Afferent Synaptic Contacts Research Articles

Absence of microglial CX3CR1 impairs the synaptic integration of adult-born hippocampal granule neurons

Microglia are immune cells that play a crucial role in maintaining brain homeostasis. Among the mechanisms of communication between microglia and neurons, the CX3CL1/CX3CR1 axis exerts a central modulatory role. Animals lacking CX3CR1 microglial receptor (CX3CR1−/− mice) exhibit marked alterations not only in microglia but also in neurons located in various regions of the brain. Here we show that microglial depletion of CX3CR1 leads to the deficient synaptic integration of adult-born granule neurons in the dentate gyrus (DG), both at the afferent and efferent level. Regarding the alterations in the former level, these cells show a reduced number of dendritic spines, which also exhibit morphological changes, namely enlargement and shortening. With respect to changes at the efferent level, these cells show a reduced area of axonal terminals. Both at the afferent and efferent level, synapses show ultrastructural enlargement, but they are depleted of synaptic vesicles, which suggests impaired functionality. We also show that selective increased microglial activation and extracellular matrix deposition in the zones in which the afferent synaptic contacts of these cells occur, namely in the molecular and the granule layer of the DG. In order to evaluate the impact of these structural alterations from a functional point of view, we performed a battery of behavioral tests related to hippocampal-dependent emotional behavior. We observed that female CX3CR1−/− mice exhibit a hyperactive, anxiolytic-like and depressive-like phenotype. These data shed light on novel aspects of the regulation of adult hippocampal neurogenesis by microglia that could be highly relevant for research into mood disorders.

Otoferlin, Defective in a Human Deafness Form, Is Essential for Exocytosis at the Auditory Ribbon Synapse

The auditory inner hair cell (IHC) ribbon synapse operates with an exceptional temporal precision and maintains a high level of neurotransmitter release. However, the molecular mechanisms underlying IHC synaptic exocytosis are largely unknown. We studied otoferlin, a predicted C2-domain transmembrane protein, which is defective in a recessive form of human deafness. We show that otoferlin expression in the hair cells correlates with afferent synaptogenesis and find that otoferlin localizes to ribbon-associated synaptic vesicles. Otoferlin binds Ca(2+) and displays Ca(2+)-dependent interactions with the SNARE proteins syntaxin1 and SNAP25. Otoferlin deficient mice (Otof(-/-)) are profoundly deaf. Exocytosis in Otof(-/-) IHCs is almost completely abolished, despite normal ribbon synapse morphogenesis and Ca(2+) current. Thus, otoferlin is essential for a late step of synaptic vesicle exocytosis and may act as the major Ca(2+) sensor triggering membrane fusion at the IHC ribbon synapse.

Expression of BDNF and TrkB in mouse taste buds after denervation and in circumvallate papillae during development.

BDNF (brain-derived neurotrophic factor) is a member of the neurotrophin family which affects the proliferation and survival of neurons. Using an immunocytochemical method, we examined the expression of BDNF and its receptor, TrkB, in the taste bud cells of the circumvallate papillae of normal mice and of mice after transection of the glossopharyngeal nerves. We additionally observed the expression of BDNF and TrkB in the developing circumvallate papillae of late prenatal and early postnatal mice. In normal untreated mice, BDNF was expressed in most of the taste bud cells; TrkB was detected in the plasma membrane of taste bud cells and in the nerve fibers. Double-labeling studies showed that BDNF and NCAM (neural cell adhesion molecule) or TrkB and NCAM colocalized in some of the taste bud cells, but that most taste bud cells were immunopositive for only BDNF or TrkB. NCAM-immunoreactive cells are known to be type-III cells, which have afferent synaptic contacts with the nerve terminals. Five days after denervation, the number of taste buds and nerve fibers markedly decreased; however, the remaining taste bud cells still expressed BDNF and TrkB. By 10 days after denervation, most of the taste buds had disappeared, and there were a few TrkB-immunoreactive nerve fibers in the connective tissue core. By 4 weeks after denervation, numerous TrkB-immunoreactive nerve fibers had invaded the papillae, and a few taste buds expressing BDNF and TrkB had regenerated. At E (embryonic day) 15 during development, the circumvallate papillae appeared, and then TrkB-immunoreactive nerve fibers entered the connective tissue core, and some of these fibers further invaded among the dorsal epithelial cells of the papillae. TrkB-immunoreactive oval-shaped cells were occasionally found in the dorsal epithelium. Such TrkB-immunoreactive nerve fibers and cells were also observed at E16-18. However, BDNF was not expressed in the papillae through the late prenatal days of E15 to E18. At P (postnatal day) 0, a cluster of BDNF-and TrkB-immunoreactive cells appeared in the dorsal epithelium of the papillae, and was presumed to be primitive taste buds. We conclude that TrkB-immunoreactive nerve fibers are necessary for papillary and taste bud formation during development and for the regeneration of taste buds after denervation. BDNF in the taste bud cells may act as a neurotrophic factor for innervating sensory neurons--through TrkB receptors of the axons of those neurons, and also may exert autocrine and paracrine trophic actions on neighboring taste bud cells by binding to their TrkB receptors.

Afferent synaptic contacts on glycine-immunoreactive neurons in the rat cuneate nucleus.

This study was aimed to clarify whether the primary afferent terminals (PATs), GABAergic terminals, and glutamatergic terminals made direct synaptic contacts with glycine-IR neurons in the cuneate nucleus of rats. In this connection, injection of the anterograde tracer WGA-HRP into brachial plexus, antiglycine preembedding immunoperoxidase, and anti-GABA, along with antiglutamate postembedding immunogold labeling, were used to identify the PATs, glycine-IR neurons, GABA-IR terminals, and glutamate-IR terminals, respectively. The present results showed that HRP-labeled PATs, immunoperoxidase-labeled glycine-IR terminals, immunogold-labeled GABA-IR, and glutamate-IR terminals made axodendritic synaptic contacts with immunoperoxidase-labeled glycine-IR neurons. The latter three presynaptic elements also formed axosomatic synapses with glycine-IR neurons. Statistical analysis has shown that the minimum diameter of the glycine-IR dendrites postsynaptic to the above-mentioned four presynaptic elements did not differ significantly. In addition, the synaptic ratio of the glutamate-IR terminals on the glycine-IR dendrites was higher than that of GABA-IR terminals. The synaptic ratio of the GABA-IR terminals on glycine-IR dendrite was in turn higher than that of the PATs and glycine-IR terminals. It is suggested that the PATs and glutamate-IR terminals on the glycine-IR neurons may be involved in subsequent postsynaptic inhibition for spatial precision of lateral inhibition. On the other hand, the GABA-IR and glycine-IR terminals which make synaptic contacts with the dendrites of glycine-IR neurons may provide a putative means for disinhibition or facilitation to maintain the baseline neuronal activity in the rat cuneate nucleus. The results of quantitative analysis suggest that glutamate act as the primary excitatory neurotransmitter, while GABA, when compared with glycine, may serve as a more powerful inhibitory neurotransmitter on glycine-IR neurons in the rat cuneate nucleus.

Multipotent neural precursors can differentiate toward replacement of neurons undergoing targeted apoptotic degeneration in adult mouse neocortex.

Neurons undergoing targeted photolytic cell death degenerate by apoptosis. Clonal, multipotent neural precursor cells were transplanted into regions of adult mouse neocortex undergoing selective degeneration of layer II/III pyramidal neurons via targeted photolysis. These precursors integrated into the regions of selective neuronal death; 15 +/- 7% differentiated into neurons with many characteristics of the degenerated pyramidal neurons. They extended axons and dendrites and established afferent synaptic contacts. In intact and kainic acid-lesioned control adult neocortex, transplanted precursors differentiated exclusively into glia. These results suggest that the microenvironmental alterations produced by this synchronous apoptotic neuronal degeneration in adult neocortex induced multipotent neural precursors to undergo neuronal differentiation which ordinarily occurs only during embryonic corticogenesis. Studying the effects of this defined microenvironmental perturbation on the differentiation of clonal neural precursors may facilitate identification of factors involved in commitment and differentiation during normal development. Because photolytic degeneration simulates some mechanisms underlying apoptotic neurodegenerative diseases, these results also suggest the possibility of neural precursor transplantation as a potential cell replacement or molecular support therapy for some diseases of neocortex, even in the adult.

A comparative and ultrastructural study of synaptic contacts established by primary sensory fibers in the spinal trigeminal nucleus of the rat

A quantitative evaluation of the types of afferent synaptic contacts in the pars oralis, using transganglionic degeneration and a comparison of previous data obtained from the pars interpolaris ( Lapa & Bauer, 1992), of the rat was performed. Following left inferior alveolar nerve transection or partial pulpectomy of the first and second left lower molar teeth well-defined degenerating terminals appeared bilaterally. In both experiments, the majority of these afferent synapses formed single asymmetric contacts with intermediate and distal dendritic segments in the pars oralis. Fewer contacts were observed with dendritic spines, proximal dendritic segments, perikarya, and other terminals. Double and multiple synaptic contacts, preferentially with small dendritic profiles, were also found. Pars oralis showed higher density of degenerating terminals and higher proportion of the contralateral contacts than pars interpolaris suggesting that it is a prime input area and that it may play a role in the bilateral management of sensory information. Pars oralis showed a higher density of contacts with intermediate and distal dendritic segment and a lower density of double contacts in comparison to the pars interpolaris. Partial pulpectomy revealed a distribution in synaptic types similar to that following IAN transection suggesting that sensory fibers conveying pain-related stimuli are not distinguished from fibers of other sensory modalities as to preference of synaptic contacts. The overall pattern demonstrates a structural organization of the sensory inputs to the spinal trigeminal nucleus regarding the bilateral handling of sensory information.

TEM analysis of neural terminals on autoradiographically identified regenerated hair cells

Regenerated tall and short hair cells identified by autoradiography ([ 3H]thymidine) were analyzed for their neural contacts using transmission electron microscopy. Ears from mature Coturnix quail ( N = 5) exposed to pure tone ovcrstimulation (1500 Hz, 115 dB, 12 h) and treated with [ 3H]thymidine for 10 days were fixed, embedded, sectioned serially in 100 μ intervals and prepared for autoradiography. At fifty percent length along the papilla, alternating semi-thick (1 μm) and thin (70 nm) sections were taken at 50 μm intervals. Semi-thick sections were analyzed at the light microscope level for autoradiographic labeling of [ 3H]thymidine over the hair cell nucleus. When an autoradiographically labelled hair cell was identified the corresponding serial thin sections were analyzed in the transmission electron microscope. Seven autoradiographically labeled hair cells in semi-thin sections were positively identified in immediately adjacent thin serial sections. Labeled hair cells were morphologically similar to adjacent cells with no label and generally appeared to receive similar innervation. Regenerated short hair cells showed large chalice shaped, efferent terminals, intermediate hair cells received both afferent and efferent innervation and tall hair cells were contacted by two to three afferent terminals with synaptic specializations. These results provide conclusive evidence of both efferent and afferent synaptic contacts on newly regenerated hair cells of all types 10 days following acoustic trauma.

Streptozotocin 誘発糖尿病ラットの味蕾の電子顕微鏡的研究

Diabetic dysgeusia has been revealed since the first Fox's report (1932). In diabetes mellitus, however, ultrastructural study of the taste buds has not reported yet. In this point of view, the present study was perfomed to investigate morphological changes of the taste buds in foliate papillae in streptozotocin (STZ) induced diabetic rats. Diabetes was induced by intra abdominal injection of STZ (60 mg/kg b. w.). After raising for 3 months, 6 months and 12 months, the foliate papillae were collected and observed by means of electron microscopy. The results were as follows : 1. The taste buds of STZ rats were characterized by swelling cytoplasm, vacuole formation, and enlarged nerve fiber. The swelling taste buds contrasted strikingly with the atrophied foliate papillae in the whole STZ rats group. 2. Vacuole formation and pyknosis were seen in the taste buds in 3-month STZ rat group, and 6-month STZ rat group. Increased lipid droplets and a lot of filament bundles are seen in 12-month STZ rat group. 3. A great number of autophagic vacuoles were recognized morphologically as membranous cytoplasmic body (MCB) and zebra body. 4. Swelling mitochondria, rough ER, Golgi apparatus were observed as change of organelle. 5. Intragemmal fibers were enlarged and myelin-like body appeared in subsurface cisternae (ssc) in the Type II cell. Plasma membrane of the nerve ending degenerated in the afferent synapse in the type III cell. On the contrary, afferent synaptic contacts were observed in 12-month STZ rat group as well. 6. Dense substance in the taste pore decreased remarkably in 3-month STZ. It was suggested environment of taste pore changed in the early stage. With the foliate papillae atrophying, taste pore was apt to show stricture. There were also many imperfolate taste pore in 12-month STZ rat. 7. Diabetes showed strong infectiosity. Many taste pores infected with bacteria were observed. On account of these changes in taste buds and nerve element in STZ rats, functional disorder of taste buds were suggested in diabetes mellitus.

The early causal influence of cell size upon synaptic number: the mutant gigas of Drosophila.

The number of synaptic contacts formed by a neuron is known to vary with its surface area. This could be because large neurons are able to establish more synaptic sites, or because those neurons that are able to establish more sites are subsequently able to enlarge. To test between these two possibilities clones of enlarged ommatidia were generated in the retina of the Drosophila mutant gigas, by mitotic recombination following gamma-irradiation in the third-instar larva. The numbers of afferent synaptic contacts formed by the photoreceptor terminals in the first optic neuropil, or lamina, were then counted in the adult. The terminals of mutant photoreceptors were also enlarged, but by varying degrees. The sizes of their profiles in single sections merged with the size distribution of terminals having a wild-type phenotype, lying outside the clone in the same lamina. A perimeter of 6.0 microns for the profiles of receptor terminal in cross section was established as a criterion for distinguishing between normal and mutant phenotypes. The mutant terminals had more presynaptic sites. Because only the gigas terminals are mutant and because they enlarged at a time long before synapse formation occurred in the lamina we may conclude that cell enlargement preceded elevated synaptic number. The increase in synaptic number roughly matched the increased membrane surface of the terminals, so as nearly to preserve a constant areal density of synaptic sites over a 5-fold range in synaptic frequency.

Light and Electron (SEM, TEM) Microscopy of Taste Buds in the Tench Tinca tinca (Pisces: Cyprinidae)

Abstract The ultrastructure and quantitative distribution of the taste buds (TBs) were studied in the oropharyngeal cavity and in skin from the head of the tench. All TBs are of similar structure, following an orthodox plan: the basal cells (1–2) are the basis of the bud, and vertically elongated gustatory cells and supporting cells span from the basal membrane to the apex where they form a sensory zone (known as the gustatory pore).The basal cells have finger‐like processes pointing towards the nerve plexus. They do not show any hemidesmosomal connections with the basal membrane. Typical afferent synaptic contacts were found only at the basal cells and gustatory cells while no such contacts were found at the supporting cells.The highest concentration of TBs (up to 170 TBs mm 2) occurs in the epithelial lining of the distal part of the pharynx, the least (12 TBs mm 2) in the epidermis of the distal part of the head. The tops of most TBs protrude above the epithelium but their gustatory pores are slightly sunken, thereby protecting the apical processes of the gustatory cells from mechanical stimulation.

Neural Cell Adhesion Molecule of Taste Buds

The distribution of the neural cell adhesion molecule (N-CAM) in the taste buds of adult and developing mice was studied by use of an immunocytochemical technique. In adult mice, nerve fibers and some elongated taste bud cells reacted with anti-N-CAM antibody. Immunoelectron microscopic observation showed that only type-III (gustatory) cells, which have afferent synaptic contacts with the nerves, were N-CAM positive in the taste buds. At day 0 after birth, taste buds of the developing mice first appeared in the circumvallate papillae, and N-CAM positive cells, which corresponded to type-III cells having afferent synapses, were found. This result suggests that N-CAM is involved in formation of the synaptic contacts between type-III cells and nerves in the taste buds.

Synaptic contacts established by primary sensory fibers innervating the teeth of rats: An ultrastructural study of the pars interpolaris of the spinal trigeminal nucleus

Transganglionic degeneration was used in an electron microscopic study of afferent synaptic contacts in the dorsomedial region of the pars interpolaris of rats. In one experiment, the left inferior alveolar nerve was transected and in the other, partial pulpectomy of the first and second left lower molars was performed. Well defined degenerating terminals, almost completely occupied by round synaptic vesicle profiles were found in both ipsi and contralateral sides. In both experiments, approximately 70% of these terminals formed single asymmetric contacts with intermediate or distal dendritic segments. Fewer contacts were observed with proximal dendritic segments, dendritic spines, perikarya and other terminals. In addition, double and multiple synaptic contacts (synaptic glomeruli), accounting for 10% of the total, were also observed. Quantitative data regarding ultrastructural synaptic elements suggest that there is no preference for post-synaptic sites of fibers related to different sensory modalities such as pain, conveyed by dental fibers or other modalities, conveyed by the interior alveolar nerve fibers.

Colocalization of ion channels involved in frequency selectivity and synaptic transmission at presynaptic active zones of hair cells

Calcium ions serve as intracellular messengers in 2 activities of hair cells: in conjunction with Ca2(+)-activated K+ channels, they produce the electrical resonance that tunes each cell to a specific frequency of stimulation, and they trigger the release of a chemical synaptic transmitter. Our experiments indicate that both of these functions are conducted within a region that extends a few hundred nanometers around each presynaptic active zone. In focal electrical recordings from the plasma membranes of isolated anuran hair cells, we found nearly all of a cell's Ca2+ channels and Ca2(+)-activated K+ channels clumped at a fixed ratio in an average of 20 clusters on the basolateral membrane surface. Because serial-section electron microscopy indicated that each hair cell has approximately 19 afferent synaptic contacts with a similar distribution upon its basolateral surface, we conclude that the channel clusters coincide with synaptic active zones. Ensemble-variance analysis of current fluctuations indicated that each cell has a total of approximately 1800 Ca2+ channels and approximately 700 Ca2(+)-activated K+ channels; if these are uniformly divided, we estimate that each channel cluster contains approximately 90 Ca2+ and approximately 40 Ca2(+)-activated K+ channels. Freeze-fracture electron microscopy demonstrated an average of 133 large intramembrane particles in the presynaptic membrane at each active zone, an observation that suggests that the particles are the clustered channels. We used the K+ channel's sensitivity to intracellular Ca2+ to assay the concentration of free Ca2+ in the presynaptic cytoplasm, which we found to vary between 10 microM and 1 mM over the physiological range of membrane potentials. The inferred concentrations agreed with the values predicted for free diffusion of Ca2+ away from Ca2+ channels scattered randomly within a 300-nm-diameter synaptic active zone. The close association among Ca2+ channels, Ca2(+)-activated K+ channels, and synaptic active zones is necessary both for the rapid activation of K+ currents required in electrical resonance and for the transmission at afferent synapses of information about the phases of high-frequency stimuli.

Firing patterns and synaptic potentials of identified giant aspiny interneurons in the rat neostriatum

Intracellular recordings were made in vivo from 9 giant aspiny neurons in the neostriatum of urethane-anesthetized rats. The cells were identified by intracellular staining with HRP or biocytin. The neurons exhibited morphological features typical of neostriatal cholinergic interneurons. Six of the cells were obtained from intact animals, while 3 were recorded from rats with ipsilateral hemidecortications. Giant aspiny neurons were characterized by their slow irregular but tonic (3-10/sec) spontaneous activity and long-duration action potentials. Examination of the underlying membrane potential trajectories during spontaneous firing revealed that individual action potentials were triggered from spontaneous small (1-5 mV) depolarizing potentials. These spontaneous potentials exhibited the voltage sensitivity of ordinary EPSPs. They were much less frequent during the 80-200 msec pause in tonic afferent input that follows the excitation evoked by cortical or thalamic stimulation, and were decreased in frequency in decorticate animals. Their rise times and half-widths matched those expected for unitary synaptic potentials placed proximally on the surface of the neurons. Low-intensity stimulation of neostriatal afferents produced small short-latency EPSPs that appeared to be composed of responses identical to the spontaneous depolarizing potentials. The latencies of the EPSPs evoked from the cerebral cortex and thalamus were consistent with a monosynaptic input from both structures, but the maximal size of the EPSPs was much smaller than that evoked in spiny neurons, suggesting that a smaller number of afferent inputs make synapses with each of the aspiny cells. Giant aspiny neurons exhibited much larger input resistances and longer time constants than spiny neostriatal neurons. They also exhibited relatively linear steady-state current-voltage relationship compared to spiny projection cells. Input resistances ranged from 71-105 M omega, and time constants ranged from 17.8-28.5 msec. Analysis of the charging transients in response to current pulses yielded estimates of dendritic length of approximately 1 length constant. Repetitive firing of the neurons was limited by a powerful spike afterhyperpolarization and by a strong spike frequency adaptation. The sensitivity of the giant aspiny interneuron to a relatively small number of proximal afferent synaptic contacts, its tonic firing, and its widespread dendritic and axonal fields place it in an excellent position to act as a modulator of the excitability of neostriatal projection neurons in advance of the onset of movement-related neostriatal activity.

Fly photoreceptor synapses: their development, evolution, and plasticity.

Recent studies are reviewed on the synapses of photoreceptor terminals in the first optic neuropile of the flies, Musca and Drosophila. Afferent synaptic contacts are of uniform dimensions; they have a postsynaptic tetrad with a membrane organization of P-face particles, resembling other inhibitory synapses. A distributed population of such contact sites forms progressively during synaptogenesis by the selective, sequential accretion of identified postsynaptic elements at the receptor terminal. The comparative anatomy of this synapse indicates that elements have also been added during its phylogeny from an ancestral dyad. All cells are homologs of those in other species of Diptera. The number of synaptic sites is regulated by both pre- and postsynaptic cells, in proportion to their cell surfaces; an independent size increase in the receptor terminals (procured in the Drosophila mutant gigas) produces an increase in their synaptic population. The number of sites declines with age, however, accompanied by an increase in size of those synaptic sites remaining; this occurs for both afferent and feedback photoreceptor synapses. Lastly, the number of sites changes with visual experience; the frequency of feedback synapses is larger following dark rearing during early adult life than following visual experience.

Patterns of afferent synaptic contacts in the alligator lizard's cochlea.

Afferent synapses on both free-standing and tectorial hair cells in the alligator lizard's cochlea are described quantitatively. Semiserial sections were photographed with a transmission electron microscope. Hair cells together with their afferent nerves were reconstructed and morphometrically analyzed with the aid of a computer. Each afferent nerve forms many synapses with its hair cell. Tectorial afferents make more than twice the number of synapses with their hair cells as do free-standing afferents. This suggests a possible neuroanatomical basis for the physiological difference in synchrony reported in these two types of auditory nerve fibers; namely, the greater the number of synapses the better a fiber is able to follow faithfully the response of its hair cell.

Synaptic organization of a multifunctional interneuron in the central nervous system of Helix pomatia L.

The morphology, axonal arborization and ultrastructure of synaptic connections of the V21 giant neuron in the visceral ganglion of the snail Helix pomatia has been investigated following intracellular labelling with horseradish peroxidase. The V21 neuron establishes several afferent and efferent axo-axonic connections, mainly along the first half of the primary axon. Collaterals of 200-300 micron length originate from the primary axon, which shows further arborization, and both afferent and efferent synaptic contacts are formed on these fine axon profiles. Afferent and efferent contacts of the cell occur within very short distances of a few micrometers. On the basis of ultrastructure and vesicle and granule content, several afferent terminals can be distinguished on V21 labelled axon profiles. The majority of these afferent terminals resembles peptidergic-(neurosecretory)-like terminals. This finding supports the possible transmitter role of neuropeptides in the central nervous system of gastropods. Our results are consistent with and provide morphological evidence for recent electrophysiological observations suggesting that, in addition to integrating input, the V21 neuron functions as an interneuron in Helix central nervous system.

Fetal development of primate chemosensory corpuscles. II. Synaptic relationships in early gestation.

Fungiform papillae contained well-developed chemosensory corpuscles in macaque monkey fetuses with crown-rump lengths of 5.0-9.0 cm. These fetuses corresponded to stages from the last part of the first and beginning of the second trimester of gestation. Chemosensory cells extended from the epithelial basal lamina to the taste pore anlage at the apex of the corpuscle and had typical afferent synaptic contacts with presumptive gustatory axons. Apical secretory granules, the distinctive cytologic feature of sustentacular cells, were absent. Cells with cytologic characteristics intermediate between chemosensory cells and basal extragemmal cells were considered to be differentiating chemosensory (DC) cells and appeared to arise from existing basal cells as well as undifferentiated postmitotic cells. Although extragemmal cells were extensively coupled by gap junctions which are apparent in electron micrographs of thin sections, similar junctions were absent from chemosensory and DC cells. While having minimal structural requirements for transduction and transmission of sapid stimuli (i.e., an intraoral surface and afferent synaptic contacts) chemosensory cells lacked efferent synapses and subsurface cisternae, which can be observed at later stages of gestation. Axoaxonic synapses, present between gustatory axons in the intragemmal and extragemmal epithelium and in the subgemmal connective tissue, were observed with greater frequency than at later stages of gestation. We conclude that the differentiation of chemosensory cells precedes the differentiation of sustentacular cells by several weeks. This fact lends support to the concept that chemosensory cells are a unique identifiable cells type in all chemosensory corpuscles.

Afferent synaptic connections between hair cells and the somata of intramacular neurons in the gravity receptor system of the statocyst of Octopus vulgaris.

In the sensory epithelium (macula) of the gravity receptor system of the statocyst of Octopus vulgaris, there are two types of afferent neurons, distinguished according to their position in the epithelium. The somata of one type lie accumulated in a ring peripheral to the hair cell layer of the epithelium; these are designated as perimacular neurons. The somata of the other type lie among the hair cells, below the level of their nuclei; these are designated as intramacular neurons. Axons of afferent neurons which touch the hair cells are postsynaptic to some of the hair cells touched. As the somata of the intramacular neurons also touch the hair cells, they were investigated by serial electron microscopic reconstruction to determine if afferent synaptic contacts between hair cells and these somata occur. An average of about 600 intramacular neurons was counted in two maculae. Afferent synapses were seen to occur between hair cells and the somata of 76% of the intramacular neurons investigated. The postsynaptic processes of the intramacular neurons' somata were of two morphological types; one with a finger-like and one with a flat postsynaptic process (average of one synapse of each type per soma). The soma of an intramacular neurons can be postsynaptic to more than one hair cell simultaneously (average of 1--2 hair cells per soma). In addition to being presynaptic to only one neuron's soma, a hair cell could be simultaneously presynaptic to the axons of one or more afferent neurons. The morphological findings are discussed as to their possible physiological consequences.

Biogenic monoamines in developing taste buds of mouse circumvallate papillae.

After administration of monoamine precursors, developing taste buds of newborn and young mice were observed by means of electron microscopy and fluorescence histochemistry. Gustatory (type III) cells occurred in the primitive taste buds during stage 1 (0-1 day after birth). These cells had an immature type of afferent synaptic contacts with nerve terminals; however, no specific fluorescence was found in the taste buds after administration of 5-HTP or L-DOPA. During stage 2 (2-7 days), mature types of afferent synapses, taste pores, type I cells and type II cells appeared in the taste buds, and fluorescent cells also appeared following treatment of 5-HTP or L-DOPA. During stage 3 (14-21 days), the gustatory cells underwent ultrastructural changes following injection of 5-HTP; i.e. small dense-cored vesicles (30-60 nm) appeared scattered throughout the cytoplasm and were found to intermingle with small clear vesicles accumulated at the presynaptic membranes of afferent synapses, and the electron densities of large dense-cored vesicles (80-100 nm) were elevated as compared with those of untreated mice. Consequently the ability of gustatory cells to take up amine-precursors started simultaneously with the formation of taste pores and mature afferent synapses between the gustatory cells and the sensory nerves.