A quantitative electron microscopic study of synaptogenesis in the dentate gyrus of the rat

Histochemical evidence of altered development of cholinergic fibers in the rat dentate gyrus following lesions. II. Effects of partial entorhinal and simultaneous multiple lesions

It has been concluded previously that the septohippocampal fibers which project to the rat dentate gyrus extend or branch in the denervated area of the molecular layer following a complete ipsilateral entorhinal lesion. The septohippocampal fibers thus appear to replace some of the perforant fibers which degenerate as a result of the lesion. The reactive fibers eventually become localized to a much smaller and more superficial area after lesions of immature rats than after lesions made in adulthood. To determine whether this difference in the response results from a selective reaction to loss of the lateral perforant path in the immature rat, various portions of the entorhinal cortex were removed at the age of 11 days, and the cholinergic septohippocampal fibers were visualized by acetylcholinesterase histochemistry. An alternative possibility, that the difference between immature and adult rats is attributable to an interaction with other reactive afferents, was tested by removing other sources of input (the contralateral entorhinal cortex, contralateral hippocampal formation or both) along with the ipsilateral entorhinal cortex at the age of 11 days and then demonstrating the septohippocampal fibers histochemically. Lesions of the lateral part of the ipsilateral entorhinal cortex (source of the lateral perforant path) at 11 days of age evoked a septohippocampal reaction along the outer edge of the molecular layer, where the lateral perforant path fibers normally terminate. This result matched that produced by a complete entorhinal lesion. Lesions of the medial entorhinal cortex evoked no obvious reaction. In contrast, the septohippocampal fibers in adult rats proliferated in the denervated area of the molecular layer after lesions of either part of the entorhinal cortex. Combining lesions of other sources of innervation to the dentate gyrus with an ipsilateral entorhinal lesion at 11 days of age did not alter the response of septohippocampal fibers, as determined histochemically. Neither did the septohippocampal fibers react to removal of commissural afferents alone. The response at any age was unaffected by prior or subsequent removal of the contralateral entorhinal cortex. These results indicate that in immature rats the septohippocampal fibers respond only to loss of the lateral perforant path, but these same fibers can later react to loss of any part of the perforant path. They are regarded as support for the hypothesis that the reactive septohippocampal fibers preferentially interact with dendritic growth cones. Our results do not support explanations based on a hypothetical attraction between septohippocampal and crossed perforant path fibers (which react in the same area) or on competition with commissural fibers (which reinnervate an adjacent area). We suggest further that proximity to the degenerating elements does not in itself determine the pattern of reinnervation after lesions of the central nervous system.

Histochemical evidence of altered development of cholinergic fibers in the rat dentate gyrus following lesions. I. Time course after complete unilateral entorhinal lesion at various ages

The entorhinal cortex of rats was removed at various times during development, and the reaction of the cholinergic septohippocampal input to the dentate gyrus was examined by use of acetylcholinesterase histochemistry. When the ipsilateral entorhinal cortex is completely removed, the outer 70-75% of the molecular layer of the dentate gyrus is almost completely denervated. After such a lesion at 5 to 33 days of age, the acetylcholinesterase staining initially intensified throughout the denervated area, indicating that the septohippocampal fibers branched or elongated. This reaction could be detected within one day after a lesion at 11 days of age and within three or five days after lesions at earlier or later times. Whereas the initial response of the septohippocampal fibers was independent of the age at which the lesion was made, their final localization depended on the developmental state of the animal. After lesions at the age of 5 or 11 days, the reactive septohippocampal fibers became restricted to the outer one-sixth to one-third of the molecular layer within two days after appearance of their initial reaction. A similar concentration of reactive fibers was demonstrable after lesions at 16, 18 or 21 days of age, but some reaction persisted in the middle third of the molecular layer. Finally, after lesions at 26 or 33 days of age the proliferating cholinergic fibers ultimately were uniformly distributed throughout the outer 60% of the molecular layer. These results suggest that septohippocampal fibers initially extend or sprout throughout the denervated area to replace the lost perforant path fibers. However, the reactive fiber population becomes restricted to the outer edge of the molecular layer if the entorhinal lesion is made before the period of cholinergic synaptogenesis and concentrates in this same zone if it is made while cholinergic synapses are forming. We suggest that either the proliferative reaction continues in the outer part of the molecular layer and subsides in other parts of the denervated area or septohippocampal fibers move outward through the molecular layer to assume a more superficial location. After entorhinal lesions at 16 days of age or later the pale-staining zone (containing fibers that originate in hippocampus regio inferior) immediately deep to the denervated area widened. If the lesion was made earlier, this zone never developed at most septotemporal levels of the dentate gyrus. These results are probably related to the extension of regio inferior fibers into the denervated area.

Development of afferent lamination in the fascia dentata of the rat

In the present study we examine the development of afferent lamination in the fascia dentata of the postnatal rat, as a first step in determining possible mechanisms controlling synaptic specificity in this system. This analysis is based on degeneration-induced argyrophilia as well as autoradiographic labeling of the entorhinal and commissural/associational afferents. Both methods show that in spite of the immaturity of the neonatal fascia dentata, these afferent systems have already established territorial relationships by 4 days of age which persist into adulthood. At 4 days, the entorhinal projection is restricted approximately to the outer 45 mum of the 80 mum wide molecular layer. The commissural/associational projection occupies appoximately the inner 35 mum of the molecular layer. At older ages the commissural/associational zone increases in width very slowly relative to the entorhinal zone. We also discuss these results in relation to potential mechanisms of afferent development and dendritic differentiation.

Mouse brain protein composition during postnatal development: an electrophoretic analysis

Changes in the concentrations of mouse brain proteins during postnatal maturation were characterized by a combination of subcellular fractionation and electrophoresis. Sodium dodecyl sulfate gel electrophoresis revealed changing protein concentrations in fractions enriched in nuclei, mitochondria plus synaptic endings, microsomes and cytosol. Postnatal maturational changes in protein concentrations were most pronounced in fractions of purified myelin membranes. The use of exponential gradient gels resulted in increased resolution of low molecular weight myelin proteins. Nuclei treated with Triton X-100 exhibited no change in relative histone concentrations during brain maturation. Nonnuclear contamination of untreated nuclear fractions was shown to be a potential source of erroneous interpretations. These findings are discussed in terms of genetic products and sodium dodecyl sulfate polyacrylamide gel electrophoresis resolution.

Some aspects of the central nervous system of the rat during aging

Literature pertaining to structural and functional changes in rat brain taking place with increasing age has been reviewed. For the most part, only young rats have been studied. Brain weight does not appear to change dramatically throughout the life span of rats beyond early development. Neuronal and neuropil elements appear to undergo age-related changes with respect to size, number, and intercellular relationships. Intracellular organelles show evidence of degeneration. Additional studies on central nervous system function in relation to age are required in order to ascertain the meaning of particular structure changes.

Synapse formation in the mouse olfactory bulb. I. Quantitative studies

A quantitative study of synapse formation in the mouse olfactory bulb has been carried out using serial sections. Volumetric synaptic density as well as absolute number of synapses per olfactory bulb for eight distinct synaptic types have been determined at 15 different ages, from the beginning of synapse formation at embryonic day 14 (E14) to postnatal day 44 (P44). Synapses are first found in appreciable numbers at E15 when both axo-dendritic and a few dendro-dendritic synapses occur in the presumptive glomerular layer. Initial synapse formation correlates closely with the reorientation of mitral cells from a primitive tangenital to a definitive radial direction. Synapse formation by mitral cell dendrites occurs after mitral cell axons have grown into the future olfactory cortical areas, either simultaneous with or before synapse formation by these axons. Virtually all synaptic types detected in adults have been found on the day of birth, consistent with the idea that olfaction is an important sensory modality for newborn mice. Volumeric density of a given synaptic type generally increases 50--100 times during development while the absolute number increases about 1,000 times. Synapses in glomeruli develop more precociously than those in the time of origin and differentiation of the principal postsynaptic elements of these two divisions (mitral cells and internal granule cells). Correlation of the time of synapse formation of various synaptic types with their putative excitary or inhibitory role determined in adult studies suggests that excitatory synapses generally form somewhat earlier, although throughout nearly all of synaptic development, both excitatory and inhibitory synapses are present. Reciprocal dendro-dendritic synapses in the external plexiform layer appear to have a special mode of formation. It is suggested that a granule-to-mitral dendro-dendritic synapse only forms next to an already existing mitral-to-granule synapse on the same gemmule.

Synapse formation in the mouse olfactory bulb. II. Morphogenesis

Serial thin sections of the mouse olfactory bulb from the fourteenth day of gestation (E14) to postnatal to 44 (P44) have been examined in order to study morphogenesis of individual synaptic junctions. At the initiation of synapse formation structures are found that resemble postsynaptic densities but are facing extracellular space or unmodified processes. Transition forms between these isolated postsynaptic densities and undoubted synapses have been found. These observations as well as quantitative studies support the hypothesis that isolated postsynaptic densities can form independently and be converted to synapses when a presynaptic specialization develops opposite them. Detailed studies of olfactory axodendritic synaptogenesis throughout the entire developmental period suggests strongly that these asymmetrical synapses pass through an immature symmetrical phase: (1) symmetrical olfactory axodendritic synapses are found in significantly higher concentration on axonal and dendritic growth cones than on more common processes; (2) the number of symmetrical synapses is correlated with the rate of formation of new synapses determined previously. The time for a recognizable symmetrical synapse to be transformed into a recognizable asymmetrical one has been calculated to be 9--10 hours. A scheme of synapse formation in the CNS has been proposed in which a post-synaptic structure forms independently followed by aggregation of pre-existing presynaptic components into a presynaptic specialization. Different morphogenetic sequences of synapse formation from region to region are attributed simply to different relative rates in the development of the postsynaptic density and the presynaptic specialization.

Electron microscopy of synaptic structures in olfactory cortex of early postnatal rats

Layer 1 of the rat olfactory cortex has been studied with the electron microscope at birth and at several consecutive postnatal days up to 14 days of age. Special attention was directed towards synaptic structures and axons of the lateral olfactory tract (LOT). Numerous mature synapses are seen at birth and estimates were made of their subsequent increase in number. In addition, immature synapses are seen and mature postsynaptic sites occur with atypical, partial, multiple or no contact. The findings suggest: (1) considerable prenatal synaptogenesis in contrast to other cortical systems; (2) the maturation of the postsynaptic site may precede that of the presynaptic contact and vesicle accumulation; (3) there may be competition by more than one process for one postsynaptic specialization; (4) the non-innervated sites may result from deafferentation caused by prenatal cell death, although no degeneration was seen, and the atypical contacts may be a stage in the reinnervation of these sites; (5) the LOT develops in parallel with the synaptic neuropil and (6) by 14 days of age the area closely resembles adult tissue.

Some aspects of synaptogenesis in the spinal cord of the chick embryo: a quantitative electron microscopic study

We have quantitatively examined the development of synapses in the ventral part of the lumbar spinal cord of the chick from embryonic day 4 until adulthood. The first synapses occur on day 4 and are of the axo-dendritic type; they are invariably located adjacent to the border between the intermediate and marginal zones. Initially there are more synapses in the presumptive white matter than in the motoneuron neuropil, but this trend is later reversed; however, we found numerous axo-dendritic synapses throughout much of the ventrolateral white matter even in the adult stage. The first axo-dendritic synapses always contain spherical synaptic vesicles and have symmetric membrane specilizations. By day 7 a few of these synapses were found to have mixed populations of spherical and flattened vesicles and asymmetric membrane specilizations. After hatching there are still considerably more axo-dendritic synapses with symmetric membrane specializations. Axo-somatic synapses were first found on embryonic day 6 and were typically located on motoneurons lying adjacent to the marginal zone. These axo-somatic synapses contain a few spherical synaptic vesicles and have symmetric membrane densities. Flattened synaptic vesicles were first found on day 10 and increased throughout development. Although a few axo-somatic synapses with asymmetric membrane specializations were found at practically all stages, the symmetric type was always in the majority. An attempt was made to relate these observations with physiological, behavioral and neuroembryological findings from birds and other forms. For example, the fact that axo-dendritic synapses always appear prior to axo-somatic contacts would seem to rule out the role of somatic synapsesin the initial induction of dendritic growth in the spinal cord.