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Medial giant (MGA) and lateral giant (LGA) axons of crayfish were doubly cut in order to selectively isolate axonal segments from perikaryal and transsynaptic sources of trophic input. Isolated MGA segments remained morphologically intact for over 43 days, whereas isolated LGA segments usually degenerated within one week. The glial sheaths around isolated MGA segments had significantly increased in thickness within one week, but severed LGA segments showed no increase in sheath thickness at any time after lesioning. These data suggest that cells of the surrounding glial sheath can provide trophic support to isolated MGA segments but not to isolated LGA segments. Extent of glial hypertrophy seems dependent upon specific spatiotemporal parameters. The diameters of isolated MGA segments decreased more rapidly than the diameters of singly cut MGA segments. These data suggest that the MGA also receives some trophic support from pre- or postsynaptic sources. Conversely, some singly cut LGA segments completely degenerated within one week, whereas other singly cut LGA segments remained intact for at least 43 days after lesioning. Such results suggest that the LGA receives a significant trophic input from pre- or postsynaptic structures.

The properties of the giant cerebral serotonin-containing neurons of the opisthobranch mollusc Aplysia californica were studied and were compared to the existing data on the giant serotonin-containing neurons (metacerebral cells) of pulmonate mulluscs. Among the properties examined were: axonal distribution, synaptic input and output, pharmacological responses, biophysical characteristics, and plasticity. With only minor exceptions, the properties of the serotonin-containing neurons of Aplysia and of pulmonate molluscs were remarkably similar, and it was concluded that these identified neurons are true homologues. The establishment of the homology of the metacerebral cells of Aplysia to the metacerebral cells of pulmonate molluscs extends the known distribution of these neurons to a second major subclass (Opisthobranchiata) of molluscs. Since pulmonate and opisthobranch molluscs differ substantially in behavioral and anatomical features, the study of the metacerebral cells of these two groups may promote the understanding of the evolutionary adaptation of the nervous system to different environmental pressures.

The migration of Bergmann glia cells from the external granular layer to the Purkinje cell layer and the differentiation of their processes were analyzed in Golgi-Cox preparations of the cerebellum of the rat. During their migration they showed numerous cytoplasmic excrescences and long thick processes, which suggested that they may follow ameboid form of movement for their migration. The differentiation of their processes was observed to involve a series of complex events related to retraction of the thick apical processes from the tips of which sheaves of filiform processes emerged, growth in the number and the thickness of the filiform processes, resorption of some of these filiform processes, cyclic changes in the appearance, resorption and reappearance of excrescences on the filiform processes, and changes in size and shape of the growth cones. These findings are discussed in the context of the role played by the Bergmann glia cells in the postnatal developmental events in the cerebellum, and the dynamic nature of differentiation of their processes.

The morphologic and functional properties of the Purkinje cells (P-cells) grown for 10-11 weeks in organotypic cultures from newborn kitten cerebella were studied and compared to cultures which had been grown for 4-5 weeks under the same standard conditions. Electrophysiological and morphological data were obtained from HRP iontophoretically labeled neurons and were quantified by means of computerized techniques. Extracellular recordings of spontaneous activity showed that the 10-11-week-old P-cells had a pacemaker-like firing rate whereas the P-cells aged 4-5 weeks in vitro displayed a bursting activity. The qualitative morphological data evidenced abnormal swellings both on dendritic and axonal processes of the 10-11-week-old P-cells which were not present on the 4-5-week-old P-cells. The quantitative data revealed a significant decrease in the overall size of the dendritic network of the 10-11-week-old P-cells mainly due to a reduction in the total dendritic length and in the total number of dendritic segments, whereas the individual segment lengths remained almost unchanged. Dendritic spine counts showed no decrease in the dendritic density of these older P-cells. Such data suggest that the changes observed in 10-11-week-old cultured P-cells may be compared to the age-related changes occurring in vivo and that such in vitro models could be useful tools in the study of the pathology of aging. However, alternative factors other than senescence are discussed since they may account for some degenerative changes observed in the older cultured P-cells.

Schwann cell mitosis has been demonstrated in chronically denervated cat tibial nerves re-innervated by axons regenerating from the proximal stump of a coapted peroneal nerve. Thymidine incorporation rose above baseline levels at the axon front, with no detectable increase in more distal regions occupied by denervated Schwann cells. Schwann cells therefore enter S phase upon the arrival of a regenerating axon in vivo as previously described in tissue culture. Intraneural treatment of the denervated distal stump with Mitomycin C prior to re-innervation delayed the subsequent appearance of myelin formation. This supports the notion that axonally stimulated division of Schwann cells is a prerequisite for myelination during nerve regeneration. Axonal advancement was also retarded by drug treatment, possibly because of a reduced level of trophic support provided by the compromised Schwann cells. A comparable absence of myelin and poor re-innervation was found in chemically untreated distal stumps that had been maintained in the denervated state for prolonged periods when Schwann cell columns are known to undergo progressive atrophy. These observations suggest that nerve repair should be delayed for limited periods if efficacious regeneration is desired.

It is known that development of the interhemispheric pathway is accompanied by a dramatic reduction in the number of callosal fibers. By injecting neuroanatomical tracers into the cortex of newborn and adult rats, we examined the distribution of fibers within the corpus callosum both before and after this loss takes place. We found that in both newborn and mature rats, axons from restricted regions of the cortex are grouped into relatively tight bundles within the corpus callosum and that the location of these bundles depends upon the region of cortex from which they originate. These results indicate that callosal fibers that are destined to disappear are not distributed diffusely throughout the corpus callosum of the neonate, but rather that they traverse this commissure through the sector that is appropriate for the area of the cortex from which they originate. This finding suggests that loss of axons does not play a significant role in establishing the overall topographic pattern of the fibers within the callosal commissure.

The finding of motoneuron dendrites organized into small compact bundles in cats, monkeys and pigs suggested that a study of this phenomenon in rats should be undertaken. An analysis was performed with electron microscopy, light microscopy and Golgi methods. An extensive dendrite bundle organization was found in the sixth lumbar segment of the spinal cord. Two discrete bundles were localized bilaterally: a lateral bundle in the ventrolateral gray substance, and a medial bundle in the ventral funiculus. The lateral bundle was found to consist of longitudinally oriented dendrites, neurocytons, glial cells and capillaries. As many as 1678 closely packed dendrites were observed in the lateral bundle. The medial bundle contained dendrites directed across the midline and also longitudinally oriented dendrites. Neurocytons in the medial dendrite bundle were found singly or in clusters, and many radiating bundles of dendrites were observed projecting toward the lateral bundle. Golgi analysis confirmed that neurons in the lateral bundle had most of their dendrites oriented longitudinally. It was possible to trace several dendrites into the lateral bundle from dorsally and medially lying neurons. Electron microscopy substantiated the fact that the bundles were composed of dendrites. It also revealed numerous dendrodendritic and dendrosomatic contacts which were desmosomal in type as well as an abundance of small unidentified processes. Various functions which have been attributed to the dendrite bundles are discussed.

Autoradiographic tracing methods were used to demonstrate a well-defined projection from the superior colliculus to the inferior olivary complex in the monkey. This projection originates within the deep layers of the superior colliculus, descends within the contralateral tecto-spinal tract, and terminates within the caudal 1/3 of the medial accessory nucleus. The terminal field is restricted to a densely packed, darkly stained group of cells located in the most dorsal segment of subnucleus b. In one animal, another group of olivary afferents was identified. These fibers also descend within the contralateral tecto-spinal tract, and terminate within the dorsal cap of Kooy. While it was not possible to determine the origin of this projection, our data suggest that it arises within a region adjacent to the rostral pole of the superior colliculus. The present study further indicates that in the monkey relatively few axons which course within the classical tecto-spinal tract pass caudal to the medulla.

In the process of an extensive Golgi analaysis of the inferior region of the rat hippocampus, a hitherto undescribed cell type was discovered. The cell has a round, elliptical, or fusiform cell body and an apical dendritic plume reminiscent of dentate granule cells. The axon is thick, with many collaterals and ramifies within, above and below the pyramidal layer. The proximal dendrites have stubby spines whereas the distal dendrites have long thin spines. All impregnated cells of this type were found in the inferior region (CA3 and CA4 of Lorente de Nó) of the hippocampus and most were found in a circumscribed suprapyramidal region at the mouth of the hilus. The majority of impregnated cells of this type were found in the middle to temporal portion of the hippocampus. Nissl-stained sections confirmed the predominant occurrence of this cell type in the inferior region of the middle to temporal hippocampus. In these preparations, the cells have a large nucleus, several nucleoli and very scanty cytoplasm with Nissl substance essentially confined to the initial dendritic segements. The unique morphology of this cell type allows relatively easy identification using Nissl staining.