudinal zones cannot be distinguished in the mesencephalon nor can the sulcus limitans be recognized here. Functionally, however, the medial part of the tegmentum mesencephali may be considered the rostral extremity of the somatomotor column, whereas the remainder of the midbrain contains a number of somatosensory centers. © 1994 Wiley-Liss, Inc.

The efferent connections of the hypothalamic area, where grooming can be elicited by local electrical stimulation or injection of various substances, were studied using iontophoretic injections of Phaseolus vulgaris leucoagglutinin. This hypothalamic "grooming area" consists of parts of the hypothalamic paraventricular nucleus and of the dorsal hypothalamic area. The specificity of these efferents for the hypothalamic "grooming area" was investigated by comparison with efferents of hypothalamic sites adjacent to this area. In addition, the distribution of oxytocinergic fibres was studied, since oxytocinergic neurons are present in the hypothalamic "grooming area" and oxytocin is possibly involved in grooming behaviour. The efferents of the hypothalamic "grooming area" as well as of hypothalamic sites surrounding this area and the oxytocinergic fibres studied do not form well determined bundles, but rather spread out throughout the hypothalamus. Clusters of fibres could be traced rostrally and caudally, forming diffuse fibre "streams". Three rostral, two thalamic and three caudal fibre "streams" have been distinguished along which efferent fibres innervate different brain areas. The many varicosities on labelled fibres "en passant" suggest that hypothalamic fibres are able to influence many parts of the brain along their way. The anterior periventricular area, the median preoptic nucleus, the ventral tegmental area and nucleus of the solitary tract were found to be more or less specifically innervated by hypothalamic "grooming area" fibres and oxytocinergic fibres. Other brain areas, like the septum, the medial amygdaloid nucleus, the central gray and the paraventricular nucleus of the thalamus were found to receive efferent projections from the hypothalamic "grooming area" and hypothalamic loci outside this area, as well as from the oxytocinergic system. Within the septum and the mesencephalic central gray, differences in the spatial organization of terminating fibres from the hypothalamic "grooming area" and hypothalamic "non-grooming" sites have been found. Fibres from the grooming area clustered in the ventral part of the lateral septal nucleus, while fibres from surrounding hypothalamic loci innervated other parts of that brain area. In the central gray, fibres from the hypothalamic "grooming area" clustered in rostrodorsal and caudoventral parts. A number of brain areas, that are innervated by hypothalamic "grooming area" fibres and oxytocinergic fibres, like central gray, ventral tegmental area and the noradrenergic A5 area, have been reported previously to be involved in grooming behaviour. It is concluded from the present findings, that the hypothalamic "grooming area" has preferential connections with a number of brain sites, not shared with hypothalamic projections from outside the "grooming area".(ABSTRACT TRUNCATED AT 400 WORDS)

Projections descending from the hypothalamus to the PAG are in the rat characterized by the fact that they have in common with other ascending and descending projections the principle of termination within restricted parts of the PAG (see Blomqvist and Craig; Shipley et al., this volume). For that reason, we have to pay first some attention to the cytoarchitecture and the possible subdivisions of the PAG. On that basis, the descending projections will be described and compared with data obtained from the literature.

The ventricular sulcal pattern and the cellular structure of the brain stem of the single surviving crossopterygian species Latimeria chalumnae have been studied in transversely cut Nissl, Klüver-Barrera and Bodian stained serial sections. Five longitudinal sulci, the sulcus medianus inferior, the sulcus medianus superior, the sulcus intermedius ventralis, the sulcus limitans and the sulcus intermedius dorsalis could be delimited. The latter three of these sulci are confined to the rhombencephalon. The walls of the mesencephalon also display some longitudinal grooves, but none of these could be traced into continuity with any of the rhombencephalic sulci. Although the neuronal perikarya in many places show a diffuse arrangement, 27 cell masses could be delineated; eight of these are primary efferent nuclei, seven are primary afferent centers, seven nuclei are considered as components of the reticular formation, and the remaining five cell masses may be interpreted as "relay" nuclei. In order to study the zonal pattern of the brain stem, this structure was subjected to a topological analysis (cf., Nieuwenhuys, '74, and fig. 16). This analysis yielded the following results. The sulcus limitans divides the greater part of the rhombencephalon into a basal plate and an alar plate. In the basal plate the sulcus intermedius ventralis marks the boundary between an area ventralis and an area intermedioventralis. The area ventralis contains two somatic motor centers (i.e., the rostral end of the spinal motor column and the nucleus of IV) and most the entire rhombencephalic medial reticular formation. The latter may be primarily considered as a somatic motor coordinating center. The area intermedioventralis contains the visceral motor nuclei of V, VII, IX and X. However, the basal plate also harbours a number of non-motor centers, for example the sensory princeps nucleus of V and the inferior olive. The alar plate is subdivided by the sulcus intermedius dorsalis into an area intermediodorsalis and an area dorsalis. The area intermediolateralis is largely occupied by the common visceral sensory center of VII, IX and X; however, this area also contains a number of somatic sensory cell masses, as e.g. the nucleus descendens of V and the magnocellular vestibular nucleus. The area dorsalis is entirely occupied by two large lateral line centers. The cell masses in the isthmus region do not exhibit a clear-cut morphological pattern. As regards the mensencephalon, the medial part of the tegmentum, which contains a primary somatic motor center (the nucleus of III) and a somatic motor coordination center (the nucleus of the f.l.m.) may be considered a direct rostral continuation of the area ventralis. The remainder of the midbrain contains a number of somatic sensory centers of primary and higher order.

The ventricular sulcal pattern and the cellular structure of the brainstem of the reedfish Erpetoichthys calabaricus were studied in transversely cut Nissl-, Kluver-Barrera-, and Bodian-stained serial sections. Six longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis, the sulcus medianus superior, and the sulcus lateralis mesencephali could be distinguished. A seventh groove, the sulcus isthmi, clearly deviates from the overall longitudinal pattern of the other sulci. Although in Erpetoichthys most neuronal perikarya are contained within a diffuse periventricular gray, 32 cell masses could be delineated; six of these are primary efferent or motor nuclei, seven are primary afferent or sensory centers, nine are considered to be components of the reticular formation, and the remaining ten may be interpreted as “relay” nuclei. In order to study the zonal pattern of the brainstem, this structure was subjected to a topological analysis (cf. Nieuwenhuys, '74 and Fig. 15) which yielded the following result. In the rhombencephalon the gray matter is arranged in four longitudinal columns or areas termed area ventralis, area intermedioventralis, area intermediodorsalis, and area dorsalis. In many places the sulcus intermedius ventralis, the sulcus limitans, and the sulcus intermedius dorsalis mark the boundaries between these morphological entities. These longitudinal areas coincide largely, but not entirely, with the so-called functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the nucleus of the solitary tract, three nonviscerosensory cell masses, namely, the nucleus vestibularis magnocellularis and parvocellularis and the nucleus of the tractus descendens of V. The four longitudinal zones cannot be distinguished in the mesencephalon nor can the sulcus limitans be recognized here. Functionally, however, the medial part of the tegmentum mesencephali may be considered the rostral extreme of the somatomotor column, whereas the remainder of the midbrain contains a number of somatosensory centers.

This paper is the first of a projected series of studies on the structure and composition of the medial forebrain bundle (MFB) of the rat and the relations of this fiber system to its bed nucleus, the lateral hypothalamic area. The first part of the paper comprises an extensive review of literature on the MFB from its discovery by Ganser in 1882 to the present. This review serves as the basis for an evaluation of our present-day knowledge of the organization of the MFB, which is presented in the second part of this paper. Despite the wealth of information available on the origins and sites of termination of the axons that constitute the MFB, surprisingly little attention has been given to the bundle itself, to its topographic boundaries, its fiber composition, or to the spatial arrangement of its constituent components. These features of the MFB as it extends through the lateral preoptic and lateral hypothalamic areas have been analyzed in normal Klüver-Barrera- and Bodian-stained material. From this analysis, a detailed atlas of the MFB and some of the surrounding structures has been prepared. This atlas, which forms the third section of this paper, illustrates the appearance and organization of the MFB at ten equidistant levels through the lateral preoptic and lateral hypothalamic continuum.

A quantitative electron microscopic analysis was undertaken of the development of the pyramidal tract, at the level of the third cervical spinal segment, in rats ranging in age from the day of birth to three months old. The axon number was calculated as the product of axon density, determined in a systematic random sample of electron micrographs, and tract area. During the first postnatal week the tract contains thin unmyelinated axons and growth cones. Growth cones are abundant in neonatal rats, but can still be observed occasionally at the end of the first postnatal week, indicating a continuous addition of pyramidal tract axons during the first postnatal week. Myelination starts around P10. By the end of the first postnatal month approximately 50% of the axons have already been myelinated. Myelination proceeds during further maturation, but in the three month old rat 28% of the axons are still unmyelinated. The total number of axons increases rapidly after birth up to 153,000 at the fourth postnatal day. Subsequently, the number of axons is reduced by nearly 50% to 79,000 in the adult rat. The axon loss is most prominent during the second postnatal week, when 32,000 axons are eliminated, but continues for several weeks at a slower rate.

The morphogenesis and histogenesis of the spinal cord of Xenopus were examined. The study encompasses the developmental period between stage 41 and stage 66 (stages according to Nieuwkoop and Faber 1967). This period can roughly be divided into three phases. From stage 50 up to stage 53 strong proliferation and rapid growth are the most striking features. This developmental phase is preceded and followed by less dynamic periods. From stage 41 up to stage 50 the rate of proliferation is relatively low. The numbers of cells in the matrix and in the mantle layer are very small. In the mantle layer two classes of early differentiated transient neurons can be distinguished: primitive giant sensory or Rohon-Beard cells and primitive motor neurons. From stage 46 onward the originally tube-shaped spinal cord swells at the thoracic level into a thoracic enlargement. After stage 50 the proliferation strongly increases until a maximum at stage 53. Concomitantly a considerable acceleration of growth takes place. The major part of the mitoses are always concentrated in the dorsal part of the matrix. From stage 51 onward the cervical and lumbar regions show much more mitoses than the thoracic part. Distinct cervical and lumbar enlargements develop and are going to mask the thoracic swelling of the cord. From stage 54 on proliferation continues on an increasingly low level. The period between stage 54 and stage 66 is characterized by differentiation of the spinal neuronal elements.

The ventricular sulcal pattern and the cytoarchitectonic organization of the brain stem of the frogs Rana esculenta and Rana catesbeiana have been studied in transversely cut, Nissl stained serial sections. Four longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans and the sulcus medianus superior could be distinguished in both species. A fifth longitudinal groove, the sulcus intermedius dorsalis, was found only in Rana esculenta. With the aid of the usual cytoarchitectonic criteria 25 cell masses have been delineated in Rana esculenta and 27 in Rana catesbeiana. These cell masses can be distributed over the following categories )numbers added in brackets for Rana catesbeiana, if different from those in Rana esculenta(: primary efferent or motor, 8: primary afferent or sensory, 4 )6(; “relay” centers, 7. Contrary to statements in the literature the reticular formation can be divided into six separate cell groups. The majority of the nuclei form part of the central gray, which constitutes a rather wide zone in anurans; three reticular nuclei lie partly within the stratum griseum and partly within the stratum album; six nuclei are entirely embedded in the stratum album. The morphological pattern of the cell masses and their relationship to the ventricular sulci were studied with the aid of a graphical reconstruction procedure termed topological analysis )cf. Nieuwenhuys, '74 and figs. 15, 16(. This analysis yielded the following results: The sulcus limitans extends throughout the rhombencephalon, dividing this brain part into a basal plate and an alar plate. The cell masses in the basal plate fit into two longitudinal zones, a medial area ventralis and a lateral area intermedioventralis. The area ventralis contains three somatic motor nuclei )IV, VI and XII( and the rhombencephalic medial reticular zone. The latter may be primarily considered as a somatic motor coordinating center. The area intermedioventralis contains the visceral motor nuclei of V, VII, IX and X. However, the basal plate also contains a number of non-motor centers, for example the superior olive. The alar plate contains visceral sensory, general somatic sensory and special somatic sensory centers. Two cell masses, the nucleus fasciculi solitarii and the nucleus visceralis secundarius, represent together a discontinuous visceral sensory zone. Both of these nuclei are situated immediately dorsal to the sulcus limitans. The special somatic sensory area, i.e., the area of termination of the eighth nerve, occupies a considerable part of the alar plate. This area comprises, apart from a large zone of diffuse gray, three distinct cell masses. The general somatic sensory nuclei, i.e., the nucleus tractus descendens and the nucleus princeps of V, constitute a zone which largely overlaps the nucleus fasciculi solitarii and one of the octavus nuclei. In the frogs investigated the cell masses in the midbrain do not exhibit a clear-cut morphological pattern. Functionally, however, the medial part of the tegmentum mesencephali may be considerd as the rostral extreme of the somatic motor area, whereas the lateral part of the tegmentum and the tectum are chiefly occupied by special somatic centers of primary and higher orders.

SYNOPSIS. The brain of actinopterygian fishes can be subdivided into five principal parts, rhombencephalon, cerebellum, mesencephalon, diencephalon and telencephalon, each of which contains a number of separate morphological entities: nuclei, areas or zones. Analysis of the origin and termination of the cranial nerves and their components reveals that many of the morphological entities distinguished in the actinopterygian brain can be interpreted in terms of elementary sensory and motor functions. Experimental anatomical and physiological studies on the fiber connections of the entities thus defined have led to a functional interpretation of many other parts of the brain. Thus, the central circuitry related to such sensory functions as hydrodynamic pressoreception, electroreception, vision, gustation and olfaction are well-known, and the same holds true for the motor systems related to feeding and locomotion. However, there are still many regions in the actinopterygian brain the functional significance of which is poorly understood, and it should be emphasized that most of our knowledge on the organization of the brain of this group rests merely on observations in teleosts. One of the most interesting aspects of actinopterygian neurobiology is that the telencephalon in this group develops in a way which differs fundamentally from that found in all other craniates, and that the telencephalon shows a marked progressive differentiation in the series: polypterids—chondrosteans—halecomorphs—teleosts.

The superior colliculus (SC) or optic tectum of mammals consists of seven layers, numbered I-VII from superficial to deep, each of which has distinct connectivity patterns and electrophysiological response properties. The present study is devoted to a morphometrical analysis of neuronal diameters, densities, and numbers in different layers and regions of the SC of albino as well as pigmented rats in order to present a quantitative characterization of the collicular neuronal population involved in the different connectivities and functions of these compartments. The morphometric parameters were calculated from tracings of nuclei and cell bodies by means of Kontron-Videoplan equipment and a Micro PDP 11/23 computer. The mean soma diameter per superior colliculus appears to be 12.0 microns, the average neuronal density 70 cells per 0.001 mm3, and the total number of neurons about 600,000. The mean soma diameter gradually increases from superficial to deep layers (i.e., from 10.0 to 14.0 microns). Cellular density is highest in layer III, the retinal afferent layer (90 cells per 0.001 mm3), and decreases both in more superficial layers (to about 80 in layer I) and deeper layers (to about 44 in layer VII). About 25% of all collicular neurons are situated in layer II whereas layer I contains the lowest percentage of cells (4%). Rostrally within each collicular layer, cellular volumes are about 25% larger than caudally. On the other hand, neuronal densities are rostrally about 38% lower than caudally in all layers except for layers VI and VII. We conclude that collicular neurons, in contrast to collicular axons, are not arranged in distinct layers or clusters but basically establish a random network with only gradual transitions. In this respect, no statistically significant differences were observed between albino and pigmented rats.

The ventricular sulcal pattern and the cytoarchitecture of the brainstem of Xenopus laevis, a pipid frog which retains a lateral line system throughout life, were studied in transverse Nissl- and Kluver-Barrera-stained serial sections. Four distinct longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, and the sulcus medianus superior, could be distinguished. With the aid of the usual cytoarchitectonic criteria 42 cell groups were delineated; seven of these are primary efferent or motor nuclei, 13 are primary efferent or sensory centers, seven nuclei are considered to be components of the reticular formation, and the remaining 15 cell masses can be indicated as “relay” nuclei. In order to provide a basis for experimental work, the topographical position of the nuclei is illustrated in photomicrographs of representative levels and in graphical reconstructions. The distribution of the cell masses and their relations to the ventricular sulci were studied with the aid of the reconstruction procedure termed topological analysis (cf. Nieuwenhuys, '74; Fig. 19). This analysis yielded the following results. The sulcus limitans extends throughout almost the entire brainstem, dividing this part of the brain into a motor basal plate and a sensory alar plate. The cell masses in the rhombencephalic basal plate fit into two longitudinal zones, a medial area ventralis and a lateral area intermedioventralis. The former contains somatomotor centers of primary and higher order, whereas the latter is composed of three primary visceromotor nuclei and one visceromotor coordinating center. The rhombencephalic alar plate is occupied by viscerosensory, general somatosensory, and special somatosensory cell masses. Two centers, the nucleus fasciculi solitarii and the nucleus visceralis secundarius, represent together a discontinuous viscerosensory zone, which is situated immediately dorsal to the sulcus limitans. The general somatosensory nuclei, i.e., the gracile and cuneate nuclei, the nucleus tractus descendens of V, and the nucleus princeps of V constitute a zone which largely overlaps the viscerosensory zone. The special somatosensory area, i.e., the area of termination of (1) primary vestibular, (2) primary acoustic, and (3) lateral line nerve fibers, is strongly developed and occupies a considerable part of the alar plate. Presumably, the fibers of each of the three categories mentioned terminate in three separate zones of gray. The midbrain can also be subdivided bilaterally into a basal plate and an alar plate. The former occupies the medial part of the tegmentum mesencephali and may be considered as the rostral extreme of the somatomotor zone. The latter comprises the lateral tegmentum and the tectum, areas which are chiefly occupied by special somatosensory (visual, acoustic, lateral line) centers.

An anterograde tracer study has been made of the developing corticospinal tract (CST) in the rat using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Analysis of normal Rager stained material revealed that corticospinal axons reach upper cervical spinal cord levels at the day of birth (PO). Postnatal rats ranging in age from one (P1) to fourteen (P14) days received multiple WGA-HRP injections into the cortex of their left hemisphere and were allowed to survive for 24 h. The first labeled CST fibers caudally extend into the third thoracic spinal cord segment at P1; into the eighth thoracic segment at P3; into the first or second lumbar segment at P7 and into the second to third sacral segment at Pg. Thus the outgrowth of the leading 'pioneer' fibers of the CST is completed at P9 but later developing axons are continuously added even beyond P9. Quantitative analysis of the amount of label along the length of the outgrowing CST revealed a characteristic pattern of labeling varying with age. The most striking features of that pattern are: the formation of two standing peaks at the level of the cervical and lumbar enlargements respectively and the transient presence of a smaller running peak which moves caudally with the front of the outgrowing bundle. The standing peaks are ascribed to the branching of the axon terminals at both intumescences, whereas the running peak probably arises by the accumulation of tracer within the growth cones at the tips of the outgrowing CST axons. Factors such as the number of axons, the varying axon diameters, the branching collaterals, the presence of varicosities, the transport rate of the tracer, the uptake of the tracer at the injection site, which possibly may affect the amount of label present in both the entire bundle and in the individual axons are discussed. Current research is focused upon an analysis of the relation between the site of injection within the cortex and the pattern of labeling of the CST. A delay of two days was found between the arrival of the CST axons at a particular spinal cord level and their outgrowth into the adjacent spinal gray. However, combined HRP and electronmicroscopic experiments are necessary to determine the factors behind the maturation of the CST as well as the maturation of the spinal gray.

The ventricular sulcal pattern and the cellular structure of the brain stem of the axolotl Ambystoma mexicanum have been studied in transversely cut Nissl and Bodian stained serial sections. Six longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis, the sulcus medianus superior and the sulcus lateralis mesencephali could be distinguished. A seventh groove, the sulcus isthmi, clearly deviates from the overall longitudinal pattern of the other sulci. Although most neuronal perikarya are contained within a diffuse periventricular gray, 19 cell masses could be delineated; seven of these are primary efferent or motor nuclei, four are primary afferent or sensory centers, four nuclei are considered as components of the reticular formation, and the remaining four cell masses can be interpreted as "relay" nuclei. In order to study the zonal pattern of the brain stem, this structure was subjected to a topological analysis (cf, Nieuwenhuys, '74 and fig. 13). This analysis yielded the following results. In the rhombencephalon the grisea are arranged in four longitudinal zones which, following Kuhlenbeck, have been termed area ventralis, area intermedioventralis, area intermediodorsalis and area dorsalis. Where present the sulcus intermedius ventralis, the sulcus limitans and the sulcus intermedius dorsalis mark the boundaries between these four morphological entities. The zonal areas in question coincide largely, but not entirely, with the so-called functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the nucleus fasciculi solitarii and the nucleus visceralis secundarius, two non-visceral sensory cell masses, namely the nucleus vestibularis magnocellularis and the nucleus cerebelli. The four morphological zones delineated in the rhombencephalon cannot be distinguished in the mesencephalon and it is of particular importance that the sulcus limitans does not extend into this part of the brain. Functionally, however, the medial part of the tegmentum mesencephali may be considered the rostral extreme of the somatic motor column, whereas the tectum primarily represents a somatic sensory correlation area.

Some projections from the lateral hypothalamic area in the rat have been investigated, using combinations of fluorescent tracers, injected into several different parts of the central nervous system. Projections appear to arise from loosely organized assemblies of neurons, called sets and from more densely packed assemblies, called clusters. The sets and clusters vary considerably in position and in distinctness of their borderlines. Even within extensive and vaguely denned sets, however, high concentrations of labeled neurons may be present at specified sites in the lateral hypothalamus. Such concentrations are observed in the transitional area of the zona incerta and the dorsal part of the lateral hypothalamus and in the ventrolateral part of the hypothalamus, bordering the cerebral peduncle and the subthalamic nucleus, in both cases after injections into some “autonomic centers” in the brainstem, such as the parabrachial nuclei and the dorsal vagal complex. Sets and clusters may overlap considerably. Within the fields of overlap the number of double labeled neurons may vary from almost zero up to more than 50%, depending on the injection sites. The results show that different parts of the lateral hypothalamus in the rat have different efferent relationships. Combination of the results of the present study with known data concerning the afferent relationships, the cytoarchitecture and behavioral functions of the lateral hypothalamic area, suggests that different parts of this entity are involved in different regulatory and behavioral functions.

The macroscopic anatomy of the brain of the single surviving crossopterygian species Latimeria chalumnae is described and depicted. The brain of this fish is slender and elongated. The rhombencephalon is well developed; its ventricular aspect shows four longitudinally arranged ridges which roughly correspond to the functional zones of Herrick and Johnston. The cerebellum comprises two extremely large auriculae and an unpaired, evaginated corpus cerebelli. The mesencephalon is small and does not show any marked differentiation of its surface. In the diencephalon, ventricular sulci mark the boundaries between the epithalamus, dorsal thalamus, ventral thalamus and hypothalamus. The dorsal thalamus protrudes into the ventricular cavity. The telencephalon can be clearly divided into a dorsal pallium and a ventral subpallium. The pallium is represented by a thickened, solid body. It is partly covered by a membranous roof, which in the median plane constitutes an ependymal septum. The subpallium is thin-walled and clearly evaginated. This structure and the ventral part of the pallium enclose a distinct lateral ventricle. The olfactory bulbs are connected with the telencephalon proper by extremely long olfactory peduncles.

In this paper the serotonin-containing neurons in the diencephalon and mesencephalon of the trout have been studied by immunocytochemistry. Serotonin-immunoreactive perikarya, fibres and terminals were visualized by using well-characterized antibodies to serotonin. Six areas could be demonstrated to contain serotonin-immunoreactive cellbodies . 1. A group of cells, situated ventral to the commissura posterior; 2. A region within the confines of the nucleus recessi lateralis and a few scattered cells dorsally to this nucleus; 3. Neurons, situated ventral to the recessus lateralis hypothalami, within the nucleus tuberis inferior; 4. Cells which are situated within the borders of the nucleus recessi posterioris ; 5. A number of cells, situated within the nucleus raphes superior and finally: 6. Perikarya, located dorsolateral to the nucleus raphes superior. The following three regions showed a very high density of serotonin-immunoreactive fibres and terminals: A. the most dorsal portion of the nucleus diffusus tori lateralis; B. the area dorsal to the recessus lateralis and C. the area surrounding the recessus posterior.

The corpus cerebelli of mormyrid fishes is strongly developed and differentiated into four lobes: C1−C4. Although all of these lobes contain the characteristic cerebellar layers: granular, ganglionic and molecular, they show distinct architectonic differences. A previous study revealed that the ganglionic layer of C1, in addition to Purkinje elements contains conspicuous giant cells. In the present paper the results of a further analysis of C1 are reported. This analysis is based on serially sectioned brains of Gnathonemus petersii, stained according to Nissl, Bodian and Haggquist. Semi-thin sections were stained with p-phenylenediamine. Routine EM techniques were used to visualize synaptic relations. Mossy fibres and granule, Golgi, Purkinje and stellate cells are located characteristically throughout C1. It appeared that the giant cells of a previous study represent the largest elements of a population which has been termed now the eurydendroid cells. The average size of the latter is somewhat larger than that of the Purkinje cells, but both groups of cells show a considerable overlap in the size of their somata. Purkinje cells and eurydendroid cells are present throughout the ganglionic layer and both have a flattened, sagittally oriented, dendritic tree that extends into the molecular layer. Yet, the eurydendroid cells (EC) display the following characteristics which distinguish them from Purkinje cells (PC): (1) In EC the Nissl substance is dispersed diffusely throughout the soma, whereas in PC it tends to be concentrated around the nucleus, (2) The soma/nucleus ratio for EC is distinctly larger than for PC, (3) The dendritic trees of EC extend over a larger stretch of the molecular layer than those of PC, hence the term EC, (4) The dendrites of EC are more widely spaced and oriented less strictly parallel to each other than those of PC, (5) The dendrites of EC are somewhat irregular in outline and not covered with spines, in contrast to those of PC, (6) The axons of EC are oriented radially and join bundles of coarse fibres which leave the cerebellum whereas the axons of PC extend and ramify within the ganglionic layer, (7) The somata of EC, contrary to those of PC, are enveloped by a dense axonal plexus which forms numerous synaptic terminals on them. The numerical ratio of EC: PC was 1:5.5. The circuitry in C1 and the possible functional roles of its constituent neurons are discussed. It is pointed out that in this lobe the axons of PC impinge on EC and that the latter constitute its output system.

In this paper the telencephalon of Latimeria chalumnae, the only surviving crossopterygian, is described and compared to that of other lower vertebrates. It is concluded that Latimeria cannot be related to a particular group of vertebrates, but stands intermediate between the dipnoans and the actinopterygians in its forebrain structure. With respect to the shape of the subpallium, the structure of the telencephalon medium, and the arrangement of its fiber systems, the latimerian forebrain closely approaches the dipnoan condition. The pallium and membranous structures of the telencephalon of Latimeria, on the contrary, are reminiscent in gross form and histological structure of their actinopterygian homologues. However, not all the structural features of the latimerian forebrain can be related to either the actinopterygian or the dipnoan plan. The subpallium, for instance, is more primitive than that of either group mentioned; in fact, it is more simply organized than that of any other living gnathostome. The forebrain of Latimeria appears to display no special structural affinities to the amphibian forebrain. This is not too surprising, since the Coelacanths, among which Latimeria is classified, represent only a side branch of the Crossopterygii, and are not in the main line of evolution to higher forms. It is known that members of the same class of lower vertebrates may vary considerably in their forebrain structure. Hence, the Rhipidistia, totally extinct Crossopterygii which are believed to have given rise to the terrestrial vertebrates, may have possessed a forebrain quite different from that of Latimeria.

This paper introduces a research project within the framework of which the brain stems of twelve representative vertebrates will be analysed according to a new procedure termed topological analysis. The first part reviews the development of the concept that the brain stem essentially consists of a number of longitudinal zones or columns. From this review it appears that, although during the past 80 years a considerable amount of evidence favoring this concept has been accumulated, a number of important questions pertaining to number, delimitation and content of the zones in the brain stem are still awaiting a definitive answer. The second part of this paper is devoted to a detailed description and a critical evaluation of the topological procedure. This procedure, which has been especially designed to facilitate the study of the zonal pattern of the brain stem, essentially involves two steps. First the cell masses are projected back to their site of origin at the ventricular surface, and next the ventricular surface, with its sulci and the projections of the cell masses marked upon it, is flattened out, i.e., is subjected to a one-to-one topological transformation. In the third part of this paper the choice of the species which will be studied within the framework of our program is discussed. The species selected are: the lamprey Lampetra fluviatilis, the shark Squalus acanthias, the actinopterygians Acipenser fulvescens, Lepisosteus osseus and Salmo irideus, the brachiopterygian Polypterus ornatipinnis, the lungfish Lepidosiren paradoxa, the crossopterygian Latimeria chalumnae, the amphibians Ambystoma mexicanum and Rana esculenta, the turtle Testudo hermanni, and the opossum Didelphis virginiana. In the final part the objectives of the project are discussed. Most important among these are: (1) furnishing clear and succinct analyses of the brain stems of representatives of all groups of vertebrates, (2) specification of the fundamental plan of the brain stem, (3) facilitation of the establishment of homologies and simplification of the nomenclature and (4) presentation of a sound basis for experimental hodological work.

The grey matter of the spinal cord consists of two lateral, wing-like expansions in cyclostomes, whereas in chondrichthyans it is differentiated into distinct dorsal and ventral horns, an arrangement similar to that found in all other vertebrates. The spinal grey matter in cartilaginous fishes contains cells of a wide size range, although in adults the larger elements are mainly confined to the ventral horns. Myelinated fibres are absent from the cyclostomes but the white matter of the chondrichthyan cord comprises numerous well-myelinated fibres.

The ventricular sulcal pattern and the cytoarchitectonic organization of the brain stem of the turtle Testudo hermanni have been studied in transversely cut, Nissl stained serial sections. Four longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans and the sulcus medianus superior could be distinguished. With the aid of the usual cytoarchitectonic criteria 54 cell groups have been delimited; 12 of these are primary efferent or motor nuclei, 13 are primary afferent or sensory centers; 10 of the nuclei distinguished are considered as belonging to the reticular formation. The remaining 19 cell masses can be indicated as “relay” nuclei. In order to provide a basis for experimental hodological work the topographical position of the nuclei is illustrated in photomicrographs of representative levels and in graphical reconstructions. In order to elucidate the morphological pattern of the cell masses and their relationship to the ventricular sulci the brain stem was subjected to a topological analysis. Essentially this method requires that (1) the cell masses are projected upon the ventricular surface and that (2) this surface, with its sulci marked on it and with the outlines of the subjacent cell masses projected upon it, is flattened out. This analysis yielded the following results: The sulcus limitans extends throughout the rhombencephalon, dividing this brain part into a basal plate and an alar plate. In the basal plate the sulcus intermedius ventralis marks the boundary between an area ventralis and an area intermedioventralis containing, respectively, the somatic motor nuclei of primary and higher order, and the visceral motor centers. Yet the terms somatic motor zone and visceral motor zone are not applicable to these two areas as a whole since both contain a number of non-motor centers. Thus the intermedioventral zone harbours two general somatic sensory centers, i.e., the spinal- and princeps nuclei of V. The medial parts of the tegmentum isthmi and of the tegmentum mesencephali contain somatic motor centers of primary and higher order, and thus may be considered rostral continuations of the rhombencephalic ventral zone. The rhombencephalic alar plate contains visceral sensory, general somatic sensory and special somatic sensory centers, but these do not show a simple ventrodorsal arrangement. A prominent feature is that the special somatic sensory nuclei of termination of VIII, the cerebellar nuclei, and a number of special somatic sensory centers of higher order together constitute a continuous zone which extends from the rhombencephalic alar plate, via the isthmus into the lateral part of the tegmentum mesencephali. In the isthmus region the sulcus limitans does not mark the ventral boundary of this zone, but rather passes over its middle.

The medial forebrain bundle (MFB) is a complex fiber system that courses through and partly arises and partly terminates within the lateral preoptic and lateral hypothalamic areas. It consists mainly of thin fibers and may be comprised of as many as 50 descending and ascending components of varying lengths and of different origins and/or destinations (Nieuwenhuys et al., '82). With the aid of an an atlas of the MFB and the surrounding brain areas in the rat presented in the preceding paper (Nieuwenhuys et al., '82), the position and topographic relationships of some 21 components of the bundle have been analyzed in detail, in brains that had been prepared for autoradiography following injections of tritiated amino acids into a number of structures that are known to contribute fibers to the MFB. From this analysis it is clear that most of the labeled components occupy specific and rather constant positions within the MFB. For example, the ascending components are largely confined to the dorsal half of the bundle; those arising from the medial preoptic area and the various hypothalamic nuclei are distributed rather diffusely over much of the MFB; and the descending components that arise from the olfactory tubercle and the magnocellular preoptic nucleus are confined to restricted parts of the bundle. These findings indicate that the neurons which occupy different parts of the lateral hypothalamic area probably receive distinctive inputs, and to a first approximation these are likely to be determined principally by their position within the MFB.

We have investigated the localization of oxytocin (OXT) and vasopressin (AVP) release sites in the neural lobe and median eminence (ME) of the rat. Ultrastructural and post-embedding immunocytochemical studies with protein A-gold as a marker were used for the identification of OXT and AVP neurosecretory granules (NSG). Release of the contents of these OXT and AVP NSG by exocytosis has been clearly demonstrated with the tannic acid-Ringer incubation method in axon terminals of the neural lobe and in varicose fibres with OXT and AVP NSG in the internal zone of the ME. As these fibers lack synaptic specializations, and are not located in the direct vicinity of capillaries of the primary portal plexus, these observations suggest that OXT and AVP are released in a paracrine way in the ME of the rat.

Acetylcholine was the first neurotransmitter to be identified. Choline acetyltransferase (ChAT) is the enzyme which is responsible for the synthesis of ACh, whereas the enzyme acetylcholinesterase (AChE) degrades this substance (Fig. 1). In numerous older studies, among which those of Shute and Lewis (Shute and Lewis 1967; Lewis and Shute 1967) especially should be mentioned, it was attempted to study the anatomy of central cholinergic neurons by means of AChE histochemistry. However, it is known that although cholinergic neurons generally show a very high content of AChE, a high activity of this enzyme is in itself not a sufficient characteristic for identifying cholinergic neurons (Lehmann and Fibiger 1979; Butcher and Woolf 1982; Eckenstein and Sofroniew 1983). The successful purification of ChAT and the subsequent procurement of antibodies against this enzyme for use in immunochemistry have recently yielded a reliable means of visualizing cholinergic neurons in the central nervous system (cf. McGeer et al. 1984 a; Wainer et al. 1984; Rossier 1984). The following survey of the major central cholinergic neuron populations and their efferent projections (Fig. 2) is based primarily on the studies of Kimura et al. (1981), Fibiger (1982) and Mesulam et al. (1983a, b, 1984a).