Synapse formation in the mouse olfactory bulb. II. Morphogenesis

Development of synapses on pyramidal and multipolar non-pyramidal neurons in the visual cortex of rabbits. A combined Golgi-electron microscope study

A combined Golgi-electron microscope method was used to study the ultrastructural maturation of synapses on identified pyramidal and multipolar non-pyramidal neurons in the visual cortex of young and adult rabbits. In samples of 10 (time of eye opening), 14, 20 day old and 7 month old animals, fully impregnated pyramidal neurons within the layers II-V and multipolar non-pyramidal neurons mainly located in lower layer III and layer IV was studied. We found that synapses in 10 and 14 day old animals were occasionally immature in appearance. They were characterized by either a poorly defined postsynaptic band or equal rims of pre- and postsynaptic electron-dense material and could therefore not be classified as Gray type I or II. The distinction between both types of synapses was easier at day 20 and in the adults when the postsynaptic band of the asymmetrical (type I) synapses had become remarkably thicker. In pyramidal neurons the cytoplasmic organelles increased in number during development. Although a few symmetrical synapses were present on dendritic spines of pyramidal neurons in 14 and 20 day old animals, all pyramidal neurons exhibited the same types of synapses on specific sites of their neuronal surface. They received exclusively type II synapses on their somata, type I synapses on their dendritic spines and both types of synapses on their dendritic shafts. However, in the adult animals the frequency of occurrence of type II synapses, especially on basal dendritic shafts, had increased. In some cases only type II and no type I synapses were present. A striking finding in all young and adult animals was that synapses at the borderline between somata and apical dendritic shafts as well as on dendritic spines were frequently complex or interrupted. The characteristic ultrastructural features of adult spine-free and sparsely spiny multipolar non-pyramidal neurons e.g. the many cytoplasmic organelles and type I and II synapses on somata and on dendrites were already present at day 10. After day 10 the number of organelles and synapses increased prominently and in adult animals the different types of synapses on dendrites were located at relatively short intervals of about 4 microns. In contrast with the dendritic shafts of pyramidal neurons many asymmetrical synapses were observed on dendritic shafts of the non-pyramidal neurons analysed in the adult animals. Furthermore, it appeared that the number of synapses on these non-pyramidal neurons is about twice that on pyramidal neurons in day 20 old animals and about four times in adult animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Differentiation of cerebellar mossy fiber synapses in the rat: a quantitative electron microscope study

The differentiation of cerebellar glomeruli was investigated by quantitative electron microscopy, starting with the period at which mossy fibers made their first appearance. Two developmental stages could be delineated in the maturation of the mossy fiber-granule cell synapse. 1. A primary growth stage (postnatal days 6--15) characterized by the rapid enlargement of mossy rosettes and the intense proliferation of post-synaptic dendrites. The synaptic perimeter of mossy terminals, i.e. percentage of membrane surface occupied by synaptic junctions, exhibited a simultaneous, rapid increase in that stage, reaching a peak at postnatal day 15. 2. Establishment and stabilization of differentiated glomeruli (15th--45th day). Because the size of mossy rosettes did not change in this period, the increase of glomerular size was due exclusively to the continuing multiplication of postsynaptic dendrites. The characteristic feature of this stage was the massive elimination o synaptic junctions. The synaptic perimeter of 14.4% at day 15 decreased to 5.7% by day 30. Since the size of individual synaptic junctions and the size of mossy terminals did not decrease while the number of postsynaptic dendrites even increased during the same period, the elimination of synaptic junctions represents a net loss of the synaptic perimeter of mossy terminals. The quantitative analysis suggests that the stabilization of the synaptic perimeter of mossy rosettes at about 6% is due to the elimination during the second developmental stage of immature synaptic junctions, produced in excess during the first growth phase. Also, the observation that synapse elimination and the subsequent stabilization of synaptic perimeter occurs in spite of a steady increase of available postsynaptic dendrites is indicative that the standard 6% value of synaptic perimeter is defined by the presynaptic mossy terminal itself. On the basis of these observations, it is also proposed that elimination of synaptic junctions may well occur without the concomitant disappearance of presynaptic and/or postsynaptic neuronal processes.

Unilateral odor deprivation: differential effects due to time of treatment

Rats were unilaterally deprived of odor stimulation by surgically sealing one naris 1, 30 or 60 days after birth. Thirty days later the laminar volumes of the olfactory bulb ipsilateral to the occluded naris were compared with those ipsilateral to the normal side. Deprivation from days 1-30 resulted in approximately 25% reductions in "deprived" bulb size, corroborating results reported by others. Altered experience at older ages had little effect on laminar volume suggesting that deprivation does not produce atrophy per se, but interacts with early developmental processes to result in differential bulb size.

The formation and maturation of synapses in the visual cortex of the rat. I. Qualitative analysis

Synapse formation and maturation were examined in the visual cortex of albino rats from birth to maturity. During the first few days of postnatal life, synapses were sparsely scattered in the subplate zone and in layer I. They appeared immature as judged by the irregular shapes of the presynaptic and postsynaptic profiles, the relatively poorly defined membrane specializations and the presence of only a few synaptic vesicles in the presynaptic structures. As the neuropil matured, synapses were observed throughout the cortex, showing increased thickening of the membrane specializations and more vesicles. However, it was not until the end of the fourth postnatal week that they appeared qualitatively indistinguishable from synapses identified in the adult material. A feature characteristic of the developing visual cortex was the presence of vacant membrane specializations that resembled type I postsynaptic densities. These specializations, which were located either opposite extracellular space or opposite another neuronal process, were only evident during the initial stages of synaptogenesis and their frequency decreased as the number of synapses increased. In addition, transitional forms between these densities and true type I synapses were identified during the first two postnatal weeks. Structures that resembled vacant postsynaptic densities typical of type II synapses were not observed. The earliest identified forms of type II synaptic contacts identified consisted of two profiles that exhibited symmetrical membrane specializations and cleft material. Based on these observations, a scheme has been proposed for the formation of type I and type II synapses in the visual cortex of the rat.

Olfactory bulb maturation in Acomys cahirinus: is neural growth similar in precocial and altricial murids?

Olfactory bulb maturation was assessed in the precocial murid Acomys cahirinus by quantifying developmental changes in laminar volume and glomerular and mitral cell size. At the time of birth (38 days postconception), the Acomys bulb is in a developmental state similar to that of altricial murids of the same gestational age. Rapid growth occurs in Acomys during the first 10 postnatal days with more gradual changes occurring thereafter. Altricial murids also exhibit accelerated bulb development in early postnatal period, but growth rates have declined by 38 days after conception. These results suggest that maturation is not similar in precocial and altricial murids and that rapid postnatal development is a necessary feature of murid brain development.

Development of olfactory receptor neuron selectivity in the rat fetus

Olfactory receptor neurons begin to differentiate from stem cells on day E10 of embryonic life in the rat. By day E16, the receptor epithelium is well populated. On this day single neuron action potentials could be recorded with some ease and the electro-olfactogram was well developed. The receptor neurons were functional in that they responded to the vapors of odorous substances. However, they were not selective. Each cell responded to nearly all of the substances in the stimulus set. The first synaptic connections between receptors and mitral cells are established on day E18. The olfactory marker protein is reported to appear first in the receptors on the same day. By day E21, single unit responses changed dramatically. The cells became selective, responding to about half of the substances in our set. The electro-olfactogram reached its limiting amplitude well before this time.

Postnatal proliferation and maturation of olfactory bulb neurons in the rat

Mitral cells are formed prenatally whereas most granule cells originate postnatally. Material was taken from 2-day-old, 14-day-old, 28-day-old, and adult rat olfactory bulbs and processed for rapid Golgi or Cresyl Violet staining. We show that the number of granule cell bodies/mitral cell body increases from 7.0 to 46.3 during the first two weeks of life; most mitral cells appear morphologically functional during the first postnatal week; few granule cells appear to be functional until the second postnatal week; and the number of short axon interneurons increases dramatically during the second postnatal week. We conclude the newborn rats have an intact afferent pathway from olfactory receptors to primary cortex that lacks the extensive interneuronal circuitry characteristic of adults.

Turnover of brain postsynaptic densities after selective deafferentation: detection by means of an antibody to antigen PSD-95

Antibodies to antigen PSD-95, a neuronal protein present only in postsynaptic densities (PSDs), have been used to follow immunohistochemically the loss and recovery of PSDs after selective deafferentation of both the hippocampal formation and the lateral septal nucleus of the rat. Three days after unilateral entorhinal ablation, densitometry of brain tissue sections stained by the peroxidase-antiperoxidase (PAP) method showed a decrease of about 25% in the outer 2/3 of the ipsilateral dentate gyrus molecular layer (ML), whereas staining in the inner 1/3 of the same ML layer actually increased about 16%. The intensely staining inner 1/3 of the deafferented ML expanded over time so that by 30 days postlesion the expansion had reached 60-70% of the ML. In the lateral septal nucleus, unilateral fimbria transection did not change either the pattern of anti-PSD PAP staining or that of [125I]protein A binding as revealed by autoradiography or by microdissection of the lateral septal nuclei to determine bound radioactivity by gamma-counting. Bilateral intraventricular injection of kainic acid to destroy area CA3 of the ipsilateral hippocampus caused very little loss of anti-PSD stain in the dentate gyrus ML, but decreased by 45% and 34% the intensity of stain in stratum radiatum and stratum oriens of the hippocampus, respectively. However, bilateral injections of doses of kainate high enough to destroy areas CA3, CA4 and part of CA1, caused 53% loss of stain in the inner 1/3 of the dentate gyrus ML. Recovery from the PSD loss caused by kainate in area CA1 was slow, only 67-81% of control by 90 days post-lesion. The immunohistochemical results in dentate gyrus and septum corresponded closely with quantitative data obtained by electron microscopy. Therefore, the response of PSDs to the loss of their presynaptic counterpart appears to depend on the overall extent of deafferentation of the neuron, the zone of the dendritic tree where deafferentation occurs and, perhaps, other specific features of the target cells.

The developmental morphology of Torpedo marmorata: the electric lobes

The development of the electric lobes of Torpedo marmorata has been investigated using light and electron microscopical techniques. The lobe Anlagen become visible in the rhombencephalon along the floor of the 4th ventricle at the 10-mm stage. Many of the neuroepithelial cells in the Anlagen differentiate, Becoming postmitotic and axonic by the 24 mm stage. Proliferative zones of neuroepithelial cells disappear from the electric lobes by the 30-mm stage. After their initial, early differentiation the electromotor neurons remain monopolar until the 40-mm stage when dendrite formation begins. The differentiation of the electromotor neuron from a mono- to an immature multi polar form occurs between the 40- and 55-mm stages and involves, in addition to dendrite formation, a change from a pear-shaped to a spherical cell body, a dramatic increase in cytoplasmic volume, a centralization of the nucleus, an enlargement of the nucleolus and its migration away from the nuclear membrane, and differentiation of the axon hillock. The electric lobes are invaded by sinusoids at the 24-mm stage but formation of the capillary network by sprouting cords of endothelial cells begins later at the 40-mm stage. Neuronal cell death (26-74-mm stages) appears to be mainly an autolytic process and the debris is removed by immature glial cells. Afferent fiber growth cones are first recognized in the lobes at the 60-mm stage but synapses are not observed until the 78-mm stage. Myelination begins in the electric lobes concomitantly with the onset of synaptogenesis. A twofold increase in dendrite length occurs over the period when synapses begin to form in the lobes but dendritic maturation is not complete until the neonatal (120-mm) stage. The results are discussed in relation to the development of the electric organs.

Development of apparent presynaptic elements formed in response to polylysine coated surfaces

Formation of apparent presynaptic elements on polylysine-coated surfaces was examined with both scanning and transmission electron microscopy. Neurons from 2- or 8-day-old rat cerebellums were used in dissociated cell cultures as a source of growing neurites. An apparent presynaptic element is a defined swelling on a neurite with 40 nm diameter vesicles accumulated at the membrane which has a slight thickening. Apparent presynaptic elements do not have a normal postsynaptic element. In place of the postsynaptic element was the polylysine-coated surface of a large diameter Sepharose bead. The first apparent presynaptic elements were seen at 2 h of incubation, suggesting that morphologically identifiable synapses may form in this short a time. The number of apparent presynaptic elements on beads increased from 2 h to 24 h incubation and decreased from 5 to 9 days incubation. At all times non-neuronal cells grew up on to the beads and often covered both neurites and apparent presynaptic elements. In the longer incubations degenerating apparent presynaptic elements were seen engulfed by non-neuronal cells, suggesting that non-neuronal cells may have the ability to remove presynaptic elements that are not functioning. The number of synaptic vesicles per apparent presynaptic elements increased continuously between 2 h and 9 day incubation, eventually surpassing the number of synaptic vesicles seen in other presynaptic elements in cultures. This result suggests that an interaction between presynaptic and postsynaptic elements may be necessary to limit the number of synaptic vesicles found in presynaptic elements. Cultures grown for only 5 days in vitro (DIV) and incubated for 1 day with coated beads had the most apparent presynaptic elements, while those at 28 DIV and incubated for 1 day had the least. In the cultures at 5 DIV, neurons formed many apparent presynaptic elements, but these neurons could form only a very few normal synapses between themselves. Thus the beginning of the formation of synaptic contacts may depend on the availability of future postsynaptic elements and not on the lack of future presynaptic elements.

The postnatal development of the substantia nigra: a light and electron microscopy study

The early postnatal development of neurons, dendrites and synaptic connectivity in kitten substantia nigra (SN) was studied by light and electron microscopy. The compact and reticular divisions of the SN are present at birth but boundaries are indistinct. Most nigral neurons stain deeply in routine histological sections and their diameters increase slightly with age. Ultrastructurally, cell bodies are characterized by eccentrically located and often invaginated nuclei surrounded by cytoplasm rich in well-formed organelles. Axosomatic synapses are infrequent and cell surfaces are enveloped by glial processes. Immature dendritic features, including growth cones and filiform processes, are commonly observed during the first 10 days. Gradually the dendritic profiles elongate and thicken and contours become smoother, retaining only scattered spinelike appendages. Clear examples of the three synaptic types described in cat are found in newborn kittens, but immature terminals contain fewer synaptic vesicles and mitochondria. Approximately 90% of synapses present at birth in both nigra subdivisions are Type I, which contain large pleomorphic vesicles and contact dendrites symmetrically. Asymmetrical contacts characterize most of the remaining definable synapses. The postnatal increase in synaptic connectivity, which was estimated from random photographs of pars reticulata neuropil, is twofold during the first 50 days of life. Initially young dendrites are enveloped by glia and then gradually become ensheathed by axon terminals. Synaptogenesis in pars reticulata reflects the postnatal increase of neostriatal inputs to this subdivision and can be correlated with functional changes in strionigral connectivity.

Axosomatic synapses in the visual cortex of adult rat. A comparison between GABA-accumulating and other neurons

Differences in axosomatic synapses between GABA-accumulating [G(+)N] or non-accumulating [G(-)N] neurons have been investigated in the visual cortex of adult rat. The neurons were classified and localized in light microscopic autoradiograms after [3H]GABA injections. The cells resectioned for electron microscopic identification of type 1 synapses (T1S) and type 2 synapses (T2S). A total of 167 neurons [45 G(+)N, 122 G(-)N] situated in laminae II-VI were evaluated. The two groups of neurons were not uniform populations. G(-)N included both pyramidal and non-pyramidal neurons, whereas no typical pyramidal neurons were found among G(+)N. A total of 691 synaptic contacts was evaluated for these groups of somata. The density of synapses was higher on G(+)N than on G(-)N. This was mainly due to a difference in the number of T1S. On G(-)N the frequency distributions of both types of synapses represented Poisson distributions, indicating that there were stochastic variations around mean values. In contrast, on G(+)N the distribution was exponential which suggests that G(+)N include several subpopulations with different densities of T1S. On all cortical neurons the average density of T2S was 509-60 T2S per 1000 microns2 of soma surface, which resembles the density in the neuropil. In contrast, T1S varied from zero to a mean of 12 per 1000 microns2 on G(-)N and to a mean of 51 per 1000 microns2 on G(+)N, i.e. the density of T1S, unlike T2S, is much smaller on neuronal somata than in the surrounding neuropil. It is suggested that the formation of axosomatic T1S, but not of T2S, is suppressed to a variable degree on almost all cortical neurons. Only on pyramidal neurons does the suppression of T1S seem to be complete.

Perisomatic changes in the maturing hypoglossal nucleus after axon injury

The perisomatic retrograde reactions to peripheral nerve injury during postnatal maturation were investigated by quantitative light and electron microscopy. The hypoglossal nerve was crushed, ligated or transected in 10 and 21 day postnatal (dpn) rats. Only crush injury was made in 7 dpn rats. Survival periods ranged from 3 to 40 days postoperative (dpo). Normal and sham operated animals of corresponding ages served as controls. A remarkable, transient response of neuronal somata in the young (7-10 dpn) rats, following all three types of axon injury, was the formation of excrescences which engulfed neuronal processes: boutons, dendrites, small neurites and a few myelinated axons in adjacent neuropil. The somal engulfment was rarely evident after nerve injury to 21 dpn rats. Bouton displacement from the somal surface, accompanied by glial incursion, followed each type of nerve injury but was less extensive and occurred later in the young rats. There seemed to be no association in the amount of boutons displaced from neuronal somata with the type of nerve injury for any of the three experimental age groups. However, the rate and intensity of the perineuronal glial reaction were related to the severity of the nerve injury in the older (21 dpn) but not in the younger (7-10 dpn) rats. Substantial loss of neurons occurred in the affected nucleus after each type of trauma to the young neurons. The degree of neuronal loss was related to the age and the volume of axoplasm disrupted. Only nerve transection in 21 dpn rats resulted in appreciable neuronal loss. The responses of axotomized neurons, their afferent axon terminals and the surrounding glia are quantitatively and qualitatively different in the hypoglossal nucleus of rats of increasing postnatal ages.

Junctions in rat neocortical explants cultured in TTX-, GABA-, and Mg++-environments

The occurrence of various cell-to-cell contacts and membrane specializations was quantitated in neocortical explant cultures prepared from 18-day-old rat embryos and exposed continuously to tetrodotoxin (TTX), gamma-aminobutyric acid (GABA) and an elevated level of magnesium ions (Mg++), respectively. Chronic TTX treatment resulted in a decrease in the number of synapses, paired neuronal membrane thickenings and nerve terminals with synaptic vesicles; the area of the neuronal compartment also decreased. By contrast, the gap junctions between glial cells increased, although the glial paired membrane thickenings decreased in number per unit area. Long-term GABA and Mg++ exposures did not alter significantly the occurrence of any of the cell contacts and membrane specializations analyzed when compared to control values. The results suggest an inhibitory effect of TTX on neuronal maturation and synapse formation in explant cultures of rat neocortex; this may lead secondarily to an increased demand for glial cell-to-cell communication.

Axoglial synaptoid contacts in the neural lobe of the human fetus

Axoglial synaptoid contacts as well as junctional specializations were examined in the neural lobe of five human fetuses ranging from 7.5 to 19 weeks of ovulation age. Because synapselike contacts (SLC), which have both presynaptic and postsynaptic densities, were observed only in the younger fetus (7.5 weeks), they may be transient structures. Axoglial SLC were also found in older fetuses, but their structural characteristics were different from those found in a 7.5-week fetus in that the cytoplasmic membrane density was found only on the presynaptic side and its extent was decreased. In the youngest fetus observed here (7.5 weeks), puncta adherentia were observed. In the fetuses older than 11 weeks, gap junctions, long adherentia-type junctions with thick filamentous structures, and mixed junctions with both types of contact were found between pituicytes.

Quantitative study of synapse formation in the duck olfactory bulb

Synaptic emergence and development in the duck olfactory bulb was quantitatively studied by electron microscopy from the 14th day of incubation (E 14) to the adult stage. Overall synaptic density in this bulb grew considerably during the last weeks of embryonic life and the first postnatal week. The pattern of synaptic density development was similar in the four main architectonic layers of the bulb. However, lower density values were observed in the mitral and inner granule cell layers. In the glomerular layer (GL), axodendritic synapse density was always higher than dendrodentritic synapse density. In the external plexiform layer, most synapses were dendrodendritic and were established between the gemmules of the granule cells (GC) and the dendrites of the mitral cells (MC) or tufted cells (TC). Synapses established by MC and TC on GC gemmules, or by GC on MC and TC dendrites had densities very similar to each other at all the stages studied. Reciprocal synapses already appeared at E 14; their density grew until a week after birth (P7) and thereafter remained stable. In the internal granular layer, the density of asymmetrical synapses was always higher than that of symmetrical synapses. Excitatory synapses formed earlier on MC and TC than inhibitory synapses. The ratio of inhibitory-to-excitatory synapses rose rapidly after birth, reaching 2.5 in the adult duck. The density of excitatory synapses received by granule cells was as high in the external plexiform layer as in the inner granule layer, at all stages of GC development. However, the ratio of received-to-formed synapses fell in these cells from 8.42 at E 14 to 2 after birth. These results are discussed as a function of the evolution of the different synaptic balances during olfactory bulb development. Synaptic development in the duck olfactory bulb at birth is relatively close to the adult state. It appears sufficiently advanced to enable the olfactory system to function in a way compatible with the relatively independent behavior displayed by the duckling.

Ontogenesis of the functional activity of rat olfactory bulb: autoradiographic study with the 2-deoxyglucose method

An ontogenetic study of the uptake of [14C]2-deoxyglucose (2-DG) within the olfactory bulb was performed on rats of 3 age groups, 1, 9 and 21 days. Animals were exposed either to ethyl acetoacetate (EAA) or to a nest odor. Already in newborns, a glomerular activity characterized by small spots of increased 2-DG uptake, appeared in response to either odor stimulation. An ontogenetic development of this glomerular activity was observed with age: whereas the labeled glomerular foci were scanty in newborns, their number constantly increased with age until weaning; moreover, an increased proportion of foci overlaying 2 or 3 single glomeruli was noted in parallel with the postnatal maturation of the olfactory system. For the 3 age groups, whatever the odor stimulus, the glomerular activity seemed located in 2 main regions of the olfactory bulb, the lateral aspect and the posterior part of the medial aspect of the bulb. In newborns, no clear difference could be brought out between the patterns of glomerular activation related to EAA and to nest odor, respectively. In 9-day-old rats, the spatial patterns of distribution associated with the 2 odors were overlapping, but nevertheless different. These data are correlated with the postnatal maturation of the olfactory system.

Fine structure of the nigrostriatal anlage in fetal rat brain by immunocytochemical localization of tyrosine hydroxylase

The developmental morphology and synaptic associations of neurons in the nigrostriatal anlage are examined by the electron microscopic immunocytochemical localization of tyrosine hydroxylase at embryonic (E) day 13.5 and 14.5 in rat brain. At E 13.5, immunoreactivity for the enzyme is localized throughout the cytoplasm of neuronal perikarya and processes including somatic, dendritic, and axonal growth cones. The cytoplasmic organelles in perikarya include primarily ribonucleic-protein particles, mitochondria and an immature Golgi apparatus. At E 14.5, the tyrosine hydroxylase labeled processes are detected in the lateral hypothalamus and ventrolateral caudate-putamen. The axonal processes showing immunoreactivity in the ventral mesencephalon and more rostral portions of the nigrostriatal bundle are frequently attached to unlabeled neurites by puncta adherentia. In the hypothalamus and caudate-putamen presumably transient synaptic junctions are also detected between the labeled axons and unlabeled neurons. The immature morphological features of neurons showing immunoreactivity for tyrosine hydroxylase thus indicate, that the biochemical differentiation of the nigrostriatal neurons precedes complete cytological differentiation.

Light-microscopic immunocytochemical localization of tyrosine hydroxylase in prenatal rat brain. I. Early ontogeny

The immunocytochemical localization of tyrosine hydroxylase is examined during early ontogeny in the fetal rat brain in order to determine the age of first detection and subsequent cellular localization of the enzyme and the developmental characteristics of the immature catecholaminergic neurons. Fetal atlases of the tyrosine hydroxylase-labeled neurons are presented at embryonic day (E) 12.5, 13.5, and 14.5. Tyrosine hydroxylase is first detected immunocytochemically at E 12.5. At this stage, the labeled neurons have completed final mitosis, but are still migrating and are cytologically immature. Tyrosine hydroxylase can also be detected in axons and axonal growth cones at this stage of development. The age of first immunocytochemical detection of the enzyme precedes the demonstration of catecholamine fluorescence by 1 to 2 days in certain nuclear groups. At later stages of development (E 13.5 and E 14.5), the major groups of perikarya and processes labeled for tyrosine hydroxylase have a distribution similar to that previously described by catecholamine fluorescence. At E 14.5, the perikarya undergo considerable changes in their cytology and exhibit the first dendrites immunocytochemically labeled for the enzyme. The first terminal fields are also detected in the rudimentary caudate-putamen at this stage.

Olfactory bulb projections to the parahippocampal area of the rat

Recent evidence suggests that the main olfactory bulb projects caudally beyond the prepiriform cortex and the cortical amygdaloid nuclei to the region of the piriform lobe called the parahippocampal area. Included within this area is the entorhinal cortex, which is composed of six major subdivisions. Since questions remain as to which of these subdivisions receives centripetal fibers from the bulb, we reexamined these projections using autoradiography and HRP histochemistry and correlated the sites of termination with the cytoarchitecture of the entorhinal cortex. The results indicate that olfactory bulb axons reach all parts of the parahippocampal area, including the cortex which forms the medial and lateral banks of the amygdaloid sulcus (area TR), and both subdivisions of the laterally located entorhinal cortex (28L' and 28L). Also, label is observed over the more medially located fields of the entorhinal cortex, including the cortex posterior to the cortical amygdaloid nucleus (28M'), as well as the ventrolateral parts of medial entorhinal cortex (28M). In addition, evidence of label occurs over the full extent of the transition zone (28i) which separates areas 28L and 28M. These results suggest that the olfactory bulb has a more extensive projection to the parahippocampal area in the rat than previously thought, and may provide at least some input to all of the parahippocampal areas which project to the hippocampal formation.

Isolation and characterization of postsynaptic densities from various brain regions: enrichment of different types of postsynaptic densities

Postsynaptic densities (PSDs) have been isolated from cerebral cortex, midbrain, cerebellum, and brain stem by the Triton X-100 method previously used in the isolation of cerebral PSDs (Cohen et al., 1977, J. Cell Biol. 74:181). These PSDs have been compared in protein composition, protein phosphorylation, and morphology. Thin-section electron microscopy revealed that cerebral cortex and midbrain PSDs were identical, being approximately 57 nm thick and composed of apparent aggregates 20-30 nm in diameter. Isolated cerebellar PSDs appeared thinner (33 nm) than cerebral cortex PSDs and lacked the apparent 20- to 30-nm aggregates, but had a latticelike structure. In unidirectional and rotary-shadowed replicas, the cerebrum and midbrain PSDs were circular in shape with a large central perforation or hole in the center of them. Cerebellum PSDs did not have a large perforation, but did have numerous smaller perforations in a lattice like structure. Filaments (6-9 nm) were observed connecting possible 20- to 30-nm aggregates in cerebrum PSDs and were also observed radiating from one side of the PSD. Both cerebral cortex and midbrain PSDs exhibited identical protein patterns on SDS gel electrophoresis. In comparison, cerebellar PSDs (a) lacked the major 51,000 Mr protein, (b) contained two times less calmodulin, and (c) contained a unique protein at 73,000 Mr. Calcium plus calmodulin stimulated the phosphorylation of the 51,000 and 62,000 Mr bands in both cerebral cortex and midbrain PSDs. In cerebellar PSDs, only the 58,000 and 62,000 Mr bands were phosphorylated. In the PSDs from all brain regions, cAMP stimulated the phosphorylation of Protein Ia (73,000 Mr), Protein Ib (68.000 Mr), and a 60,000 Mr protein, although cerebrum and midbrain PSDs contained very much higher levels of phosphorylated protein than did the cerebellum. On the basis of the morphological criteria, it is possible that PSDs isolated from cerebrum and midbrain were derived from the Gray type I, or asymmetric, synapses, whereas cerebellum PSDs were derived from the Gray type II, or symmetric, synapses. Since there is some evidence that the type I synapses are involved in excitatory mechanisms while the type II are involved in inhibitory mechanisms, the role of the PSD and of some of its proteins in these synaptic responses is discussed.

Fine structural studies of synaptogenesis in the superficial layers of the chick optic tectum

Synaptogenesis in the superficial layers of the rostral pole of the optic tectum has been studied in the chick from embryonic day six (E6) to seven days post-hatching. Symmetrical membrane densities of puncta adhaerentia are observed prior to the detection of synapses and throughout development. Immature synaptic contacts are observed by E7. These early synapses are primarily axodendritic; however, somatodendritic, dendrodendritic, axosomatic and axoglial synapses are also observed. The majority of these synapses have asymmetrical membrane densities and the presynaptic terminals contain clear, spherical, synaptic vesicles. Synaptic terminals containing pleomorphic vesicles and making symmetrical synaptic contacts are not commonly observed until the third week of embryonic development, and may represent the onset of inhibitory function within the tectum. Comparison of the number of synapses per unit area in control versus experimental tecta, after unilateral eye enucleations of E3, indicates that the presynaptic terminals of some synapses present at E8 are of retinal origin. It is suggested that the development of retinotectal synapses follows a rostrocaudal gradient in the tectum and corresponds to the intrinsic tectal pattern of cytoarchitectonic differentiation.

Induction and maintenance of free postsynaptic membrane thickenings in the adult superior cervical ganglion

The superior cervical ganglion (SCG) of adult rats was exposed to GABA, either by long lasting microapplication (implantation of glass bulbs for 1-24 days) or in short term experiments (external application up to 6 h). Autoradiography showed that [3H] GABA accumulated selectively in satellite cells. The GABA produced the following effects: (1) Specialized membrane thickenings--similar in fine structural appearance to those seen as postsynaptic membrane thickenings at Gray type I synapses--were formed at the extrasynaptic dendritic surface of principal ganglion cells. (2) Morphometry revealed that the surface to volume ratio of dendrites increased significantly corresponding to an enlargement of their extrasynaptic surface as a result of the formation of spine-like projections. (3) Electrophysiology confirmed that, at least after short term application, the action potentials induced by preganglionic stimulation were heavily suppressed. These results suggest that, in the course of depressed ganglionic activity, so-called free postsynaptic membrane thickenings are generated and maintained in the SCG of adult rats even in the absence of significant axonal degeneration. The discussion focuses on two points: (1) possible similarities between the conditions of neurons after denervation and under the influence of GABA; (2) a possible role of GABA and other substances with inhibitory action in synaptogenesis.

Postnatal dendritic development in the rabbit visual cortex

Golgi preparations of rabbit visual cortex aged 1-25 days, as well as similar tissues from adults, were examined for the growth of the dendritic arbor, and in particular the development of dendritic spines. The layer 5 pyramidal neurons and layer 4 stellate neuron were chosen as representatives of larger classes of neurons in the visual cortex. It was determined that the growth of the dendritic arbor, determined by counts of total number of dendritic and total dendritic length, is quite similar for pyramidal and stellate neurons. Dendritic spine development, however, is more rapid in pyramidal neurons than in stellate. This disparity in the rate of dendritic spine development is discussed in the light of physiologic studies on the development of receptive field properties in the rabbit visual cortex.

Synaptic development in the rabbit superior colliculus and visual cortex

The development of synapses in the visual cortex (VC) and superior colliculus (SC) of the rabbit has been examined with the electron microscope. In both areas, the number of synapses reaches adult levels by 20--25 days of postnatal age, but the development in the visual cortex is delayed in comparison to that in the superior colliculus. When S synapses (spheroidal vesicles, asymmetric thickening) are compared with F synapses (flattened vesicles, symmetric thickening), even greater differences are seen. In both the VC and SC, S synapses develop earlier than F synapses, though there is considerable overlap. Of interest is that fact that synapses in the visual cortex seem to overshoot their adult levels late in development, suggesting that an excess of synapses may be formed in this system. Multiple synapses, probably of retinal origin, increase in the first 3 weeks of synaptic development in the SC, but never are present in significant proportions in the VC. Synapse formation most often is characterized by formation of a junction and a postsynaptic thickening, followed by acquisition of synaptic vesicles. After 15 days, there is only a small number of such "non-vesicle synapses" in either the SC or VC.

Postnatal synaptogenesis in the rat interpeduncular nucleus

The rat interpeduncular nucleus (IPN) was studied by electron microscopy from the day of birth to 33 days of age. The habenulointerpeduncular (H-IPN) axons, the principal afferents to IPN, were prominent at birth, occurring in large groups. They formed occasional S synapses en passant at this time. S synapses subsequently increased progressively in number and maturity until 28 days of age. Crest synapses, also formed by the H-IPN axons, appeared at eight days without a recognized formative stage. They exhibited their diverse adult morphology by 14 days of age. F synapses, endings of uncertain origin that contain flattened vesicles, were seen at 21 days, although endings containing small vesicles at 14 days represented the same population. Axosomatic synapses were first seen at four days and remained infrequent subsequently. Dendritic growth cones had their maximal prevalence at birth, decreased at four days and were absent by six days of age. Thus each of the four synaptic types previously described in the adult rat IPN exhibited a characteristic time of appearance and pattern of development in normal rat pups. This well-ordered sequence of synaptogenesis makes the H-IPN system well suited for a study of synaptic plasticity.

Quantitative effects of methylzoxymethanol acetate on Purkinje cell dendritic growth

A quantitative analysis was made of alterations in the dendritic organisation of Purkinje cells in the cerebellum of the rat following the administration of the degranulating agent, methylazoxymethanol acetate (MAM). This drug depleted the granule cell population of the cerebellar cortex and disturbed Purkinje cell alignment such that a number of Purkinje cells became inverted and grew in the white matter. The quantitative changes that occurred in the dendritic trees of these cells (increase in segment length, decrease in segment numbers, trichotomy and branching probability) were similar to those seen following other degranulation procedures. The size of the Purkinje cell dendritic tree was found to be related to the number of parallel fibres present in the molecular layer. These results were discussed in relation to current theories of neuronal development and were shown to lend further support to the filopodial attachment hypothesis of dendritic growth.

Synapse development within the spinal trigeminal nucleus

Pars interpolaris of the spinal trigeminal nucleus of kittens has been studied with the electron microscope at birth and at several subsequent ages during the first month of life. Attention has been given to ultrastructural maturational changes that occur in this neuropil, especially events in synaptogenesis. The results of this investigation include the following observations: (1) the neuropil, even at the earliest ages studied (three-hour-old kittens), is strikingly mature, necessitating a quantitative assessment in order to determine subtle developmental changes in synaptic patterns; (2) the number of axoaxonic contacts at birth are few, and their emergence is essentially a postnatal phenomenon; (3) it appears that the immature Gray type II or symmetrical synapse possesses distinct cleft material and dense, parallel membrane specializations. Synaptic vesicle accumulation at this contact appears to occur after the membrane specializations have formed. A previous study by Kerr26 has shown a reduced potential for primary afferent reorganization with the spinal trigeminal nucleus when kittens are subjected to trigeminal rhizotomy after three days of age. Our observations on the development of axoaxonic synaptic arrangements in the neonatal period may provide an explanation for these earlier results.

Denervation of the primary olfactory pathway in mice. V. Long-term effect of intranasal ZnSO4 irrigation on behavior, biochemistry and morphology

Intranasal irrigation of mice with 0.17 M ZnSO4 solution results in the immediate and total loss of the ability to find a buried food pellet. This anosmia persists for 6 weeks in at least 80% of the treated mice and for 4 months in half of the animals. This marked behavioral effect is matched by a long-term reduction of the levels of carnosine synthesis and transport in the primary olfactory pathway. These biochemical parameters are virtually undetectable at two weeks after treatment and even at one year after treatment do not exceed 5-10% of average control values. Light microscopic observations of tissues of the primary olfactory pathway at various times after treatment are consistent with these observations and indicate a substantial destruction of the olfactory epithelium with subsequent atrophy of the olfactory bulb. At very long intervals after treatment, some receptor regeneration is apparent with accompanying reinnervation of the olfactory bulb. Estimates from microscopy and biochemistry suggest that much less than 10% of the normal complement of functioning receptor cells is adequate to give apparently normal food-finding behavior.

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.