The fine structure of the hypothalamic secretory neurons of the white-crowned sparrow, Zonotrichia leucophrys gambelii (Passeriformes: Fringillidae). II. Magnocellular and parvocellular nuclei of the rostral hypothalamus

The fine structures of the neurons and neuropils of the magnocellular supraoptic nucleus and the parvocellular nuclei of the rostral hypothalamus, including the suprachiasmatic and medial, lateral and periventricular preoptic nuclei, and the neuronal apparatus of the organum vasculosum laminae terminalis, have been examined in the male White-crowned Sparrow, Zonotrichia leucophrys gambelii, by correlated light and electron microscopy. The magnocellular supraoptic nucleus is characterized by large neurosecretory perikarya which contain a well developed Golgi complex and dense-cored granules 1,500-2,200 A in diameter. The neuropil displays axons, dendrites and glial fibers. Some axonal profiles contain dense-cored vesicles 800-1,000 A in diameter and clear vesicles 500 A in diameter. Axo-somatic and axo-dendritic synapses are conspicuous in this nuclear region. The suprachiasmatic nucleus is characterized by an accumulation of small neurons with moderately developed cellular organelles and some dense-cored granules, approximately 1,000 A in diameter. The profiles of axons within the neuropil contain dense-cored granules 800-1,000 A in diameter and clear vesicles 500 A in diameter. The neurons of the medial preoptic nucleus are relatively large and exhibit well developed cellular organelles and dense-cored granules 1,300 to 1,500 A in diameter. Granular materials are formed within the Golgi complex. The medial preoptic nucleus is rich in secretory perikarya. Occasionally, neurons with granules 1,500-2,200 A in diameter are encountered in the lateral preoptic and periventricular preoptic nuclei. They may be considered as scattered elements of the magnocellular (supraoptic and paraventricular) system. The organum vasculosum laminae terminalis consists of three layers, i.e., ependymal, internal and external zones, and exhibits a vascular arrangement similar to that of the median eminence. The perikarya of the parvocellular neurons and their axons in the internal zone contain numerous secretory granules ranging from 1,300 to 1,500 A in diameter.     

Neurons and synaptic patterns in the deep layers of the superior colliculus of the cat. A Golgi and electron microscopic study

Golgi and electron microscopic observations were made on the neurons in the deep layers (below the stratum opticum) of the cat superior colliculus. Large neurons, 35-60 micrometers in somal diameter, occur mainly in the lateral two-thirds of the colliculus. They have numerous somatic and dendritic spines and receive a large number of axon terminals (bouton covering ratio: more than 70%). The medium-sized neurons (20-30 micrometer), with a moderate number of dendritic spines, show a lower bouton covering ratio (25-30%). The ratio for small neurons (8-15 micrometers), with very few dendritic spines, is less than 10%. The medium-sized and small neurons are distributed throughout the colliculus and show marked variability in the dendritic arrangement. Seven different types of axon terminals were distinguished: types I, II, V, and VII form asymmetrical and types III, IV, and VI symmetrical synapses. Type I terminals represent small boutons containing predominantly spherical vesicles, and are in contact mainly with small dendritic profiles. Type II terminals are medium-sized and slender, contain a mixture of spherical and slightly oval vesicles, and make synaptic contacts with small to medium-sized dendrites and somatic spines. This type of terminal is occasionally presynaptic to vesicle-containing dendrites (type VIII). Type III terminals are small, contain flattened vesicles predominantly, and are presynaptic to a wide variety of neuronal elements in the deep layers of the superior colliculus. Type IV terminals are represented by medium to large-sized boutons that contain pleomorphic vesicles and make synaptic contacts chiefly with the large neurons. Type V and VI terminals exhibit a quite dense axoplasmic matrix and mainly contact the large neurons. Type VII terminals are often in the form of boutons en passant and contain numerous large granular vesicles. Pleomorphic vesicle-containing dendrites (type VIII terminals) are also observed to participate in the axodendrodendritic serial synapses.     

Ultrastructure of the organum vasculosum of the lamina terminalis of the Japanese quail, Coturnix coturnix japonica

The structure of the organum vasculosum of the lamina terminalis (OVLT) of the Japanese quail has been studied by light and electron microscopy. The OVLT has a palisade arrangement. It forms a part of the terminal plate extending from the optic chiasma to the anterior commissure and is characterized by a special vascular arrangement. The organ consists of ependymal, internal and external zones. The ventricular surface of the organ is covered by non-ciliated ependymal cells characterized by the presence of raspberry-like ventricular protrusions. The ependymal zone is composed of two types of ependymal cells, one being a large, cuboidal cell with columinous cytoplasmic protrusions suggesting secretion into ventricle, and the other a slender tanycyte with long balar plexus of the external surface. The internal zone contains two types of secretory neurons, parvocellular neurons containing a few dense-cored granules 1,000 A in diameter, and mediocellular neurons containing in their perikarya many larger granules 1,300-1,500 A in diameter and well developed granular endoplasmic reticulum and Golgi apparatus. The granular formations are usually observed in the Golgi area of both types of cells, thus indicating of their secretory activities. There are three types of nerve terminals in the neuropil: (1) nerve endings containing spherical, dense-cored granules 800 A in diameter, which display axosomatic synapses with perikarya of the neurons, (2) nerve endings containing dense-cored granules 1,000 A in diameter and clear vesicles 400 A in diameter, and (3) nerve endings containing dense granules 1,300 A in diameter and clear vesicles. Types 2 and 3 do not display axo-xomatic synapses but often show axo-dendritic synaptic contacts with dendrites in the neuropil; thus they are probably axons originating from the parvocellular and mediocellular neurons of this organ. The functional significance of the neurons and axons of the OVLT is discussed and it is suggested that two kinds of neurohormones may be secreted from the OVLT of the Japanese quail.     

Morphological aspects of the elimination of polyneuronal innervation of skeletal muscle fibres in newborn rats

The ultrastructure of neuromuscular junctions of rat soleus muscles 1-40 days postnatally was examined for possible morphological correlates of the transient polyneuronal innervation which is present in newborn rats. Several vesicle-laden profiles of terminal axons are seen to contact each muscle fiber up to 8 days postnatally. Axon terminals often lie close together, without Schwann cell intervention. Between days 8 and 16 the number of profiles of terminals on each muscle fibre is reduced, and both Schwann cells and ridge-like extensions of muscle fibre cytoplasm intervene between and separate axon terminals. No signs of degenerating intramuscular axons or axon terminals could be found. It is suggested that the redundant terminals are eliminated by retraction into the parent axons. This process is apparently accomplished without any morphological signs of degeneration.     

The synaptic organization of the cerebello-olivary circuit

Section of the superior cerebellar peduncle just rostral to the deep cerebellar nuclei results in degenerating axon terminals within the contralateral inferior olive. The nuclear origin of this fiber system and its distribution within the subdivisions of the inferior olive were described in a companion study (Martin et al., 1976). Precise localization of these degenerating terminals within the nucleus was accomplished by the examination of 1 mu plastic sections cut from each tissue block prior to thin sectioning. Degenerating axon terminals are present in all the nuclear subdivisions and when seen with the electron microscope they frequently are localized in the previously described synaptic clusters (King, 1976). These terminals demonstrate an electron dense reaction at survival times of 2 and 3 days. By day 4, they are shrunken and irregular in shape, and typically are surrounded by astrocyte processes. Cerebello-olivary axon terminals measure 1-3 mu, contain spherical, clear synaptic vesicles and typically contact spiny appendages within the synaptic clusters (glomeruli). Thus, we have demonstrated that one of the primary axon systems which terminates within the synaptic clusters is from the cerebellar nuclei. We have yet to determine the origins of the remaining terminals within the synaptic clusters which include endings with either smaller spherical, pleomorphic or numerous dense core vesicles.     

Sensory terminals on extrafusal muscle fibres in myotendinous regions of developing rat muscles

Axon terminals were observed to form neuromuscular contacts with extrafusal muscle fibres in myotendinous regions of developing rat muscles up to 5 days after birth. These neuromuscular contacts are found in fascicles of muscle fibres connected with differentiating Golgi tendon organs. Axon terminals establishing these contacts are obviously sensory, since they do not degenerate after de-efferentation performed in neonatal rats. The terminals contain mainly clear and dense core vesicles and form neuromuscular connections resembling developing motor endplates, with a cleft about 60 nm wide and basal lamina interposed between the axolemma and the sarcolemma. Each terminal, however, also forms a close contact in a restricted region where the basal lamina is missing; there the cleft is reduced to 20 nm and the axolemmal and sarcolemmal membranes are linked by desmosome-like attachment plaques. After the fifth postnatal day, axon terminals become detached from muscle fibres and are only found among collagen bundles of the tendon organ. The functional significance of these temporary neuromuscular contacts is not clear.     

Inhibition of the acetaldehyde dehydrogenases in rat liver by a cyanamide derivative present in a commerical standard diet for small animals

In the rat superior cervical and coeliac-mesenteric ganglia we have observed three types of small granulated (SG) cell: Type I cells are characterised by membrane-bounded cytoplasmic granules with a core of variable, moderate to low electron-density, whose limiting membranes are rounded in profile ranging from 50-150 nm in diameter. Type II SG cells contain numerous highly electron-dense, polymorphic cytoplasmic granules ranging from 100-300 nm in diameter. The haloes of Type II cell granules are variable in shape, and the core is often eccentrically located or fragmented. Type III SG cells contain membrane-bounded granules with a core of variable moderate to low electron-density. In profile these granules appear oblong or circular with average dimensions of 170 x 50 nm. All three SG cell types receive cholinergic-type pre-ganglionic terminals whose afferent nature is confirmed by their degeneration following pre-ganglionic neurectomy. Only Type I cells have been observed to donate efferent synapses to dendrites of principal ganglionic neurones and are thus interneuronal.     

Dual ultrastructural immunocytochemical labeling of mu and delta opioid receptors in the superficial layers of the rat cervical spinal cord

The delta opioid receptor (DOR) and mu opioid receptor (MOR) are abundantly distributed in the dorsal horn of the spinal cord. Simultaneous activation of each receptor by selective opiate agonists has been shown to result in synergistic analgesic effects. To determine the cellular basis for these functional associations, we examined the electron microscopic immunocytochemical localization of DOR and MOR in single sections through the superficial layers of the dorsal horn in the adult rat spinal cord (C2-C4). From a total of 270 DOR-labeled profiles, 49% were soma and dendrites, 46% were axon terminals and small unmyelinated axons, and 5% were glial processes. 6% of the DOR-labeled soma and dendrites, and < 1% of the glial processes also showed MOR-like immunoreactivity (MOR-LI). Of 339 MOR-labeled profiles, 87% were axon terminals and small unmyelinated axons, 12% were soma and dendrites, and 2% were glial processes. 21% of the MOR-labeled soma and dendrites, but none of the axon terminals also contain DOR-LI. The subcellular distributions of MOR and DOR were distinct in axon terminals. In axon terminals, both DOR-LI and MOR-LI were detected along the plasmalemma, but only DOR-LI was associated with large dense core vesicles. DOR-labeled terminals formed synapses with dendrites containing MOR and conversely, MOR-labeled terminals formed synapses with DOR-labeled dendrites. These results suggest that the synergistic actions of selective MOR- and DOR-agonists may be attributed to dual modulation of the same or synaptically linked neurons in the superficial layers of the spinal cord.     

Morphologically heterogeneous met-enkephalin terminals form synapses with tyrosine hydroxylase-containing dendrites in the rat nucleus locus coeruleus

Physiological and anatomical studies have suggested that the endogenous opioid peptide, methionine-enkephalin (ENK), may directly modulate noradrenergic neurons. Additionally, chronic opiate administration has been shown to increase the levels of a number of G-proteins and phosphoproteins including the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). We combined immunogold-silver localization of tyrosine hydroxylase and immunoperoxidase labeling for ENK in single sections through the nucleus locus coeruleus (LC) in the rostral pons to determine potential substrates for the divergent actions of this opioid peptide. Light microscopic analysis of ENK immunoreactivity in the LC area indicated that ENK fibers are dense and highly varicose. In coronal sections, ENK-immunoreactive processes were punctate and appeared to envelop LC-cell bodies. More rostrally, in the region of catecholamine-immunoreactive extranuclear dendrites, ENK-immunoreactive varicose processes were interdigitated with TH-labeled processes. Electron microscopy of this rostral region revealed that ENK-immunoreactive axon terminals contained small clear as well as large dense core vesicles. The large dense core vesicles (1-10/terminal) were consistently the most immunoreactive and were identified toward the periphery of the axon terminal distal to the active zone of the synapse. Unlabeled axon terminals and glial processes were the most commonly observed elements located adjacent to the plasmalemma of axons containing the labeled dense core vesicles. Axon terminals containing ENK immunoreactivity varied in size (0.3 micron to 2.0 microns) as well as formation of synaptic specializations (i.e., asymmetric versus symmetric). The ENK-labeled terminals formed synapses with dendrites with and without detectable TH immunoreactivity. These results provide the first direct ultrastructural evidence that morphologically heterogeneous terminals containing ENK immunoreactivity form synapses with catecholamine dendrites within the LC. The formation of asymmetric and symmetric synaptic specializations suggests that the opioid peptide, ENK, may be colocalized with other neurotransmitters. Furthermore, the distribution of ENK immunoreactivity in axon terminals apposed to other unlabeled afferents or astrocytic processes suggests that actions of ENK may also include presynaptic modulation of other transmitters and/or effects on astrocytes.     

Gap junctions and nerve terminals among stromal cells in human fallopian tube ampullary mucosa

The purpose of the present study is to investigate the ultrastructure and immunohistochemistry of the stromal cells and terminal nerve fibers in human fallopian tube ampullar mucosa to achieve a detailed characterization of this tissue to permit a better assessment of possible functions. Tissues were obtained during surgery or at autopsy from 26 patients. Specimens were studied by the conventional histologic technique, immunohistochemistry (Cx43, synaptophysin, neurofilament proteins, and S-100 protein), and electron microscopy. Gap junction and nerve terminal frequency between stromal cells were studied by direct assessment on ultrathin sections in the transmission electron microscope. Gap junctions were observed between the cytoplasmic processes of subepithelial stromal cells. There were approximately 23 gap junctions per 73 nucleated stromal spindle cells. Immunohistochemistry using Cx43 antibody confirmed the dot-like distribution of gap junctions. The frequent and intimate association of stromal cell processes with nerve terminals was also demonstrated. Nerve terminals were immunostained by antibodies to nerve-specific molecules and ultrastructurally as axonal profiles containing dense-cored granules or empty vesicles. Analysis of nerve terminal frequency revealed 18 nerve profiles containing 51 axonal profiles per 73 nucleated stromal spindle cells. The present paper documents the participation of autonomic nerve endings and gap junctions in the stromal cell network in human fallopian tube stroma. Similarities to the unique anatomical unit referred to as the 'neuro-reticular complex' in bone marrow tissue (Yamazaki and Allen, 1990) are discussed.     

Development and application of a new method for amplification and detection of human rhinovirus RNA

Motor nerve terminals on abdominal body-wall muscles 6A and 7A in larval flesh flies were investigated to establish their general structural features with confocal microscopy, transmission electron microscopy, and freeze-fracture procedures. As in Drosophila and other dipterans, two motor axons supply these muscles, and two morphologically different terminals were discerned with confocal microscopy: thin terminals with relatively small varicosities (Type Is), and thicker terminals with larger varicosities (Type Ib). In serial electron micrographs, Type Ib terminals were distinguished from Type Is terminals by their larger cross-sectional area, more extensive subsynaptic reticulum, more mitochondrial profiles, and more clear synaptic vesicles. Type Ib terminals possessed larger synapses and more synaptic contact area per unit terminal length. Although presynaptic dense bars of active zones were similar in mean length for the two terminal types, there were almost twice as many dense bars per synapse for Type Ib terminals. Freeze-fractures through the presynaptic membrane showed particle-free areas indicative of synapses on the P-face, within which were localized aggregations of large intramembranous particles indicative of active zones. These particles were similar in number to those found at active zones of several other arthropod neuromuscular junctions. In general, synaptic structural parameters strongly paralleled those of the anatomically homologous muscles in Drosophila melanogaster. In live preparations, simultaneous focal recording from identified varicosities and intracellular recording indicated that the two terminals produced excitatory junction potentials of similar amplitude in a physiological solution similar to that used for Drosophila.     

Endothelin in perivascular nerves. An electron-immunocytochemical study of rat basilar artery

The endothelium is recognized as a major source of endothelin in the vasculature. In this report we show for the first time that endothelin can be demonstrated with immunoelectronmicroscopy in perivascular nerves. About 36% of the axon profiles (varicosity/intervaricosity) examined in the rat basilar artery showed immunolabelling with a polyclonal antibody to endothelin-1. Endothelin-labelled axon varicosities were characterized by the presence of small spherical agranular vesicles (42 +/- 1 nm); some varicosities also contained large granular vesicles (92 +/- 5 nm) with labelled cores. Immunolabelling was mostly distributed in the axoplasm and in association with the membrane of synaptic vesicles (the lumen of the small agranular vesicles was immuno-negative). The presence of endothelin in perivascular nerves of the basilar artery suggests a neuronal as well as endothelial role in the physiological control of the vessel wall.     

Identification of a secretomotor centre in the brain of Locusta migratoria, controlling the secretory activity of the adipokinetic hormone producing cells of the corpus cardiacum

After retrograde filling of axons terminating in the glandular lobe of the corpus cardiacum (CC) of Locusta migratoria with cobalt chloride, a paired group of about 15 cobalt containing cells was demonstrated in the lateral area of the protocerebrum. The axons of these cells run via the NCC II into the glandular lobe of the CC. These small neurons have the characteristics of secretory cells; they contain secretory granules of about 1000 A in diameter. The axon terminals in the glandular lobe, making synaptic contacts with the glandular cells, contain secretory granules of the same size. It is therefore concluded that the cell groups in the protocerebrum control the activity of the glandular cells which produce an adipokinetic hormone. Arborizations of fibers of the lateral secretomotor cells are present in the dorsal neuropile of the protocerebrum, ventral of the mushroom bodies and along the tracts of the NCC I within the brain. It is proposed that these arborizations are sites of synaptic input. It is discussed that the axons of these cells might receive additional synaptic input in the storage lobe of the CC. The localization of cell bodies, the axons of which enter the storage part of the CC is described. The course of the axon tracts of the various cell groups in the protocerebrum and their connections with the NCC I and NCC II are demonstrated.     

Developmental changes in synaptic formation in the hypothalamic arcuate nucleus of female rats

The hypothalamic arcuate nucleus (ARCN) of female rats at 5, 20, 45 and 90 days of age was examined ultrastructurally. Axodendritic and axosomatic synapses were counted in 18,000 mum2 area of the ARCN in each brain. Axodendritic and axosomatic synapses in the ARCN of day 5 rats were very small in number. Axon terminals contained small spherical vesicles (SSVs, 40-60 nm in diameter). Occasionally large granular vesicles (LGVs, 75-130 nm in diameter) were found to coexist with SSVs in the endings. Pre- and postsynaptic membranes were thin. The ARCN at this age exhibited a large extracellular space which decreased with advancing age. In day 20 rats, axodendritic and axosomatic synapses increased in number up to about one-half of those of day 45 or day 90 animals. Synaptic vesicles increased in number and mitochondria were frequently encountered in the axon terminals. Pre- and postsynaptic membranes became thicker than those of day 5 rats. Further increase in the number of axodendritic and axosomatic synapses in the ARCN of day 45 rats was observed, and there were no significant difference in the morphology and incidence of synapses between day 45 and day 90 rats. Synaptic vesicles were numerous and pre- and postsynaptic membranes were thick. In tissue incubated with 5-hydroxydopamine (5-OH-DA) before fixation, small granular vesicles (SGVs, about 50 nm in diameter) which were labeled with 5-OH-DA were detected in a certain number of endings in all material taken from each age group, but the incidence of synapses containing SGVs was usually low. From these results, it can be proposed that an increase in the number of synapses in the ARCN is correlated wihh functional maturation of the ARC neurons.     

Identification of separate receptors for adenosine and adenosine 5''-triphosphate in causing relaxations of the isolated taenia of the guinea-pig caecum

An ultrastructural investigation showed that there was a neurohaemal organ in the wall of the ampulla of the antennal pulsatile organ. The neurosecretory axon terminals occurred singly or in small groups, rather than closely packed together as in other neurohaemal organs. All axons contained the same type of neurosecretory granule. The granules had varying electron density and a diameter in the range 1000-2500 A. Some terminals contained small, elliptical electron-transparent vesicles and the axolemma was apposed to the stroma. Other terminals were large and enveloped by glial tissue and the contents of the terminals exhibited varying degrees of autolytic degeneration. Autolysis was characterized by the occurrence of dense bodies and multilaminate bodies which enclosed mitochondria and neurosecretory granules. It was suggested that the neurosecretory material affects antennal function.     

Glutamate, gamma-aminobutyric acid and tachykinin-immunoreactive synapses in the cat nucleus tractus solitarii

Neurophysiological and pharmacological evidence suggests that glutamate, gamma-aminobutyric acid and tachykinins (substance P and neurokinin A) each have a role in cardiovascular regulation in the nucleus tractus solitarii. This study describes the ultrastructural relationships between nerve terminals immunoreactive for these substances in the nucleus tractus solitarii of the cat using post-embedding immunogold (single and double) labelling techniques on sections of tissue embedded in LR White resin. The technique combines a high specificity of labelling with good ultrastructural and antigenic preservation. Glutamate-immunoreactive terminals, recognized by their high density of gold particle labelling compared to the mean tissue level of labelling, accounted for about 40% of all synaptic terminals in the region of the nucleus tractus solitarii analysed (medial, dorsal, interstitial, gelatinosus and dorsolateral subnuclei). They appeared to comprise several morphological types, but formed mainly asymmetrical synapses, most often with dendrites of varying size, and contained spherical clear vesicles together with fewer dense-cored vesicles. Substance P- and neurokinin A-immunoreactive terminals were fewer in number (9% of all terminals) but similar in appearance, with the immunoreaction restricted to the dense-cored vesicles. Analysis of serial- and double-labelled sections showed a co-existence of substance P and neurokinin A-immunoreactivity in 21% of glutamate-immunoreactive terminals. Immunoreactivity for gamma-aminobutyric acid was found in 33% of all terminals in the nucleus tractus solitarii. These predominantly contained pleomorphic vesicles and formed symmetrical synapses on dendrites and somata. Possible sites of axo-axonic contact by gamma-aminobutyric acid-immunoreactive terminals onto glutamate-or tachykinin-immunoreactive terminals were rare, but examples of adjacent glutamate and gamma-aminobutyric acid-immunoreactive terminals synapsing on the same dendritic profile were frequent. These results provide an anatomical basis for a gamma-aminobutyric acid mediated inhibition of glutamatergic excitatory inputs to the nucleus tractus solitarii at a post-synaptic level.     

Isolation of cholinergic synaptic vesicles from the myenteric plexus of guinea-pig small intestine

The acetylcholine-rich myenteric plexus-longitudinal muscle preparation of the guinea-pig small intestine has been subjected to subcellular fractionation using modifications of both classical methods and that originally devised for bulk isolation of cholinergic synaptic vesicles from the electromotor nerve terminals of Torpedo marmorata by means of density gradient centrifugation in a zonal rotor. The latter method gave a vesicle fraction with the highest acetylcholine content so far recorded for a mammalian particulate fraction, 30.9 +/- S.E.M. 1.8 (5) nmol of acetylcholine . mg of protein-1. Electron-microscopical examination showed that it consisted of a homogeneous preparation of vesicles of mean spherical diameter 61 +/- SD 4 (108) nm, with little or no contamination with other lipoprotein membrane structures, mixed however with considerable amounts of actomyosin fibrils, presumably derived from the longitudinal muscle. Slab-gel electrophoresis in sodium dodecyl sulphate showed that, in addition to prominent peaks attributable to actin and myosin, there was a relatively simple pattern of (presumably) vesicle protein among which all the proteins thought to be characteristic of Torpedo synaptic vesicles were present.     

Synapses from labeled type II axons in the mouse cochlear nucleus

This study investigates the ultrastructure and central targets in the cochlear nucleus of axonal swellings of type II primary afferent neurons. Type II axons comprise only 5-10% of the axons of the auditory nerve of mammals, but they alone provide the afferent innervation of the outer hair cells. In this study, type II axons were labeled with horseradish peroxidase, and serial-section electron microscopy was used to examine their swellings in: (1) the granule-cell lamina at its boundary with posteroventral cochlear nucleus, (2) the rostral anteroventral cochlear nucleus, and (3) the auditory nerve root. Only some (18%) of the type II terminal and en-passant swellings formed synapses. The synapses were asymmetric and contained clear round synaptic vesicles, suggesting that they are excitatory. Type II synapses were compared to those from type I fibers providing the afferent innervation of the inner hair cells. Type II synapses tended to have slightly smaller and fewer synaptic vesicles, had a greater proportion of the membrane apposition accompanied by a postsynaptic density, and often had densities that were discontinuous or 'perforated'. In all cochlear nucleus regions examined, the postsynaptic targets of type II synapses had characteristics of dendrites; in most cases these dendrites could not be traced to their cell bodies of origin. Some evidence suggests, however, that targets may include granule cells, spherical cells, and other cells in the nerve root. These results suggest afferent information from outer hair cells reaches diverse regions and targets within the cochlear nucleus.     

Regulated spacing of synapses and presynaptic active zones at larval neuromuscular junctions in different genotypes of the flies Drosophila and Sarcophaga

Synapses at larval neuromuscular junctions of the flies Drosophila melanogaster and Sarcophaga bullata are not distributed randomly. They have been studied in serial electron micrographs of two identified axons (axons 1 and 2) that innervate ventral longitudinal muscles 6 and 7 of the larval body wall. The following fly larvae were examined: axon 1--wild-type Sarcophaga and Drosophila and Drosophila mutants dunce(m14) and fasII(e76), a hypomorphic allele of the fasciclin II gene; and axon 2--drosophila wild-type, dunce(m14), and fasII(e76). These lines were selected to provide a wide range of nerve terminal phenotypes in which to study the distribution and spacing of synapses. Each terminal varicosity is applied closely to the underlying subsynaptic reticulum of the muscle fiber and has 15-40 synapses. Each synapse usually bears one or more active zones, characterized by dense bodies that are T-shaped in cross section; they are located at the presumed sites of transmitter release. The distribution of synapses was characterized from the center-to-center distance of each synapse to its nearest neighbor. The mean spacing between nearest-neighbor pairs ranged from 0.84 microm to 1.05 microm for axon 1, showing no significant difference regardless of genotype. The corresponding values for axon 2, 0.58 microm to 0.75 microm, were also statistically indistinguishable from one another in terminals of different genotype but differed significantly from the values for axon 1. Thus, the functional class of the axon provides a clear prediction of the spacing of its synapses, suggesting that spacing may be determined by the functional properties of transmission at the two types of terminals. Individual dense bodies were situated mostly at least 0.4 microm away from one another, suggesting that an interaction between neighboring active zones could prevent their final positions from being located more closely.     

Regeneration of retinal axons into the goldfish optic tectum

The growth of regenerating retinal axons into the central portion of the optic tectum of adult goldfish was examined with the light and electron microscopes. Optic tracts were cut and, two days to five months later, the animals were perfused and the tecta prepared for microscopy. Regenerating axons first reached central regions of the tectum seven to ten days postoperatively. Regenerating axons appear in very large numbers and travel in fascicles in the stratum opticum (SO) and in the adjacent neuropil, the stratum fibrosum et griseum superficiale (SFGS). In the SO, the fascicles are bordered by glial cells and degenerating debris. Within the SFGS, however, the fascicles do not seem to be similarly associated with glial cells and degenerating debris. The youngest regenerating axons are very slender processes, containing microtubules but few or no neurofilaments or dense granular material. By 10 to 14 days postoperatively, neurofilaments can be seen and, in addition, large numbers of vesicles with dense cores appear. The vesicles with dense cores increase in numbers until about 28 days postoperatively and then become quite rare. That vesicles with dense cores were seen in regenerating axons in both SO and SFGS during the period of growth into the tectum but were not seen in axon terminals at any time, suggests that they may be concerned with axon elongation. During the period one month to five months postoperatively, the regenerating axons gradually increase in diameter but do not reach preoperative sizes, suggesting that the regenerative changes may still be occurring. Remyelination is delayed and proceeds slowly. Many axons remain unmyelinated for as long as five months postoperatively.