Neuropil Volume Research Articles

Pre- and postsynaptic correlates of interocular competition and segregation in the frog.

Segregated zones of termination between converging inputs that arise from different presynaptic populations are a common property of topographically organized zones within the vertebrate central nervous system. Increasing evidence suggests that such segregation is at least in part established on the basis of competitive interactions that depend upon the activity patterns within each afferent population. However, the cellular mechanisms of these interactions are poorly understood. We have used a preparation in which a stereotyped interdigitating pattern of retina-specific termination stripes are produced in frog tecta innervated by two retinas as a result of embryonic implantation of a third eye primordia. In these animals it has been possible to examine the relationship between the number of retinal ganglion cells in each of the retinas innervating a striped tectum, the volumetric changes in the tectum as a result of this double innervation, and the pattern of eye-specific segregation that is produced. Counts of retinal ganglion cells in the retinas of the three-eyed frogs with one completely striped tectal lobe revealed no significant differences between cell numbers in the doubly innervating retinas and the normal retinas of the same animals. The average increase in retinal ganglion cell innervation to the striped tecta of these animals was 100%. However the tecta only increased in total volume by 26%. This later increase consisted of a 25% increase in the volume of the deep lying and predominantly cellular tectal laminae and a 37% increase in the superficial retinotectal synaptic zone. In many of these same animals HRP and 3H-proline were used to differentially label the set of stripes from each retina and measurements of the extent of each projection were performed. We found that the volume of tectal neuropil occupied by a striped projection is relatively unrelated to the number of ganglion cells making up that projection. Observations of the striping pattern after HRP processing to visualize stripes in whole unsectioned tecta indicate that the periodicities and rostrocaudal orientation of stripes are robust over a wide range of relative innervation densities. When one projection is much smaller than the other, stripes appear to break down into a series of "puffs" or islands of retina-specific termination zones. Nevertheless, these puffs still have a rostrocaudal alignment and the spacing of fully formed stripes. These observations suggest that the formation of exclusive termination zones may be a threshold phenomenon: so after a certain innervation density is reached one input can take over a unit of target neuropil in an all-or-none manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical localization of serotonin in the rat periaqueductal gray: a quantitative light and electron microscopic study.

The distribution of serotonin-like immunoreactivity in five regions of the rodent midbrain periaqueductal gray (PAG) was studied by using light and electron microscopic immunohistochemistry in combination with quantitative analysis. Light microscopic analysis revealed the presence of serotonin-like immunoreactive cell bodies located in the ventrolateral and ventromedial regions of the caudal PAG and serotonin-like immunoreactive processes throughout the PAG. Ultrastructural analysis showed dendritic profiles that stained positively for serotonin primarily in ventral regions, although an occasional profile was seen dorsally. Numerous synaptic contacts between unstained axon terminals and ventral dendritic profiles were seen. Axonal profiles that contained reaction product were identified throughout the PAG, but were rarely observed to make any type of specialized contact. Ultrastructural quantification of serotonin-like immunoreactive processes indicated that the highest volume fraction of serotonin immunoreactivity occurred caudoventrally where stained processes constituted 2.6% of the neuropil volume. Rostroventrally stained processes constituted only 0.14% of the neuropil volume at the level of the posterior commissure. By contrast the amount of serotonin-like immunoreactivity found dorsally remained relatively constant at all rostrocaudal levels. Analysis of serotonin staining among PAG regions demonstrated the lowest overall volume fraction in the dorsal region and the highest overall volume fraction in the ventromedial region. No significant differences were observed between medial and lateral regions. A comparison of the results of light microscopic quantitative analysis of serotoninergic processes with electron microscopic quantitative analysis indicated that both techniques produce comparable results.

Primary sensory projections from the labella to the brain of Drosophila melanogaster Meigen (Diptera : Drosophilidae)

Golgi silver impregnation of sensory neurons arising from labellar taste sensilla of Drosophila melanogaster Meigen (Diptera : Drosophilidae) revealed 7 distinct types I-VII of primary (sensory) fibres projecting to the suboesophageal ganglion (SOG) of the brain. Each fibre was classified on the bases of the neuropil volume occupied by its terminal arborisation, the shape of neuropil region receiving the arborisations and the detailed morphology of the arborisations. The primary sensory fibre projections from the labella are confined to the SOG where they project mainly in the anterior and central neuropils. No labellar sensory fibres project to posterior SOG. Of these 7 types of sensory fibres, three (III, IV and VII) show ipsilateral projections, while others have both ipsi-, and contralateral branches. Four types of interneurons are suggested to be associated with taste perception. Type A interneurons are local interneurons with arborisations confined only to the taste sensory neuropil of the SOG. The types B - D interneurons are interganglionic/output neurons with axons projecting to various brain regions-SOG, calyces of the mushroom bodies, tritocerebrum and thoracic ganglia. These projections suggest that more than one centre (SOG, tritocerebrum, calyces of the mushroom bodies and thoracic ganglia) are involved in processing gustatory information.

Size of neurons and glial cells in the enteric ganglia of mice, guinea-pigs, rabbits and sheep.

A quantitative light microscopic study has been carried out on the myenteric and submucosal ganglia of the stomach, duodenum, ileum, proximal colon and rectum of the guinea-pig; the enteric ganglia of the ileum were studied also in the mouse, rabbit and sheep. The area of the profiles of nerve cells, of nerve cell nuclei and of glial nuclei, and the proportion of the area of ganglia occupied by neuropil were measured, and the relative numbers of neurons and glial cells were estimated. The myenteric ganglia were found to be firmly anchored to the stroma of the muscle coat; their shape and the shape of their component cells varied with contraction and distension of the musculature. The range of neuronal sizes in the myenteric ganglia was extremely wide. In the guinea-pig, the myenteric neurons were on average largest in the stomach and duodenum and smallest in the ileum, with intermediate values in the colon and rectum; the submucosal neurons showed little variation in average size along the length of the gut. The average size of ganglion neurons in the ileum was greatest in the sheep and smallest in the mouse, and had intermediate values in the guinea-pig and rabbit. The percentage volume of neuropil in the myenteric ganglia was 51% in the mouse, 65% in the guinea-pig, 70% in the rabbit, and 74% in the sheep. The number of glial cells relative to the number of neurons was also ranked in the same order. In all the species examined the submucosal ganglia, when compared with the corresponding myenteric ganglia, had a smaller percentage volume of neuropil, a much smaller number of glial cells and (except in the mouse ileum) neurons of smaller average size. In all the ganglia there was a positive correlation between size of neurons and size of glial cells. The results are discussed in the light of possible relations between body size (and length of the intestine), numerical density of ganglion neurons, average size of neurons, amount of musculature, average distance between neurons, and amount of neuropil.

Hemispheric asymmetry of synapses in chick medial hyperstriatum ventrale following passive avoidance training: a stereological investigation

A stereological analysis was made of synapses in the left and right hemispheres of chick medial hyperstriatum ventrale (MHV) 24h after passive avoidance training (PAL) and in water trained controls (W-control). The synaptic parameters examined were (D̄), the mean length of the postsynaptic thickening; (N V.syn), the number of synapses per unit volume of neuropil; (V V.syn), the volume density of the pre-synaptic bouton; (N V.ves), the number of synaptic vesicles per unit volume of neuropil and ( ves .syn), the mean number of synaptic vesicles per pre-synaptic bouton. No significant differences exist in N V.syn between the left or right hemispheres of W-control and trained chicks, nor is N V.syn influenced by training. However, in W-control chicks D̄ in the right MHV is significantly greater (12%) than in the left MHV and this difference disappears on training. There are no differences in V V.syn between left and right hemispheres of W-control chicks but following training V V.syn is 22.7% greater in the left MHV than in the right MHV. Training als influences the number of synaptic vesicles; in W-control chicks N V.ves in the right MHV is 12.25% greater than in the left MHV but following training these differences are reversed. When the data are expressed as numbers of vesicles per synapse ( ves .syn), values for the left hemisphere of trained chicks exceed those in the right hemisphere by a staggering 61.38%. These results are discussed in the context of biochemical and electrophysiological studies which suggest that there is lateralization of the memory trace.

Size of neurons and glial cells in the intramural ganglia of the hypertrophic intestine of the guinea-pig.

A quantitative light microscopic study has been carried out on the myenteric and submucosal ganglia of the guinea-pig ileum, after inducing hypertrophy of the wall with an experimental stenosis. The area of the profiles of nerve cells, of nerve cell nuclei and of glial nuclei, and the percentage area of the neuropil were measured, and the relative numbers of neurons and glial cells were estimated. The average size of neurons in both plexuses was greatly increased over the control values. Nearly half of the neuronal profiles measured over 500 microns2 in sectional area (less than 3% in the controls), and less than 1% measured less than 150 microns2 (21% in controls). The average size of glial nuclei (and presumably that of the glial cells too) was increased in the hypertrophic ganglia. The number of glial cells (relative to the number of neurons) was unchanged or slightly decreased in the hypertrophic myenteric ganglia; in contrast, it was markedly increased in the submucosal ganglia. In both ganglia the hypertrophy was accompanied by a decrease in the percentage volume of neuropil.

Continuous serial thin sectioning for electron microscopy

AbstractThe process of serial sectioning for electron microscopy has been refined such that loss of thin sections is kept below 0.1% and the series is continued at will. The method relies on microscopic control of all manipulative steps, Formvar casting on plate glass for coated slot grids, coating of the block with contact cement for reliable ribboning, pickup by a one‐step method with grid support in the diamond knife trough, staining in LKB grid holders, gentle treatment of grids in the electron microscope, and a slight modification to the microscope for safe grid withdrawal. The results are particularly applicable to the reconstruction of neuronal microcircuits and larger volumes of neuropil.

Qualitative and quantitative measures of plasticity during the normal development of the Rana pipiens retinotectal projection

We have examined the following aspects of retinal development in the frog, Rana pipiens: (1) the overall pattern of cell addition to the retina; (2) the relative rate of retinal ganglion cell (RGC) accretion; and (3) the changes in RGC density during larval development. In addition, we have studied the development of the retinal projection onto the tectum by means of the anterograde transport of horseradish peroxidase (HRP) and measurements of the volume of tectal neuropil at several larval stages and in postmetamorphic frogs. We find that the addition of new cells to the retina of Rana pipiens larva is restricted to the ciliary margin and that this addition is concentric at all larval stages. Additionally, the morphometric measures of retinal and tectal growth, along with the HRP histochemistry, indicate that the retinal projection exhibits considerable plasticity during normal development. The plasticity we observe in normal development may explain why the retinotectal projection can compensate its area and volume in experimental paradigms that effect drastic changes in innervation density.

Structural comparison of a homologous neuron in gryllid and acridid insects

The fast extensor tibiae (FETi) motor neuron is responsible for exciting the extensor tibiae muscle to produce most of the force for jumping in acridids. Because of its relatively large size and crucial role in jumping, FETi has been studied in an ever-increasing number of orthopteran species. Here we describe the structure of the metathoracic FETi neuron in six species of acridids and in two species of gryllids. The morphology of FETi within the respective groups is essentially equivalent, but marked differences are apparent between acridid and gryllid FETis. There are similarities in the size and location of the cell body and the course of the neurite through the ganglion. Differences are found in the number of large branches, density of branching, and the volume of neuropil receiving branches. We propose that the gryllid FETi is an intermediate form between slow extensor tibiae motor neurons involved in walking and acridid fast extensor tibiae motor neurons specialized for jumping.

Anatomy and physiology of experimentally produced striped tecta

Transplantation of a third eye primordium to the forebrain region of a frog (Rana pipiens) embryo causes two retinal projections to converge on a single tectal lobe. These projections form stereotyped eye-specific termination bands (Constantine-Paton, M., and M. I. Law (1978) Science 202: 639-641) that are similar to the source-specific stripes found normally in many regions of the mammalian brain. In the present study, we use quantitative analyses of anatomical data and double labeling techniques to demonstrate that induced bands of three-eyed frogs are approximately 200 micrometers wide, that they invariably run in a rostrolateral to caudomedial direction, and that they represent interdigitating synaptic zones which fill the entire superficial neuropil of the dually innervated tectal lobe. This periodic segregation pattern is not seen in the superinnervated diencephalon or in the optic tracts. Morphometric measurements on animals with dually innervated tectal lobes revealed an approximate 30% hyperplasia that was restricted largely to the deeper predominantly cellular tectal layers. Each of the banded retinal projections occupied neuropil volumes that were roughly 50% smaller than the volume occupied by the noncompeting retina of the same animal. Eye-specific segregation was detected with extracellular recording techniques as an ability of one of the two eyes to elicit consistently more and larger action potentials throughout a radial penetration of the superficial tectal neuropil. In several preparations, electrolytic lesions confirmed that physiologically defined positions of eye dominance corresponded to a band from the same eye. Maps of the visual field projections within dually innervated tecta were relatively normal and their orientation was consistent with the initial embryonic orientation of the retinas. Eye laterality, time of arrival, or fasciculation during growth are not responsible for this induced banding. Instead, our results are attributed to two mechanisms that are probably fundamental to neural mapping. These are: differential affinities between retinal and tecta loci which normally align the projection by bringing together appropriate pre- and postsynaptic areas and interactions among retinal ganglion cell fibers. The latter causes axons from physically neighboring retinal ganglion cell bodies to terminate together within the tectum and consequently increases the internal order of map. We propose that these same two mapping mechanisms may be responsible for the afferent segregation found in diverse regions of many vertebrate brains.

Anatomical and neurochemical consequences of deafferentation in the development of the visual system of the moth Manduca sexta.

The primordium of the compound eye of the moth Manduca sexta was removed by surgery within 12 hours after the molt to the pupa. When the operated animals were examined as pharate adults just prior to emergence from the pupal cuticle, no eye tissue was present on the operated side. Histological examination of the brain at successive developmental stages showed that the volume of laminar neuropil on the operated side increased very little after deafferentation while the volume of the control-side laminar neuropil increased more than 20-fold. The impairment of development of the lamina was accompanied by a reduction in the synthesis and storage by the optic lobe of two neurotransmitter candidates, acetylcholine and 5-hydroxytryptamine. The endogenous content of 5-hydroxytryptamine was also reduced in these preparations. Excision of a section of the stemmatal nerve, which connects the primordium of the compound eye to the brain, also arrested the development of the lamina, but in this case the compound eye itself developed apparently normally without making contact with the brain. This finding supports the hypothesis that the centripetal growth of photoreceptor-cell axons normally is contact-guided. Results of neurochemical experiments on retinal tissue argue against the idea that acetylcholine, 5-hydroxytryptamine, or gamma-aminobutyric acid is a likely photoreceptor-cell neurotransmitter. From these and our previous studies of the metamorphosing brain in Manduca we conclude that different classes of neurons in the same central nervous system can exhibit widely different degrees of dependency on their normal innervation for their survival and differentiation.

Specific patterns of neuron arrangement and of synaptic articulation in the medial geniculate body.

Golgi and electron microscopic analysis of the known cellular layers in concentric shells of the ventro-lateral portion of the medial geniculate body revealed a flat grid of high density neuropil filling the space between the geniculocortical relay cells, forming essentially a single cell layer in each lamina. The "skeleton" of this neuropil grid is made up by the interdigitating dendritic tufts of the geniculocortical relay cells, joined together by a rich system of desmosomoid adhesion plaques. The "holes" of the "skeleton" are filled in by the multilobed dendritic appendages of Golgi type II interneurons and the grape-like terminals of the inferior collicular specific afferents. Additional axon terminals of other sources--terminals of descending corticogenicular fibers, axons of the Golgi type II interneurons and terminals of the initial collaterals of the geniculocortical relay cells--contribute only to a very insignificant fraction of neuropil volume. The Golgi type II interneurons are oriented in perpendicular direction to the cell layers so that they may bridge with their dendrites several successive layers. Although the general expression "synaptic glomeruli" used in other relay nuclei for this type of specific synaptic arrangement is hardly applicable to this grid-like neuropil, the essential synaptic articulation pattern of all thalamic relay nuclei is well maintained. The specific inferior collicular afferents are presynaptic to both relay cell dendrites and to the multilobed dendritic appendages of Golgi type II cells, which in turn are presynaptic to the same dendritic regions of the relay cells receiving the bulk of the specific afferents.

The nervous system of Loligo. II. Suboesophageal centres.

A well-marked hierarchy of centres can be recognized within the suboesophageal lobes and ganglia of the arms. The inputs and outputs of each lobe are described. There are sets of motoneurons and intermediate motor centres, which can be activated either from the periphery or from above. They mostly do not send fibres up to the optic or higher motor centres. However, there is a large set of fibres running from the magnocellular lobe to all the basal supraoesophageal lobes. The centre for control of the four eye-muscle nerves in the anterior lateral pedal lobe receives many fibres direct from the statocyst and from the peduncle and basal lobes, but none direct from the optic lobe. The posterior lateral pedal is a backward continuation of the oculomotor centre, containing large cells that may be concerned in initiating attacks by the tentacles. An intermediate motor centre in the posterior pedal lobe probably controls steering. It sends fibres to the funned and head retractors, and by both direct and interrupted pathways to the fin lobe. It receives fibres from the crista nerve and basal lobes, but none direct from the optic lobe. The jet control centre of the ventral magnocellular lobe receives fibres from the statocyst and skin and also from the optic and basal lobes. Some of these last also give extensive branches throughout the palliovisceral lobes. The branching patterns of the dendritic collaterals differ in the various lobes. Some estimates are given of the numbers of synaptic points. The dendritic collaterals of the motoneurons spread through large volumes of neuropil and they overlap. The incoming fibres spread widely and each presumably activates many motoneurons either together or serially. Many of the lobes contain numerous microneurons with short trunks restricted to the lobe, but there are none of these cells in the chromatophore lobes or fin lobes. The microneurons have only few dendritic collaterals, in contrast to the numerous ones on the nearby motoneurons.

The cerci and abdominal giant fibres of the house cricket, Acheta domesticus. II. Regeneration and effects of chronic deprivation.

The abdominal cerci of immature crickets are regenerated repeatedly after loss. The develop­mental sequence of cereal regeneration depends on the instar and the time within the instar that cerci are removed. Initial regenerates in early instars lack sensilla, which appear at the second moult, and subsequently increase in number in a definite order as in normal development, with filiform hairs appearing first, followed by large numbers of appressed hairs. Initial regenerates of later instars may bear filiform hairs. If one of the cerci is removed at hatching and all subsequent regenerates are removed as soon as they appear, a maximally asymmetric animal is produced. The general dimensions of the giant intemeurons carrying cereal information anteriorly are not altered, but the volume of the terminal abdominal ganglion neuropile on the deprived side is reduced by 30%. At least three important changes in connectivity accompany unilateral deprivation: (a) responses to air puffs in the largest intemeurons (l. g. i. and m. g. i.) contralateral to the intact cercus are 20 times greater than in control animals, where responses are virtually com­ pletely ipsilateral; (b) responses to substrate vibration, detected by non-cercal receptors, are about 10 times greater than normal on the deprived side; and (c) contralaterally pro­jecting axons from cereal receptors are found, which is never the case in normal animals. Responses on the intact side are normal. Animals which are deprived of both cerci for six instars and then regenerate only one show all these changes but to a lesser degree. In particular, the non-cercal, vibration sensitive input to the continuously deprived side is not enhanced as much as with the former treat­ment. Animals permanently deprived of both their cerci similarly do not acquire as greatly enhanced a vibration input as does the deprived side of continuously asymmetric animals. None of these rearrangements occur in animals which are deprived of both their cerci for six instars and then regenerate both; these symmetrically deprived and symmetrically regenerating animals resemble normals in all measured respects even though a period of deprivation lasting for two thirds of postembryonic development has intervened between the initial amputation and regeneration. We conclude that the capacity for correct neuronal circuit assembly is maintained throughout postembryonic development; that removal of a major input to the giant inter­neurons leads to a relative strengthening of the remaining inputs; and that additional rearrangements, beyond the effects of simple deprivation, occur as a result of asymmetry during cereal development.

A study of neuron morphology in Aplysiacalifornica using procion yellow dye

1. 1. The morphology of two neurons, L10 and R2, of the abdominal ganglion of Aplysia californica was studied in serial sections after intracellular injection of Procion Yellow dye. 2. 2. The main axons of both cells give off many finer processes ending within a restricted volume of neuropil. 3. 3. The geometry of the major interneuron L10 requires that the processes of distant follower cells converge in this restricted neuropil region. 4. 4. The branching patterns of L10 and R2 do not reflect differences in synaptic output.

On the dendritic topology and activation of cockroach giant interneurons

Each cercus of Periplaneta americana has 650 to 750 sensilla. The smaller cercal nerve contains about 1000 fibres; the larger contains about 2500. Many of these are under 0·5 μm in diameter. The sensilla found to be most effective in driving the giant fibres were cercal thread hairs and cercal bristle hairs. The cercal nerve tract leads directly into the dendritic trees of the giant interneurons. The dendritic trees of two giant interneurons were filled with Procion Yellow dye. Both display very extensive ipsilateral (to axon) ramification through a large volume of neuropile of the last abdominal ganglion. The location and extent of ramification of the two dendritic trees differs markedly, indicating that the two giant interneurons probably mediate different integrative activities. One of the giant fibres is attached to a single neuron soma, demonstrating that, at least in one case, the giant interneurons are not syncitial.

Development of fine structure of spinal cord in monkey fetuses. II. Pre‐reflex period to period of long intersegmental reflexes

AbstractA series of 13 macaque fetuses of estimated ovulation ages 42 to 51 days (17 to 41 mm) was subjected to behavioral analysis with umbilical cord intact (in and out of the amnion), and to light and electron microscopic analysis of the cervical spinal cord. This period is a critical one in terms of onset and early development of spontaneous activity and of cutaneous reflexes. Behavioral criteria, reinforced by well‐defined microscopic characteristics, suggested a grouping of specimens into three major stages, namely, a pre‐reflex group (Stage 1: 17–22 mm); a group in the period of onset of primitive spontaneous activity and of precocious local cutaneous reflexes in the trigeminal‐cervical region (Stage 2: 24 to 28 mm); and a group characterized by the development of more vigorous activity, spontaneous and otherwise, and of long intersegmental and crossed reflexes (Stage 3: 32 to 41 mm). The evidence suggests that junctional differentiation and occurrance of a cluster of agranular spheroid synaptic vesicles characterize the primitive synaptic knobs which appear on motoneuron dendrites coincident with earliest reflexes. Onset of active long intersegmental reflexes in Stage 3 is coincident with development of axosomatic synaptic knobs, and of F type (flattened) synaptic vesicles in a rapidly increasing fraction of all synaptic bulbs in the motoneuron neuropil. Total volume of all synaptic bulbs increases gradually as a fraction of the volume of the motoneuron neuropil until birth, but a steep increase in proportionate number of F type synaptic knobs occurs in Stage 3.