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Actin-myosin contraction has been shown to play a major role in early morphogenetic movements in Drosophila (fly) and Xenopus (frog) [1,2]. However, the specific role of actomyosin contractility in amniote embryos (reptiles, birds, and mammals) during primitive streak (PS) formation, the “organizing center” for gastrulation (formation of three primary germ layers), is not known. Current theories regarding primitive streak formation in higher order amniotes center around cell-cell intercalation or chemotactic cell movement [3,4]. We hypothesize that contraction via actin-myosin (AM) filaments is conserved from anamniotes and drives formation of the PS and the associated morphogenetic cell movements.Copyright © 2010 by ASME

The vertebrate eye receives innervation from ciliary and pterygopalatine parasympathetic and cervical sympathetic ganglia as well as sensory trigeminal axons. The sympathetic and parasympathetic pathways represent the classical "core" of neural regulation of ocular homeostasis. Sensory trigeminal neurons are also involved in autonomic regulation by both providing the afferent limb of various reflexes and exerting their peptide-mediated local effector function. This arrangement is remarkably conserved throughout vertebrate classes although significant modifications are observed in anamniotes, in particular their irises. In higher primates and birds, intrinsic choroidal neurons emerged as a significant additional innervation component. They most likely mediate local vascular regulation and other local homeostatic tasks in foveate eyes. This review across the vertebrate classes outfolds the complex neuronal regulatory underpinnings across vertebrates that ensure proper visual function.

Publisher Summary This chapter provides an overview of the traits of the mammalian basal ganglia (BG) that are pertinent to its identification in nonmammals. It reviews the modern findings on the organization of the basal ganglia in each extant vertebrate group and discusses the implications for BG evolution. The chapter reviews that BG evolution in vertebrates reveals that both the striatum and pallidum are ancient structures, both likely present in the jawed fish ancestral to modern jawed vertebrates. Thus, the notion that the pallidum evolved first and is older than the striatum is incorrect. The reason that the terms paleostriatum and neostriatum perpetuate outdated ideas about basal ganglia evolution, their abandonment is recommended. The actual evolutionary history of the basal ganglia seems characterized by an increase in neuron number as the telencephalon expanded during the anamniote-amniote transition, with the elaboration of prominent cortical glutamatergic inputs and midbrain dopaminergic inputs, and an increased role for telencephalic circuitry in motor control occurring in stem amniotes. In mammals, especially the primate lineage, this trend has been accelerated. Nonetheless, the basic direct-indirect pathway circuit plan by which the basal ganglia regulate movement may have already been in place in early anamniotes.

Quantitative analyses based upon the superimposition of phylogenetic and reproductive data have revealed that viviparity has originated on at least 132 independent occasions among vertebrates, with 98 of these origins having occurred among reptiles. The viviparous lineages have given rise to at least 24 matrotrophic clades, all but four of which are anamniotes. Traditional scenarios assume progressive, gradualistic evolution from oviparity to lecithotrophic viviparity to matrotrophic viviparity. However, mammalian evidence indicates that matrotrophy can precede the evolution of viviparity. Moreover, data on reptiles seem to be consistent with a punctuated equilibrium model for viviparity and a saltatory model for incipient matrotrophy and placentation. Among the specializations for fetal nutrition, strong convergence is evident at organismal, organological, and cytological levels. Examples include yolk sac placentation, trophotaeniae, and adaptations for embryonic cannibalism. Certain lizards of the genera Mabuya and Chalcides have converged strongly on eutherian mammals with respect to morphology of the chorioallantoic placenta. Placental specializations that have evolved independently in some eutherians and matrotrophic lizards include placentomes, giant binucleate cells, deciduate maternal tissue, and chorionic areolae.

The axial motor column has undergone a major reorganization during the evolution of vertebrates. In aquatic anamniotes including lampreys, goldfish, and mudpuppies, epaxial and hypaxial motoneurons are intermingled in the column. In contrast, epaxial and hypaxial motoneurons are spatially segregated in water snakes, rats, and monkeys, apparently as a consequence of an isomorphic mapping of motoneuron location onto the position of innervated muscle in the embryonic myotome. The presence of these two very different arrangements of motoneurons requires a major restructuring of the motor column during vertebrate evolution. The time of this reorganization is unknown. All amniotes studied to date have an epaxial/hypaxial segregation, and all anamniotes do not, suggesting that the map arose with the origin of amniotes. All the anamniotes examined previously were permanently aquatic, however, and the map might therefore be associated with terrestrial locomotion. If so, we would expect terrestrial anamniotes to have an arrangement of motoneurons like that in amniotes. We studied the organization of motoneurons innervating the trunk muscles of postmetamorphic, terrestrial tiger salamanders and asked whether their motor columns are more like those of amniotes or those of aquatic anamniotes. The motor column in tiger salamanders is similar to that seen in aquatic anamniotes and very like that in mudpuppies--permanently aquatic salamanders. There are several classes of motoneurons with morphological similarities to the primary and secondary motoneurons characteristic of aquatic anamniotes. Epaxial and hypaxial motoneurons show no obvious morphological differences and occupy extensively overlapping positions in the motor column. The only epaxial/hypaxial distinction is the presence of a few, small, relatively undifferentiated motoneurons located subadjacent to the ependymal layer. These motoneurons are filled only by horseradish peroxidase (HRP) applied to hypaxial nerves. They are probably newly born motoneurons, and their presence suggests continued addition of motoneurons, even in adult salamanders. We conclude that the epaxial/hypaxial segregation seen in amniotes is not necessarily associated with terrestrial locomotion. The segregation and the topographic map it reflects may have arisen in conjunction with the origin of amniotes. If they instead arose prior to the origin of extant amphibians, they must have been secondarily lost in those salamanders studied to date. An examination of the motor column of other amphibians should help to resolve this issue.

The vertebrate brain is highly complex with millions to billions of neurons. During development, the neural plate border region gives rise to the neural crest, cranial placodes and, in anamniotes, to Rohon-Beard sensory neurons, whereas the boundary region of the midbrain and hindbrain develops organizer properties. Comparisons of developmental gene expression and neuroanatomy between vertebrates and the basal chordate amphioxus, which has only thousands of neurons and lacks a neural crest, most placodes and a midbrain-hindbrain organizer, indicate that these vertebrate features were built on a foundation already present in the ancestral chordate. Recent advances in genomics have provided insights into the elaboration of the molecular toolkit at the invertebrate-vertebrate transition that may have facilitated the evolution of these vertebrate characteristics.

http://www.springerlink.com/content/w1v58u6352h0132r/?p=6833eb34cc254a6cbda28e68a398aaf3&pi=2; International audience; The telencephalon is the most anterodorsal craniate/vertebrate brain division and forms with its anteroventral complement, the hypothalamus, the most rostral portion of the neural tube. The telencephalon represents part of the ancestral craniate (and, thus, anamniote) brain Bauplan (morphotype). The craniate telencephalon includes ventrally a subpallial region (medially: septum; laterally: striatum) and dorsally a pallial region. In addition to olfactory bulb input, the pallium receives ascending sensory information in all craniates, with largely separate pallial terminations in all jawed vertebrates (gnathostomes). While it is likely that a lateral pallium (olfactory cortex homologue) and a medial pallium (hippocampus homologue) arose at the latest with the gnathostomes (and likely are ancestral for craniates), a dorsal pallium may have arisen several times independently in chondrichthyans, teleosts and amniotes. A separate pallidal subdivision of the craniate striatal formation may only have evolved with the gnathostomes.

To gain more insight into the dopaminergic system of amphibians and the evolution of catecholaminergic systems in vertebrates in general, the distribution of dopamine and tyrosine hydroxylase immunoreactivity was studied in the brains of the anuran Rana ridibunda and the urodele Pleurodeles waltlii. In both species, dopamine-immunoreactive (DAi) cell bodies were observed in the olfactory bulb, the preoptic area, the suprachiasmatic nucleus, the nucleus of the periventricular organ and its accompanying cells, the nucleus of the posterior tubercle, the pretectal area, the midbrain tegmentum, around the solitary tract, in the ependymal and subependymal layers along the midline of the caudal rhombencephalon, and ventral to the central canal of the spinal cord. Tyrosine hydroxylase (TH) immunohistochemistry revealed a similar pattern, although some differences were noted. For example, with the TH antibodies, additional cell bodies were stained in the internal granular layer of the olfactory bulb and in the isthmal region, whereas the same antibodies failed to stain the liquor contacting cells in the nucleus of the periventricular organ. Both antisera revealed an almost identical distribution of fibers in the two amphibian species. Remarkable differences were observed in the forebrain. Whereas the nucleus accumbens in Rana contains the densest DAi plexus, in Pleurodeles the dopaminergic innervation of the striatum prevails. Moreover, cortical structures of the newt contain numerous DAi fibers, whereas the corresponding structures in the frog are devoid of immunoreactivity. The dopaminergic system in amphibians appears to share many features not only with other anamniotes but also with amniotes.

The lateral-line system of gars consists of superficial neuromasts, which are arranged in lines termed pit lines, canal neuromasts and spiracular organs, which are located within diverticula of the hyoid gill pouch. Both canal and superficial neuromasts possess polarized hair cells whose directional sensitivity parallels the long axis of their respective lines. However, the apical surfaces of canal neuromasts are larger and possess far more hair cells than do those of superficial neuromasts, but superficial neuromasts have longer kinocilia and, presumably, longer cupulae. The receptors of the lateral-line system are innervated by three pairs of cranial nerves: anterior, middle and posterior lateral-line nerves. The anterior lateral-line nerves innervate neuromasts of the supraorbital, infraorbital and preoperculo-mandibular canals as well as dorsally located anterior pit lines, cheek (horizontal, vertical and mandibular) and gular pit lines of superficial neuromasts and the spiracular organ. The middle lateral-line nerves innervate dorsally located middle pit lines and a single neuromast in each temporal canal. The posterior lateral-line nerves innervate dorsally located posterior pit lines, neuromasts of the supratemporal commissures and all remaining postotic and trunk neuromasts. The ganglion of the anterior lateral-line nerve is divided into dorsal and ventral subganglia; the single ganglion of the middle lateral line nerve has no recognizable subdivisions, and the ganglion of the posterior lateral-line nerve consists of rostral and caudal subganglia. Analysis of the roots of these nerves and review of the embryonic origin of their ganglia as well as comparisons with cranial nerves in other anamniotes suggest that the anterior and posterior lateral-line nerves of gars may represent the fusion of four to five separate lateral-line nerves at some stage in vertebrate phylogeny. Thus, with the addition of the middle lateral-line nerve, and the possible existence of a ventral lateral-line nerve of the trunk, it is possible that the earliest jawed vertebrates possessed six or even seven pairs of lateral-line nerves.

Two different patterns of the condensation and chondrification of the limbs of tetrapods are known from extensive studies on their early skeletal development. These are on the one hand postaxial dominance in the sequential formation of skeletal elements in amniotes and anurans, and on the other, preaxial dominance in urodeles. The present study investigates the relative sequence of ossification in the fore- and hindlimbs of selected tetrapod taxa based on a literature survey in comparison to the patterns of early skeletal development, i.e. mesenchymal condensation and chondrification, representing essential steps in the late stages of tetrapod limb development. This reveals the degree of conservation and divergence of the ossification sequence from early morphogenetic events in the tetrapod limb skeleton. A step-by-step recapitulation of condensation and chondrification during the ossification of limbs can clearly be refuted. However, some of the deeper aspects of early skeletal patterning in the limbs, i.e. the general direction of development and sequence of digit formation are conserved, particularly in anamniotes. Amniotes show a weaker coupling of the ossification sequence in the limb skeleton with earlier condensation and chondrification events. The stronger correlation between the sequence of condensation/chondrification and ossification in the limbs of anamniotes may represent a plesiomorphic trait of tetrapods. The pattern of limb ossification across tetrapods also shows that some trends in the sequence of ossification of their limb skeleton are shared by major clades possibly representing phylogenetic signals. This review furthermore concerns the ossification sequence of the limbs of the Palaeozoic temnospondyl amphibian Apateon sp. For the first time this is described in detail and its patterns are compared with those observed in extant taxa. Apateon sp. shares preaxial dominance in limb development with extant salamanders and the specific order of ossification events in the fore- and hindlimb of this fossil dissorophoid is almost identical to that of some modern urodeles.

The dorsomedial telencephalon of lepidosirenid lungfishes has been interpreted in two divergent ways: earlier investigators regarded it as a subpallial (septal) structure; more recently, it has been reinterpreted as the medial pallium (hippocampus). To resolve this question, we identified parameters that are conclusive in their association with either the medial pallium or the septum in anamniotes. The present study examines the position of ependymal thickenings and the distribution of acetylcholinesterase (AchE) in the cerebral hemispheres of the African lungfish Protopterus, the Australian lungfish Neoceratodus, and the amphibian species Xenopus and Ambystoma. In addition, projections from the hypothalamus (paraventricular organ) to the telencephalon are investigated in Protopterus. Ependymal specializations are located dorsally and ventrally in the lateral ventricles of amphibians, but laterally and medially in lungfishes. In Protopterus, the paraventricular organ projects to the medial telencephalic hemisphere, but not to the dorsal roof. High levels of AchE are present in restricted neuropil regions of the medial hemisphere and in the ventral and ventrolateral telencephalon, but they are lacking in the dorsal roof. Intensely AchE-stained neuronal cell bodies are located in the ventral telencephalon (rostrally) and the dorsomedial telencephalon (at mid-level). In Neoceratodus, AchE staining is pronounced in the septal area, but absent in the pallium. The terminal nerve proper lacks AchE stain in Protopterus; nerve fibres of the preoptic nerve are AchE-positive in both lungfish species. In Xenopus, AchE staining of fibers and terminals is restricted to the subpallium (medial septum, tuberculum olfactorium, striatum, nucleus accumbens, and medial amygdala); cell bodies are AchE positive in parts of the subpallium and rostral pallium. Comparison of cytological, histochemical, and "connectional" parameters substantiates the interpretation that the dorsomedial telencephalon of lungfishes represents a subpallial, but not a "medial pallial" structure. The dorsomedial part of the lepidosirenid telencephalon corresponds to the septum in the most plesiomorphic living lungfish, Neoceratodus forsteri, but it differs considerably from the dorsomedial telencephalon (medial pallium) in amphibians.

To improve knowledge of the peptidergic systems in the brain of amphibians we have conducted a comparative analysis of the distribution of TRH immunoreactive cell bodies and fibers in three species of urodeles. Fiber labeling was observed in all main brain subdivisions suggesting different control functions for TRH in extrahypothalamic systems. However, as in other vertebrates, TRH neurons were abundant in the hypothalamic nuclei that presumably project to the median eminence and the neural lobe of the hypophysis. Considerable interspecies differences were noted mainly related to innervation of the olfactory and visual centers (thalamus and mesencephalic tectum) and the precise localization of immunoreactive cell bodies, which was assessed by double labeling with tyrosine hydroxylase. The comparison of the distribution of TRH immunoreactive neurons and fibers found in urodeles with those reported for other vertebrates, in particular with anamniotes, reveals a strong resemblance but also notable variations not only across vertebrate classes but also within the same class. In this respect, the virtual lack in urodeles of TRH innervation of the intermediate lobe of the hypophysis clearly contrasts with the innervation found in anurans. Therefore, the important role of skin color adaptation proposed for TRH in anurans on the basis of the direct innervation of the intermediate lobe is not applicable for urodeles.

Numerous studies over the last few years have demonstrated that the amygdaloid complex in amniotes shares basic developmental, hodological and neurochemical features. Furthermore, homologous territories of all the main amygdaloid subdivisions have been recognized among amniotes, primarily highlighted by the common expression patterns for numerous developmental genes. Thus, derivatives from the lateral pallium, ventral pallium and subpallium constitute the fundamental parts of the amygdaloid complex. With the development of new technical approaches, study of the precise neuroanatomy of the telencephalon of the anuran amphibians (anamniotes) has been possible. Current embryological, hodological and immunohistochemical evidence strongly suggests that most of the structures present in amniotes are recognizable in these anamniotes. These investigations have yielded enough results to support the notion that the organization of the anuran amygdaloid complex includes subdivisions with their origin in ventral pallial and subpallial territories; a strong relationship with the vomeronasal and olfactory systems; abundant intra-amygdaloid connections; a main output centre involved in the autonomic system; recognizable amygdaloid fibre systems; and distinct chemoarchitecture. Therefore, the new ideas regarding the amygdaloid evolution based on the recent findings in anamniotes, and especially in anurans, strongly support the notion that basic amygdaloid structures were present at least in the brain of ancestral tetrapods organized following a basic plan shared by tetrapods.

The sympathetic nervous system develops as an evolutionary trait with gnathostomes (jawed vertebrates), but not with agnathan fishes (i.e., hagfishes and lampreys). Organization of the sympathetic preganglionic neuronal columns is different in teleosts and anurans. In the teleosts so far examined, the majority of sympathetic preganglionic neurons (SPNs) are located in the dorsal part of the spinal central gray matter. In Tetraodontiformes, the cell column occupies only two rostral spinal segments, which are distinct in their cytoarchitecture and projections. On the other hand, the SPNs of anurans form two cell columns segregated mediolaterally. The lateral and medial columns are also distinct in their cytoarchitecture and projections. The neuroactive substances expressed in the SPNs both in teleosts and anurans are coded to the projections. In anurans, the SPNs containing gonadotrophin-releasing hormone and those containing calcitonin gene-related peptide are involved in the regulation of blood vessels and cutaneous glands, respectively. In the filefish, the SPNs containing galanin project specifically to non-adrenergic non-cholinergic postganglionic neurons in the cranial sympathetic ganglia. Therefore, both anuran and teleost systems have different morphological and chemical-coded patterns for functional variation, although the anuran sympathetic nervous system has more organizational similarity with that of amniotes.

The organization of the forebrain in amphibians (anamniotes) is currently being re-evaluated in terms of evolution and several evidences have corroborated numerous traits shared by amphibians and amniotes, such as the organization of the basal ganglia and the amygdaloid complex. In the present study we have analysed the organization of forebrain afferent systems to the hypothalamus of the frog Rana perezi. In vivo and in vitro tract-tracing techniques with dextran amines and immunohistochemistry for localizing nitric oxide synthase (NOS) in a series of single or combined experiments were used as NOS labelling reveals hypothalamic afferents arising from the lateral amygdala and the combination allowed analysis of the relationship between fibers of different origins in the same section. The results showed a large segregation of afferents in the hypothalamic region depending on their site of origin in the forebrain. Four highly topographically organized prosencephalic tracts reaching the anuran hypothalamus were observed: (i) the medial forebrain bundle, from the medial pallium and septal complex; (ii) the caudal branch of the stria terminalis formed by fibers arising in the lateral and medial amygdala; (iii) part of the lateral forebrain bundle with fibers from the central amygdala and (iv) the dorsal thalamo-hypothalamic tract. Fibers coursing in each tract reach the hypothalamus and terminate in distinct fields. The resemblance in pattern of forebrain-hypothalamic organization between amphibians and amniotes suggests that this feature represents an important trait conserved in the evolution of all tetrapods and therefore essential for the hypothalamic function.

This chapter focuses on the origin of Sertoli cells, during both the initial development of the testes and testicular recrudescence, and their role in the control and support of spermatogenesis. This chapter also highlights the current understanding of Sertoli cell biology in major anamniote classes, namely, the elasmobranchs (cartilaginous fishes, Chondrichthyes), teleosts (bony fishes, Osteichthyes), anurans (frogs and toads, Amphibia), and urodeles (salamanders and newts, Amphibia). A major structural difference between the testes of anamniotes and amniotes is the biological compartment in which spermatogenesis occurs. In amniotes, spermatogenesis is mediated by or involves a permanent germinal epithelium and takes place within a fixed structure, the seminiferous tubule. However, in many anamniotes, the spermatogenic unit is a transient structure, the spermatogenic cyst, and in most fishes and urodeles there does not appear to be a permanent germinal epithelium; spermiation usually accompanies Sertoli cell degeneration.

The lateral line is a placodally derived mechanosensory organ in anamniotes that detects the movement of water. In zebrafish embryos, a migrating primordium deposits seven to nine clusters of sensory hair cells, or neuromasts, at intervals along the trunk. Postembryonically, neuromasts continue to be added. We show that some secondary neuromasts arise from a pool of latent precursors that are deposited by the primordium between primary neuromasts. Interneuromast cells lie adjacent to the lateral line nerve and associated glia. These cells remain quiescent while they are juxtaposed with the glia; however, when they move away from the nerve they increase proliferation and form neuromasts. If glia are manually removed or genetically ablated by mutations in cls/sox10, hypersensitive (hps), or rowgain (rog), neuromasts precociously differentiate. Transplantation of wt glia into mutants rescues the appropriate temporal differentiation of interneuromast cells. Our studies reveal a role for glia in regulating sensory hair cell precursors.

Pineal evolution is envisaged as a gradual transformation of pinealocytes (a gradual regression of pinealocyte sensory capacity within a particular cell line), the so-called sensory cell line of the pineal organ. In most non-mammals the pineal organ is a directly photosensory organ, while the pineal organ of mammals (epiphysis cerebri) is a non-sensory neuroendocrine organ under photoperiod control. The phylogenetic transformation of the pineal organ is reflected in the morphology and physiology of the main parenchymal cell type, the pinealocyte. In anamniotes, pinealocytes with retinal cone photoreceptor-like characteristics predominate, whereas in sauropsids so-called rudimentary photoreceptors predominate. These have well-developed secretory characteristics, and have been interpreted as intermediaries between the anamniote pineal photoreceptors and the mammalian non-sensory pinealocytes. We have re-examined the original studies on which the gradual transformation hypothesis of pineal evolution is based, and found that the evidence for this model of pineal evolution is ambiguous. In the light of recent advances in the understanding of neural development mechanisms, we propose a new hypothesis of pineal evolution, in which the old notion ‘gradual regression within the sensory cell line’ should be replaced with ‘changes in fate restriction within the neural lineage of the pineal field’.

Recently, a thorough, comparative analysis of catecholamine (CA) systems in the brain and spinal cord of vertebrates was done (1). The results of that study have forced us to reconsider our view on the organization of CA systems. Evidence has been provided for the existence of extensive, putatively CA cell groups that do not fit within the A1A17/C1-C3 classification as proposed by Hokfelt and colleagues (2). Moreover, in contrast with the classical idea that the evolution of CA systems is marked by an increase in complexity going from anamniotes (fishes, amphibians) to amniotes (reptiles, birds, mammals), it is now evident that the brains of anamniotes contain CA cell groups, of which the counterparts in amniotes have lost the capacity to produce CAs. In addition, it has become clear that a segmental approach in studying the organization of CA systems is preferable. Such an approach has been fruitful in the comparison of dopamine (DA) cell groups among vertebrates, in particular between amniotes and anamniotes. In that same review data were brought together about the distribution of receptors and CA fibers as well as about developmental aspects. From these data it is obvious that there is a good match between CA fibers and receptors, but at many places volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of CA cell groups, in particular those in the retina and olfactory bulb.

Histochemical, pathway tracing, and neuropeptide/neurotransmitter localization studies in birds, reptiles and mammals during the 1970s and 80s clearly showed that the telencephalon in all amniotes consists of a prominent ventrally situated subpallial region termed the basal ganglia, and a large overlying region involved in higher order information processing termed the pallium or cortex. These studies also showed that the basal ganglia in all extant amniote groups possessed neurochemically and hodologically distinct striatal and pallidal territories. More recently, studies of the localization of genes controlling regional brain development have confirmed the homology of the basal ganglia among amniotes. In our ongoing studies, we have identified several aspects of the functional organization of the basal ganglia that birds also share with mammals. These include: (1) an extensive glutamatergic “cortico”-striatal input and distinctive, cell-type specific localization of glutamate receptor subtypes; (2) an extensive, presumptively glutamatergic intralaminar thalamic input to striatal neurons; (3) an extensive dopaminergic input from the midbrain targeting both substance P (SP) type and enkephalin (ENK) type striatal projection neurons, with SP-type striatal neurons seemingly richer in the D-1 type dopamine receptor; and (4) SP+ and ENK+ striatal outputs giving rise to functionally distinct so-called direct and indirect motor output pathways, with the direct pathway having a pallido-thalamo-motor cortex loop and the indirect pathway relaying back to the direct circuit via the subthalamic nucleus. These findings suggest that the major aspects of the cellular organization and functional circuitry of the basal ganglia in stem amniotes were already as observed in living amniotes, as therefore presumably was its key role in movement control. Because the organization of the basal ganglia of anamniotes is clearly less elaborate than in amniotes, and because the basal ganglia and cortex in amniotes are clearly extensively interconnected structures, it seems likely that stem amniotes were characterized by a major step forward in the grade of telencephalic organization of both the basal ganglia and the pallium.

PMCID: PMC1468093, The basal ganglia (BG) have received much attention during the last 3 decades mainly because of their clinical relevance. Our understanding of their structure, organisation and function in terms of chemoarchitecture, compartmentalisation, connections and receptor localisation has increased equally. Most of the research has been focused on the mammalian BG, but a considerable number of studies have been carried out in nonmammalian vertebrates, in particular reptiles and birds. The BG of the latter 2 classes of vertebrates, which together with mammals constitute the amniotic vertebrates, have been thoroughly studied by means of tract-tracing and immunohistochemical techniques. The terminology used for amniotic BG structures has frequently been adopted to indicate putative corresponding structures in the brain of anamniotes, i.e. amphibians and fishes, but data for such a comparison were, until recently, almost totally lacking. It has been proposed several times that the occurrence of well developed BG structures probably constitutes a landmark in the anamniote-amniote transition. However, our recent studies of connections, chemoarchitecture and development of the basal forebrain of amphibians have revealed that tetrapod vertebrates share a common pattern of BG organisation. This pattern includes the existence of dorsal and ventral striatopallidal systems, reciprocal connections between the striatopallidal complex and the diencephalic and mesencephalic basal plate (striatonigral and nigrostriatal projections), and descending pathways from the striatopallidal system to the midbrain tectum and reticular formation. The connectional similarities are paralleled by similarities in the distribution of chemical markers of striatal and pallidal structures such as dopamine, substance P and enkephalin, as well as by similarities in development and expression of homeobox genes. On the other hand, a major evolutionary trend is the progressive involvement of the cortex in the processing of the thalamic sensory information relayed to the BG of tetrapods. By using the comparative approach, new insights have been gained with respect to certain features of the BG of vertebrates in general, such as the segmental organisation of the midbrain dopaminergic cell groups, the occurrence of large numbers of dopaminergic cell bodies within the telencephalon itself and the variability in, among others, connectivity and chemoarchitecture. However, the intriguing question whether the basal forebrain organisation of nontetrapods differs essentially from that observed in tetrapods still needs to be answered.

The vertebrate inner ear is phylogenetically ancient. The inner ear of both extinct and extant species of the earliest vertebrates, the Agnatha (jawless fishes), has a variety of anatomical characteristics that were not retained in later vertebrates, and it is uncertain whether extant agnathans can hear (Popper and Fay, Chapter 3). This chapter describes the acoustic circuits of the three classes of jawed anamniotes: the Chondrichthyes (cartilaginous fish), the Osteichthyes (bony fish), and the Amphibia.

Bullock and Basar (1988) report that “when the extracellular ongoing, spontaneous potentials recorded from the surface or the depth of the cerebrum are compared in terms of gross wave form and power spectral density, there are no systematic differences among vertebrate classes (mammals, reptiles, birds, amphibians, bony and cartilaginous fishes) and that the differences in brain electrical activity between fish and mammals are smaller, if any, in spite of the major histological differences, than those in different states and loci within the same individual.” For instance, among various bony fishes and elasmobranchs there are no differences in the parameters of ongoing activity. On the other hand, cerebellar cortex, tegmentum, medulla, and cord show quite different forms of ongoing activity (Bullock and Basar 1988).

While a basal ganglia with striatal and pallidal subdivisions is 1 clearly present in many extant anamniote species, this basal ganglia is cell sparse and receives only a relatively modest tegmental dopaminergic input and little if any cortical input. The major basal ganglia influence on motor functions in anamniotes appears to be exerted via output circuits to the tectum. In contrast, in modern mammals, birds, and reptiles (i.e., modern amniotes), the striatal and pallidal parts of the basal ganglia are very neuron-rich, both consist of the same basic populations of neurons in all amniotes, and the striatum receives abundant tegmental dopaminergic and cortical input. The functional circuitry of the basal ganglia also seems very similar in all amniotes, since the major basal ganglia influences on motor functions appear to be exerted via output circuits to both cerebral cortex and tectum in sauropsids (i.e., birds and reptiles) and mammals. The basal ganglia, output circuits to the cortex, however, appear to be considerably more developed in mammals than in birds and reptiles. The basal ganglia, thus, appears to have undergone a major elaboration during the evolutionary transition from amphibians to reptiles. This elaboration may have enabled amniotes to learn and/or execute a more sophisticated repertoire of behaviors and movements, and this ability may have been an important element of the successful adaptation of amniotes to a fully terrestrial habitat. The mammalian lineage appears, however, to have diverged somewhat from the sauropsid lineage with respect to the emergence of the cerebral cortex as the major target of the basal ganglia circuitry devoted to executing the basal ganglia-mediated control of movement.

Intraaxonal labeling studies in the toadfish, frog, turtle, and chinchilla suggest broad evolutionary trends in the vertebrate crista ampullaris. The crista of anamniotes (fish, amphibians) contains type II hair cells innervated by bouton afferents and is longitudinally organized. Type I hair cells are first seen in reptiles and birds, where they are confined to a central zone and are innervated by calyx and dimorphic afferents. The central zone is surrounded by a peripheral zone containing only type II hair cells innervated by bouton afferents. Results in the turtle suggest that the peripheral zone in reptiles and birds is organized similarly to the entire anamniote crista. The turtle central zone finds no parallel in anamniotes but resembles the mammalian central zone in its structure and afferent physiology. With the advent of a central zone in reptiles, a concentric organization is superimposed on a linearly organized peripheral zone. The mammalian crista, in contrast, has an entirely concentric organization. This may be related to the extension of the neuroepithelium further down the slopes of the crista in mammals than in other vertebrates and to the distribution of type I hair cells throughout the mammalian neuroepithelium.

Publisher Summary This chapter reviews the unique features of the zebrafish embryo that make it attractive for the study of early neural development, and examines how experimental manipulation of this system has contributed to the understanding of key events in neurogenesis and the patterning of neural tissue. Vertebrates such as zebrafish and other anamniotes including Xenopus are characterized by the presence of an early population of neurons called primary neurons. These neurons are part of a relatively simple nervous system that differentiates early to coordinate the activity of the free living larvae. The simplicity of the early neuronal network facilitates the examination of the distribution and definition of the axonal projections of primary neurons. This chapter illustrates how a combination of experimental approaches has been used in this system to answer specific developmental questions related to neurogenesis using selected examples rather than providing an exhaustive account of the subject. A combination of different approaches has allowed the investigation of cellular and molecular mechanisms of neurogenesis in the zebrafish embryo.

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.

Over 70 years ago John Tait raised a question with regards to the certainty that the line of division between the hearing and vestibular portions of the VIIIth cranial nerve had been drawn at the proper place [J. Tait, Ann. Otol. Rhinol. Laryngol. 41, 681–704 (1932)]. His reason for raising this question was the evidence which existed at that time that the otolith organs of anamniotes (fish and amphibians), most commonly the sacculus, were demonstrably organs of hearing in many species. He went on to speculate about a possible auditory role for the otolith organs in amniotes (reptiles, birds, and mammals), including in humans, although at this time there was no direct evidence for this. Since this time, however, there has developed a significant literature documenting evidence, not only for anamniote otolithic hearing, but also for a conserved acoustic sensitivity for the amniote otoliths. In the case of humans, this sensitivity has been exploited for the development of new acoustic tests of vestibular function. The time is ripe 70 years on, therefore, for us to revisit John Tait’s question and explore the possibility that the sacculus may indeed contribute to human hearing. [Work supported by the Royal Society.]

This chapter explores the composition of animals' body, and that water is the most important substance in terms of composition, whereby animals contain 70% water by mass and 99% water by number of molecules. It also shows that the transition from Anamniotes to Amniotes is a soft-tissue transition of the postcranial skeleton and the evolutionary pressure on Paleozoic anamniote tetrapods led them out of water. The most common avenues of water gain are ingestion in the food or by drinking; osmosis through permeable skin, as in frogs and toads; and production of metabolic water de novo during oxidation of carbohydrates, fats; and protein for energy production. It highlights the water budgets of modern animals. Highly permeable skin is a feature shared by all Lissamphibia, allowing cutaneous respiration, and that the skin of Lissamphibia is modified in such a way that it does not provide barrier to evaporative water loss or gain. The accumulation of nitrogen waste is a constraint on terresstriality in extant Lissamphibia. Aquatic amphibians produce copious dilute urine because water is freely available and excrete nitrogenous wastes through this urine. Amphibians are limited in duration of their terrestrial excursions by plasma urea levels, and this problem becomes acute when the animal is feeding out of water.

Amniotes belong to a group of tetrapod vertebrates that lay eggs (keys to innovation), which are specially adapted to survive in a terrestrial environment. In this chapter, an effort has been made to explain the fundamental macro evolutionary changes that led to the radiation of reptiles, birds, and mammals. The transition from water to air breathing or the evolution from ectothermy to endothermy has been examined in various disciplines. The purpose is to bring together paleontologists' and neonatologists' knowledge to examine this very important evolutionally transition. Amniotes were the first vertebrates to complete the transition to land, adapted to behavioral changes like locomotion, fright, air respiration, and other thermoregulatory strategies. It provides a broad overview of the vertebrate body evolution and analysis of the influences physiologists and morphologists have on understanding the extinct animals that made the transition from anamniotes to amniotes. An elaboration has been illustrated on how scientific and historical perspectives have led to the discovery of fossils that has led to the understanding of the early amniotes, with the many controversial theories surrounding nomenclature and phylogeny of groups, formally known as the Amphibia and the reptilia.

This chapter explores the evolution of the Amniote. The Amniote egg presumably evolved from anamniote egg that gave rise via direct development to fully formed, precocial young. The precursor egg, like eggs of extant amphibians, were enclosed by a capsule composed of the vitelline membrane jelly layers. The shell membrane provided physical support that the capsule alone could not supply and the transfer of water from the jelly to the yolk functionally eliminated the jelly from the Yolk, which functionally eliminated spaces between the diffusion of gases between the egg and its environment. One view of the evolution of Amniotes from anamniotes concludes that the transition was made by species of relatively small adult size that presumably laid eggs similar in many respects to the direct developing eggs of a variety of extant amphibians. The ancestral egg probably was oviposited in terrestrial environments and thus might not have been in contact with free standing water. Several lines of evidence indicate that the barriers to diffuse movement of oxygen through the jelly hinders the metabolism of embryos of contemporary amphibians late in development, particularly in large eggs or in eggs naturally incubated at higher temperatures.

Spermatogenesis appears to be a fairly conserved process throughout the vertebrate series. Thus, spermatogonia develop into spermatocytes that undergo meiosis to produce spermatids which enter spermiogenesis where they undergo a morphological transformation into spermatozoa. There is, however, variation amongst the vertebrates in how germ cell development and maturation is accomplished. This difference can be broadly divided into two distinct patterns, one present in anamniotes (fish, amphibia) and the other in amniotes (reptiles, birds, mammals). For anamniotes, spermatogenesis occurs in spermatocysts (cysts) which for most species develop within seminiferous lobules. Cysts are produced when a Sertoli cell becomes associated with a primary spermatogonium. Mitotic divisions of the primary spermatogonium produce a cohort of secondary spermatogonia that are enclosed by the Sertoli cell which forms the wall of the cyst. With spermatogenic progression a clone of isogeneic spermatozoa is produced which are released, by rupture of the cyst, into the lumen of the seminiferous lobule. Following spermiation, the Sertoli cell degenerates. For anamniotes, therefore, there is no permanent germinal epithelium since spermatocysts have to be replaced during successive breeding seasons. By contrast, spermatogenesis in amniotes does not occur in cysts but in seminiferous tubules that possess a permanent population of Sertoli cells and spermatogonia which act as a germ cell reservoir for succeeding bouts of spermatogenic activity. There is, in general, a greater variation in the organization of the testis and pattern of spermatogenesis in the anamniotes compared to amniotes. This is primarily due to the fact there is more reproductive diversity in anamniotes ranging from a relatively unspecialized condition where gametes are simply released into the aqueous environment to highly specialized strategies involving internal fertilization. These differences are obviously reflected in the mode of spermatogenesis and this is particularly true of the stage of spermiogenesis where the morphology of the species-specific spermatozoon is determined. Moreover, unlike amniotes, many anamniotes display a spermatogenic wave manifest, depending upon the species, either at the level of the cyst or seminiferous lobule. This variation in the organization of the testis makes certain anamniotes perfect models for investigating germ cell development and maturation. For instance, the presence of a spermatogenic wave provides an opportunity to manually isolate discrete germ cell stages for analysis of specific Sertoli/germ cell interactions. Furthermore, for many anamniotes, germ cells mature in association with a morphologically poorly developed Sertoli cell. This seeming independence of Sertoli cell regulation allows the in vitro culture of isolated germ cells of some species of anamniotes through several developmental stages. Thus, due either to the anatomical organization of the testis, or structural simplicity of the germinal units, nonmammalian vertebrates can provide excellent experimental animal models for investigating many basic problems of male reproduction.

Although hair cells in the cochlea and in the vestibular endorgans of anamniotes are thought to release glutamate or a similar compound as their transmitter, there is little evidence in amniotes (which, unlike anamniotes, possess both type I and II hair cells) as to the nature of the hair cell transmitters in the vestibular labyrinth. We have recorded extracellularly from single semicircular canal afferents in the turtle labyrinth maintained in vitro and have bath-applied a number of transmitter agonists and antagonists to relate the effects of these substances to the actions of the endogenous transmitter substances. Both glutamate and aspartate strongly excite the afferents while GABA and carbachol have negligible or weak effects. In contrast to its lack of effect on afferent activity in some anamniotes, N-methyl-D-aspartate (NMDA) was also found to excite these afferents. Kynurenic acid reversibly reduced the resting firing rates of the afferents and the increases in firing due to the application of glutamate and aspartate. These findings provide preliminary support for the hypothesis that glutamate (or a related compound) is also a vestibular hair cell transmitter in amniotes.

In anamniotes, the dorsal thalamus comprises: (1) a caudal division, the collothalamus, which receives its predominant input from the midbrain roof and projects ipsilaterally to the telencephalon, predominantly to the striatum, and (2) a rostral division, the lemnothalamus, which predominantly receives a direct retinal (lemniscal) input and projects bilaterally to the telencephalon, predominantly to the pallium. In amniotes, collothalamic nuclei relay visual, auditory, and somatosensory-multisensory inputs from the midbrain roof to the ipsilateral telencephalon, terminating in both striatum and pallium. For example, the collothalamic visual nuclei consist of the LP-pulvinar complex in mammals and nucleus rotundus in diapsid reptiles, birds, and turtles. Among amniotes, the latter nuclei are homologous to each other as discrete nuclei, as are the collothalamic auditory and collothalamic somatosensory-multisensory nuclei. Lemnothalamic nuclei (and nuclear groups) in amniotes predominantly (and/or plesiomorphically) receive lemniscal inputs; some project to the telencephalon bilaterally, and most, in contrast to collothalamic nuclei, do not project to the striatum. In mammals, the lemnothalamic nuclei include most of those in the anterior, medial, intralaminar, and ventral nuclear groups and the dorsal lateral geniculate nucleus. In diapsid reptiles, they include the dorsomedial and dorsolateral anterior nuclei and the dorsal lateral optic nucleus; comparable nuclei are present in birds and turtles, with birds additionally having a discrete somatosensory lemniscal relay nucleus. These lemnothalamic nuclei in each amniote radiation are homologous as a field to the lemnothalamus (i.e., nucleus anterior) in anamniotes. Both divisions of the dorsal thalamus were elaborated to some degree in the common ancestral amniote stock. A further major elaboration of the lemnothalamus characterized the ancestral stock of mammals and may have been one of the key events in early mammalian evolution. Birds have independently, to a lesser degree, elaborated the lemnothalamus.

Phylogenetically persistent structures such as the mitral cells of the vertebrate olfactory bulb undergo changes in their dendritic arbor in the course of evolution. The morphology of mitral cells and the main elements of the olfactory bulb circuit in all classes of vertebrates are reviewed in this paper. Most of the neuronal elements found in the mammalian olfactory bulb are present in anamniotes. However, in contrast to those of amniotes, the mitral cells of most anamniotes lack basal dendrites, and periglomerular cells are absent in fish. This suggests a different circuitry and therefore drastic changes in the processing of olfactory information within the olfactory bulb. Lateral inhibition, conferred by basal dendrites in anamniotes, must then utilize other mechanisms in anamniotes. Moreover, the marked segregation of olfactory inputs onto mammalian mitral cells is less obvious in mitral cells of anamniotes that lack basal dendrites. The general role of dendrites, including those of mitral cells, is discussed in the light of increasing evidence for dendritic excitability. The evolutionary significance of mitral cell basal dendrites is also discussed.

The evolution of the dorsal thalamus in various vertebrate lineages of jawed vertebrates has been an enigma, partly due to two prevalent misconceptions: the belief that the multitude of nuclei in the dorsal thalamus of mammals could be meaningfully compared neither with the relatively few nuclei in the dorsal thalamus of anamniotes nor with the intermediate number of dorsal thalamic nuclei of other amniotes and a definition of the dorsal thalamus that too narrowly focused on the features of the dorsal thalamus of mammals. The cladistic analysis carried out here allows us to recognize which features are plesiomorphic and which apomorphic for the dorsal thalamus of jawed vertebrates and to then reconstruct the major changes that have occurred in the dorsal thalamus over evolution. Embryological data examined in the context of Von Baerian theory (embryos of later-descendant species resemble the embryos of earlier-descendant species to the point of their divergence) supports a new 'Dual Elaboration Hypothesis' of dorsal thalamic evolution generated from this cladistic analysis. From the morphotype for an early stage in the embryological development of the dorsal thalamus of jawed vertebrates, the divergent, sequential stages of the development of the dorsal thalamus are derived for each major radiation and compared. The new hypothesis holds that the dorsal thalamus comprises two basic divisions--the collothalamus and the lemnothalamus--that receive their predominant input from the midbrain roof and (plesiomorphically) from lemniscal pathways, including the optic tract, respectively. Where present, the collothalamic, midbrain-sensory relay nuclei are homologous to each other in all vertebrate radiations as discrete nuclei. Within the lemnothalamus, the dorsal lateral geniculate nucleus of mammals and the dorsal lateral optic nucleus of non-synapsid amniotes (diapsid reptiles, birds and turtles) are homologous as discrete nuclei; most or all of the ventral nuclear group of mammals is homologous as a field to the lemniscal somatosensory relay and motor feedback nuclei of non-synapsid amniotes; the anterior, intralaminar and medial nuclear groups of mammals are collectively homologous as a field to both the dorsomedial and dorsolateral (including perirotundal) nuclei of non-synapsid amniotes; the anterior, intralaminar, medial and ventral nuclear groups and the dorsal lateral geniculate nucleus of mammals are collectively homologous as a field to the nucleus anterior of anamniotes, as are their homologues in non-synapsid amniotes. In the captorhinomorph ancestors of extant land vertebrates, both divisions of the dorsal thalamus were elaborated to some extent due to an increase in proliferation and lateral migration of neurons during development.(ABSTRACT TRUNCATED AT 400 WORDS)

The distribution of the central neural connections of the pineal organ of the larval sea lamprey was investigated by means of anterograde and retrograde tracing with the fluorescent lipophilic dye, DiI (1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate). Pinealofugal projections are well developed in larvae, extending from the posterior commissure into the diencephalon and mesencephalon. Small numbers of neurons were retrogradely labelled in the transition zone between the diencephalon and the mesencephalic tegmentum. These cells may constitute the first pinealopetal system described in anamniotes.

Muller (glial) cells of the retina of various species of amphibia (urodeles and anurans) were investigated by means of the freeze-fracture technique. This was done because Muller cells in anamniotes were believed to differ from those in mammals in that they should lack the so-called orthogonal arrays of particles (OAP) which are a characteristic feature of Muller cells in mammalian retina. However, as we could demonstrate previously (Bergvon der Emde and Wolburg, Glia, 2 (1989) 458), fish retinal Muller cells also contain OAP in their membranes suggesting that OAP are a general marker of Muller cells in all vertebrates. As demonstrated in this study, Muller cells of urodeles (Batrachoseps attenuatus and Pleurodeles waltlii ) are OAP-positive, whereas two anurans ( Rana esculenta and Xenopus laevis) do not reveal any OAP in their Muller cell membranes. Under phylogenetic aspects, it appears very interesting that frogs are as yet the only vertebrate group that deviates from all other vertebrates in terms of Muller cell membrane morphology.

In the present account, the relation between the descending, substance P (SP)-containing projection fibers from the striatum and the dopaminergic cell bodies in the midbrain tegmentum was investigated in several species of reptiles and the domestic chicken by means of antisera against SP and dopamine. It was found that in all species studied an almost complete match of the SP and the dopamine immunoreactivity occurs in the ventral tegmental area. In contrast, considerable differences were observed in the substantia nigra and the retrorubral area. Two different patterns were recognized: one pattern in which there is little overlap between the dopaminergic cells and SP fibers (lizards, turtles) and another with an extensive overlap (snakes, chickens). Similar patterns were found in mammalian species. On the basis of an out-group comparison with anamniotes, it is suggested that the pattern with little overlap represents the primitive character. The pattern with considerable overlap, found in the three classes of amniotes, should be considered an independent development from the same primitive character and should, therefore, be defined as an example of parallel homoplasy.

tor and are distinguished from anamniotes by the form of their eggs (Carroll, 1982; Packard and Seymour, 1997). Instead of a "naked" egg, which must be incubated in an essentially aquatic environment, amniotes have an eggshell that provides some protection from desiccation (Little, 1983), and, instead of gas exchange by diffusion from the environment to the embryo, gas diffuses through the eggshell between the environment and extra-embryonic membranes where it is transported in the blood to the embryo (Packard and Seymour, 1997). Thus, the diffusion distance is essentially that of the thickness of the eggshell in eggs of amniotes. It is often postulated that the limitation on the size of the anamniote eggs of amphibians is the slow rate of diffusion of 02 from the outside of the egg to the embryo in large eggs (Seymour and Bradford, 1995). The maximum rate of 02 consumption in large amphibian eggs seems to be compromised by limitations of rates of diffusion of 02 through the gelatinous capsule of amphibian eggs (Packard and Seymour, 1997). Rates of 02 consumption and total energetic cost of development have been measured in a number of amphibian and reptilian eggs (Vleck and Hoyt, 1991; Seymour and Bradford, 1995; Packard and Seymour, 1997). However, all the reptilian eggs measured to date are larger than those of the amphibians so far measured. To make comparisons of the rates of 02 consumption and energetics of reptilian and amphibian eggs by using regressions beyond the size range for each taxon is not statistically valid. The largest of amphibian eggs has had their rates of 02 consumption and/or energetics measured (Packard and Seymour, 1997); larger eggs are not available to extend regressions of these parameters against egg mass. Thus, we chose to measure these parameters in the smallest of available reptilian (amniote) eggs, those of the Australian skink Menetia greyii, to test the null hypothesis that the maximum rate of 02 consumption during embryonic development is not greater in reptiles than in amphibians of equivalent size. Eggs of M. greyii, one of the smallest lizards in Australia (Greer, 1989), are smaller than the largest of amphibian eggs. We quantified the rates of 02 consumption and energetic cost of development to enhance understanding of the evolution of amniote eggs from anamniancestors. At the same time, we provide det iled information on the energy contents of eggs and neonates and quantify how much energy is consumed during embryonic development for comparison with other species of reptiles.

Evolutionary perspectives invertebrates aspects of hearing among vertebrates anamniotes non-mammalian amniotes mammals epilogue.

In most anamniotes the axial musculature is myomeric and is functionally subdivided into superficial red and deep white muscle. In those anamniotes that have been studied the organization of the motor column is related to this functional subdivision. The motoneurons innervating red and white muscle differ in size, distribution in the motor column, and developmental history. There is no obvious topographic relationship between the location of motoneurons in the motor column and the dorsoventral location of the muscle they innervate in the myomeres; epaxial motoneurons are not segregated from hypaxial ones. Among amniotes, the myomeres divide to form a number of discrete muscles that may be complexly arranged. This breakup of the musculature is correlated with a subdivision of the motor column into discrete motor pools serving the different muscles. Unlike anamniotes, the motor pools are topographically organized. The epaxial pools are segregated from hypaxial ones, and within the epaxial and hypaxial pools the location of motoneurons innervating any particular muscle is related to the location of the muscle's precursor in the embryonic muscle masses.Thus adjacent motor pools innervate muscles arising from adjacent positions in the myotome. These dramatic differences between the motor columns in anamniotes and amniotes imply that the medial motor column has undergone a major restructuring during the evolution of vertebrates. The available evidence — which is tentative because of the few species that have been studied — suggests that a topographically organized motor column was absent in early vertebrates. A motor column/myotome map appears to have arisen just prior to, or in conjunction with the origin of amniotic vertebrates. The details of this map were conserved in different amniotes in spite of major structural and functional changes in the musculature. The map may be important for the proper control of the many muscles arising from the myotomes in amniotes because it facilitates the development and evolution of motor systems in which anatomically and functionally different muscles have spatially separate motor pools in the cord.

Cytological, cytochemical and molecular investigations have shown different trends in DNA evolution among different lineages of reptiles. In chelonians, qualitative and quantitative differences in all the main DNA fractions (highly repetitive, middle repetitive and single-copy DNAs) have been found. These variations do not seem to be correlated with phylogeny nor with the taxonomic position of the species studied so far; they might result mainly from variations in the amount of non-functiona l DNA fractions. On the contrary, in the squamates the differences include almost exclusively highly repetitive DNA sequences, and a correlation is also noted between the degree of divergence in the DNA composition and the phyletic distance among the species studied. Moreover, the results obtained by means of C-banding techniques seem to suggest that in this order variation in highly repetitive DNA is correlated with processes such as the differentiation of sex chromosomes and speciation. The situation seen in turtles is similar to that observed in various groups of anamniotes, mainly amphibians; whereas that observed in squamates is similar to the situation found in birds and mam-, mals. It is possible that the differences observed between turtles and squamates are the consequence of differences in the amplification and preservation of non-functiona l DNA fractions.

In many animals the sensory hair cells of the inner ear are ultrastructurally variable within individual epithelia. This variation has been hypothetically related to both the function and the age of the individual cells. In this study, growth-related changes in hair cell populations were examined in the macula neglecta sensory epithelia of juvenile and adult sharks. Scanning electron microscopy demonstrated that more than 80% of the 200,000 hair cells in the adult's macula neglecta are produced postembryonically. Tritiated thymidine autoradiography and histological descriptions of the hair cells in this sound detector indicate that new sensory cells are produced in growth zones at the edges of the epithelia. The hair cells in those zones have small cell bodies, small and heterogeneous cilia complexes, and associations with small numbers of particularly thin nerve terminals. Their cytological features and their sparse innervation contrast with the features of the more numerous central cells in each epithelium, but appear to resemble the published descriptions of embryonically developing hair cells. Thus, a germinal zone at the leading edge of sensory epithelium growth appears to persist into adult life in sharks. Published reports reinterpreted in light of this evidence suggest that such hair cell population growth may be expected in other anamniotes and that latent growth zones might persist in the ears of amniotes.

If the hundred years of study on theHensen's node — i.e. on gastrulation and early determination of the embryos of amniote vertebrates — teach anything, they teach in the first place how limited and fragmentary our knowledge is about one of the most central problems of the whole developmental biology. We know that the events in early amniote development — or early avian development, on which our data and ideas are nearly all based — in many ways resemble those in early Amphibian development, which is only slightly better understood, but we also know that direct extrapolations from anamniotes to amniotes cannot be made without proper reservations and without studying the amniote embryos themselves. And we have practically no idea of what is really going on in the cells of the blastoderm when they move, invaginate, induce or are induced, interact, become determined and begin their differentiation. We know that at the stages of gastrulation, the node, and indeed the whole blastoderm, is in a very labile state and can be regulated in many ways to produce a harmonious whole — or a monster — although we only understand very poorly the modes of this regulation. The progress made during the decades, and particularly in recent years, shows, however, that useful information is accumulating to produce a coherent picture, and there is no reason to be pessimistic.

1. The organization of brain neuronal systems utilizing monoamines (primary techolamines and serotonin) as neurotransmitters and/or neuromodulators, has been studied in representatives of each major vertebrate class using fluorescence histochemical methods. This long-term investigation has led to the following conclusions: 2. (1) The monoaminergic neurons seem to have occurred early in phylogeny. These neurons, particularly the serotoninergic systems, are already well-developed in the brains of cyclostomes (lampreys) and primitive bony fishes. 3. (2) The monoaminergic cell bodies are especially abundant within the hypothalamus in anamniotes whereas they predominate at brainstem levels in amniotes. However, a similar pattern of distribution of monoaminergic cell bodies has been found in the brainstem of all vertebrates. 4. (3) The monoaminergic neurons may act as an important informative link between the cerebrospinal fluid and the neural tissue in all vertebrates through processes contacting the ventricular space. 5. (4) The monoamingeric neuronal systems are widespread and highly collateralized inall vertebrates. A single brainstem monoaminergic neuron may send divergent axon collaterals to various remote forebrain areas and, thus, could strongly influence vast neuronalassemblies. 6. (5) The terminal arborization of monoaminergic systems, particularly that of the serotoninergicneurons, displays patterns that are strikingly constant from one vertebrate class to the other. Such a constancy strongly suggests that these chemospecific neuronal systems are indeedphylogenetically ancient and subserve similar fundamental brain functions, such as the controlof sleep-waking cycle and of feeding and reproductive behaviors, in all vertebrates.

Retinofugal pathways in the spotted dogfish Scyliorhinus canicula and the thornback ray Raja clavata were studied with reduced silver techniques following unilateral eye enucleations. Optic nerve axons decussate in the chiasma opticum, except for a small ipsilateral projection to the area preoptica. After crossing, retinal projections distribute to the area preoptica, the thalamus dorsalis pars lateralis, the thalamus ventralis pars lateralis, the corpus geniculatum laterale, the nucleus pretectalis, and the superficial layers of the tectum mesencephali. In Scyliorhinus most primary optic fibers terminate in the stratum medullare externum of the mesencephalic tectum, while in Raja the zona externa of the stratum cellulare externum receives the bulk of the retinal input. A basal optic tract could be identified in Raja, but not in Scyliorhinus. The retinofugal pathways of the two species studied are compared with those of other cartilaginous fishes and other anamniotes. It is concluded that the primary visual system in chondrichthyans resembles that of actinopterygians and amphibians. However, there is a striking difference in the way in which the primary optic fibers reach the tectal target areas. In elasmobranch fish the optic nerve fibers enter the tectum through the zona interna of the stratum cellulare externum and send their axons into the more superficial tectal layers, while in actinopterygians and amphibians the majority of the optic fibers enter the tectum through the superficial layer and distribute their axons to deeper tectal layers.

Cartilage of the vertebrate jaw is derived from cranial neural crest cells that migrate to the first pharyngeal arch and form a dorsal “maxillary” and a ventral “mandibular” condensation. It has been assumed that the former gives rise to palatoquadrate and the latter to Meckel's (mandibular) cartilage. In anamniotes, these condensations were thought to form the framework for the bones of the adult jaw and, in amniotes, appear to prefigure the maxillary and mandibular facial prominences. Here, we directly test the contributions of these neural crest condensations in axolotl and chick embryos, as representatives of anamniote and amniote vertebrate groups, using molecular and morphological markers in combination with vital dye labeling of late-migrating cranial neural crest cells. Surprisingly, we find that both palatoquadrate and Meckel's cartilage derive solely from the ventral “mandibular” condensation. In contrast, the dorsal “maxillary” condensation contributes to trabecular cartilage of the neurocranium and forms part of the frontonasal process but does not contribute to jaw joints as previously assumed. These studies reveal the morphogenetic processes by which cranial neural crest cells within the first arch build the primordia for jaw cartilages and anterior cranium.

1. Studies on the genomic evolution in vertebrates have highlighted the differences existing between anamniotes and amniotes, both in quantitative and compositional terms. 2. These differences do not seem to depend on a different tendency to genic amplification, but rather on the existence of more strict and efficient constraints in amniotes. 3. Some constraints, that may be defined as "intrinsic", would act directly on the genome; among these particularly important is the chiasma frequency during meiosis. 4. Other, "extrinsic", constraints, would act indirectly through genic products or through cell morphometric parameters. 5. The genome size increase seems to depend on various mechanisms. The most wide-spread one seems to be the amplification of interspersed repetitive and non-repetitive sequences.

One of the simplest experiments available to embryologists is to subject embryos to abnormal temperatures. It is well known that raised temperatures tend to speed up development of anamniotes and reduced temperatures to lower it. To some extent the same is true for amniotes. Chick embryos incubated at slightly raised temperatures develop faster, and at reduced temperatures more slowly, (Landauer, 1948).