Showing papers in "The Journal of Comparative Neurology in 1966"

Autoradiographic and histological studies of postnatal neurogenesis. I. A longitudinal investigation of the kinetics, migration and transformation of cells incorporating tritiated thymidine in neonate rats, with special reference to postnatal neurogenesis in some brain regions.

Abstract: Thymidine-H3 was injected intraperitoneally into 6- and 13-day old rats and they lived afterwards for periods ranging from one hour to 60 days. Autoradiographic data obtained from animals surviving for short periods were used to estimate rates of regional cell proliferation. Animals with longer survival were used to deduce the movements of new cells from germinal sites, through migratory channels, to target areas, and to determine their mode of differentiation. The formation and differentiation of microneurons goes on during infancy, though in most structures at a declining rate. In the wall of the olfactory ventricle cell multiplication continued at a high rate at six days, with a decline at 13 days, and the new cells migrated to the layers of the olfactory bulb. The migration of labeled cells from the lateral ventricle was traced, by way of the fimbria, to the polymorph cell layer of the dentate gyrus, and from there to the granular layer. The “older” granule cells were located in the upper part of the granular layer, the “younger” cells at its base. Cell multiplication continued at a very high rate in the external granular layer of the cerebellar cortex, whence cells migrated to the molecular layer and internal granular layer. Speed of migration was approximately 50 μU/day in the olfactory bulb, and 60–70 μ/day in the cerebellum. The possible significance of these findings was discussed.

A study of Golgi preparations from the dorsal lateral geniculate nucleus of the adult cat

Abstract: Four cell types are distinguishable in the lateral geniculate nucleus. The largest (class 1) have their dendrites oriented in relation to the plane of the laminae. The dendrites cross laminar borders freely and bear many fine spines. Class 2 cells show less orientation of their dendrites. These dendrites bear clusters of grape-like appendages close to the branching points and fine spines on their peripheral segments. Only the peripheral segments cross laminar borders. The smallest (class 3) cells have long stalked appendages and give rise to intrinsic axons which ramify close to the perikaryon. Cell classes 1 and 2 occur in laminae A and A1, with class 1 cells mainly concentrated between laminae. Class 3 cells occur in all major laminae. Class 4 cells, with long smooth dendrites oriented parallel to the lamina, lie in lamina B. Two types of extrinsic axon are seen. Type I axons give off many short simple terminal collaterals which often innervate more than one lamina. These are regarded as extra-retinal afferents. Type II axons (probably retinal afferents) have complex flower-like terminals and each axon innervates only one lamina. Contacts between several type II axons and two or more grape-like clusters occur in laminae A and A1. In lamina B finer type II axons form complex overlapping terminal nests around the dendrites of class 4 cells.

The isodendritic core of the brain stem

Abstract: According to their degree of morphological specialization, the cell populations of the brain stem may be classified into three groups: isodendritic, allo-dendritic and idiodendritic. The isodendritic neurons, or generalized neurons, are the most frequently encountered. If one discards those isodendritic centers that by common definition are sensory or motor, an isodendritic core is left which displays very little histological variation throughout the whole extent of the brain stem. This core corresponds, with certain restrictions, to the regions that are usually regarded as reticular formation. It constitutes a continuum of overlapping dendritic fields that extends from the spinal cord to the diencephalon. In view of the similarities that exist between the histology of the isodendritic core and the relatively disorganized nervous system of the lower vertebrates, it is postulated that it represents a pool of pluri-potential neurons which in the course of phylogeny have remained relatively undifferentiated and in charge of processing afferent signals of very heterogeneous origin. By contrast, the allodendritic and idiodendritic centers can be regarded as relatively specialized centers from the point of view of their dendritic morphology, connections and functions. Attention is paid to the fact that the diffuse characteristics of the isodendritic core do not necessarily entail ill-defined physiological properties.

Increases in cortical depth and glia numbers in rats subjected to enriched environment.

Abstract: We have previously reported anatomical and chemical changes in the cerebral cortex of rats living in an enriched, stimulating environment. The present study includes additional histological measures such as more extensive depth measures, differential cell counts, and cell size measurements. Celloidin-embedded, thionin-stained sections of the visual cortex from environmentally enriched rats and their impoverished littermates were measured. Two methods for calculating cortical depths were used: one, with an ocular micrometer taking eight measures lateral to the elevation in the corpus callosum on each hemisphere; two, on enlarged photographs taking 51 measurements 2 mm apart on each hemisphere. With the ocular micrometer method the cortical depth of the enriched brains was 6.4% greater than the depth of the impoverished cortex. On the enlarged photographs the more medial depths in the enriched rats showed greater increases than the lateral depths. Overlapping photomicrographs were made of a medial cortical area 1.00 mm by 0.75 mm on each transverse section and were combined into one composite picture for each animal. Differential cell counts from the composite pictures were determined independently by two technicians. Results indicate a 14% (p<0.01) increase in glia among the enriched rats. No significant differences between enriched and impoverished groups were found in the perikarya and nuclear circumferences as measured with a planimeter from camera lucida drawings.

H3-thymidine autoradiographic studies on the cell proliferation and differentiation in the external and the internal granular layers of the mouse cerebellum.

Abstract: Cell proliferation and migration in the external granular layer of the mouse cerebellum were studied with autoradiography after cumulative labeling with H3-thymidine. The germinative cells in the external granular layer were considered as externally dislocated matrix cells. Their generation time, presynthetic time, duration of DNA synthesis, postsynthetic time and mitotic time were determined in one-, three-, seven- and ten-day-old mice. The entire sequence of the ontogeny of the external granular cell-system was separated into three consecutive stages; stage 1 or stage of pure external matrix cell proliferation, stage 2 or stage of neuroblast production, and stage 3 or stage of neuroglia differentiation. Production of neuroblasts in the external granular layer at seven and ten days of life and their migration into the internal granular layer were demonstrated by means of autoradiography. Transit times of the neuroblasts migrating across the external mantle layer and the molecular layer of ten-day-old mice were estimated at 21 and four hours, respectively. More than 50% of the inner granular cells migrated from the external granular layer later than ten days of life and almost 81 to 92% were produced later than seven days of postnatal life. In conclusion, on the basis of the matrix cell concept, the authors tried to unify observations of previous and present investigators and presented a scheme of pre- and postnatal histogenesis of the mouse cerebellum.

The projection of optic fibers to the visual centers in the cat.

Abstract: Retinal projections to the thalamus and midbrain were studied by the Nauta-Laidlaw technique. After unilateral enucleation, degeneration was found in five areas: dorsal lateral geniculate nucleus (LGNd, thalamus), ventral lateral geniculate nucleus (LGNv, subthalamus), nucleus of optic tract (NOT, pretectum), superior colliculus (S.C., tectum), and accessory optic nuclei (AON, tegmentum). Degeneration after focal lesions in nasal retina made by photocoagulation is found in four areas of the contralateral LGNd: Laminae A and B, nucleus interlaminaris centralis (NIC), and the medial part of nucleus interlaminaris medialis (NIM). Temporal lesions lead to degeneration ipsilaterally in three areas: lamina A1, NIC, and the lateral part of NIM. Central lesions are followed by bilateral projections; the area of degeneration on each side is greater than that of peripheral lesions of comparable size. Focal degeneration is found in NIM after either central or peripheral retinal lesions. Each retina projects to both NOT and both S.C. In NOT, degeneration following nasal lesions is strictly contralateral while after temporal lesions is mainly ipsilateral. A bilateral projection of temporal retina to NOT constitutes an exception to theories of chiasmal decussation. Both nasal and temporal projections to AON are wholly crossed, thus constituting another example of projection of temporal retina to the contralateral side, Nasal fibers project contralaterally and nearly all temporal fibers ipsilaterally to S.C.

An electrophysiological study of the visual projection to the superior colliculus of the rat.

Abstract: The topographical relations of the visual field with the superior colliculus of the rat was investigated using constant small visual stimuli and recording the electrical response of aggregate unit activity with fine tip metal microelectrodes. A precise essentially linear projection onto the contralateral colliculus was demonstrated after appropriate corrections for brain curvature were performed. The general pattern and arrangement of the retinotopic projection is similar to that found in infra-mammalian vertebrates. An ipsilateral projection appears to be absent. The phasic properties and rhythmic discharge patterns of single units in the coliculus were studied with stationary and moving luminous stimuli.

Sprouting of optic tract projections in the brain stem of the rat

Abstract: The present experiments examine axonal sprouting of optic tract projections as a consequence of occipital cortex removal. Eyes were removed bilaterally in experimental rats 16 months following unilateral occipital cortex ablation. Seven animals survived this procedure. Controls included rats sacrificed 16 months after unilateral occipital cortex ablation, one week after unilateral occipital cortex destruction, and one week after bilateral or unilateral eye removal. Brains were serial-sectioned and stained by the Nauta-Gygax method for degenerating axons. Sprouting occurred only at two loci, the caudal half of the ventral lateral geniculate nucleus, pars lateralis, and the caudal portion of lateral nucleus of the optic tract and subjacent medial quarter of pretectal nucleus. These loci have in common that they are regions of convergence in a major fashion of the two fiber systems, the occipitofugal and retinofugal projections on which experiments were done. However, sprouting did not occur in other similarly deafferented regions. Explanations for this specificity of sprouting are discussed. Optic tract sprouting was observed only at paraterminal portions of the axon and not as collateral sprouts along its course. A suggested relationship between axonal sprouting and functional plasticity is discussed.

Behavior of neuro-epithelial cells during closure of the neural tube

Abstract: Chick embryos ranging in age from 18 to 32 hours were treated with thymidine-H3 to examine the cell cycle and nuclear migration of the neuro-epithelial cells during closure of the neural tube. By plotting the percentage of labeled metaphases against the tune of thymidine-H3 application, the DNA-synthetic period was found to last five hours, the postduplication (G2) stage and prophase two and one-half hours and the preduplication (G1) stage to be brief or non-existent. The total cell cycle time was calculated to be eight hours. During DNA synthesis the nuclei are located near the basement membrane; subsequently they migrate to the lumen to undergo mitosis, after which they return to the outer zone to start DNA synthesis for the next cell division. To determine the life span of the neuro-epithelial cells by a different method, chick embryos were treated with vincristine, a drug known to arrest the mitosis in the metaphase. Fifteen, 26, 34 and 41% of the neuro-epithelial cells were arrested in the metaphase, one, two, three and four hours after treatment, respectively. Since the “mitotic index” of the neuro-epithelial cells was 5.2, the duration of the mitosis was calculated to last 25 minutes, and the life span of the neuro-epithelial cell to be eight hours. Immediately after closure of the neural tube, a different cell type characterized by a round nucleus, pale nucleoplasm and a dark-staining nucleolus appeared at the periphery of the tube near the basement membrane. These cells, which originate by division from the neuro-epithelial cells at the lumen, did not incorporate thymidine-H3 at any time during further development. They are considered to be primitive neuroblasts and to form the first representatives of the mantle layer.

Electron microscopic studies of the dentate gyrus of the rat. I. Normal structure with special reference to synaptic organization.

Abstract: The fine structure of the dentate gyrus of the rat has been studied in glutaraldehyde perfused material. With the exception of the short axon association cells, the major components of each of the three layers of the gyrus have been identified, and a detailed account is given of the synaptic organization of the dentate granule cells. The granule cells have most of the features typical of small neurons and although they contain no large aggregates of granular endoplasmic reticular they exhibit a variety of cisternal specializations. In the granule cell layer numerous axo-somatic and axo-dendritic synapses have been observed, and on occasion a single presynaptic fiber has been seen to contact the soma of one cell and a proximal dendrite of another. In the inner part of the molecular layer the majority of synapses are upon the main dendritic shafts of the granule cells but many also contact short dendritic spines. In the outer two-thirds of this layer most presynaptic fibers end upon long dendritic spines some of which are characterized by a prominent projection from the subsynaptic surface into the presynaptic process. The unmyelinated axons of the granule cells establish many en passant contacts with dendrites and spines of CA4 pyramids before ending in large presynaptic bags which may contain a number of large agranular vesicles (up to 2,000 A in diameter) and vesicles with dense cores.

The organization of the posterior ventral cochlear nucleus in the rat.

Abstract: The ventral cochlear nucleus was examined in 31 rats' brains prepared according to the protargol method of Bodian. The following regions were delimited on the basis of synaptic, cellular and axonal criteria. Region II, which occupies the central part of the nucleus, consists principally of cells (type g)which receive modified bulbs of Held from the acoustic nerve and which send large axons to the trapezoid body. The cells were graded in size, the smallest (high frequency) being located dorsally and the larges (low frequency) being located ventrally. Associated with region II are the cells (type b) of the acoustic nerve nucleus. These cells receive boutons from the acoustic nerve and send very large axons to the trapezoid body. Region IV is located in the posterior part of the ventral cochlear nucleus and is composed of large multipolar cells (type k). The dendrites of these cells are specifically organized with respect to the fibers of the acoustic nerve. Synaptic endings, from the descending branch of the acoustic nerve, consist of boutons on cell body and dendrites. The axons of these cells compose the stria of Held. Region V is located in the posterior part of the ventral cochlear nucleus and appeared to have only intrinsic connections. All regions received synaptic endings in addition to those which arose from the acoustic nerve.

Magnocellular neurosecretory centers in the rat hypothalamus.

Abstract: Hypothalami from normal rats were examined after preparation with Gomori's chromalum-hematoxylin stain for neurosecretory material. In addition to the supraoptic and paraventricular nucleii, five additional nuclear masses of neurosecretory cells were found. These all formed more or less compact cellular groups, clearly demarcated from surrounding neural tissue and with constant location from animal to animal. One nucleus, immediately posterior to the anterior commissure, along the ventricular wall, is termed anterior commissural. Two others, being dorso-lateral to the columns of the fornix, are termed anterior and posterior fornical. Midway between the paraventricular nucleus and the optic tract, a characteristically shaped nucleus of very compact cells, the nucleus circularis, is encountered. Finally, another group of cells is seen dorsal to the optic tract in the middle of the medial forebrain bundle, the nucleus of the medial forebrain bundle.

Fiber degeneration after lateral hypothalamic lesions in the rat.

Abstract: Fiber degeneration in the rat following lesions of the lateral hypothalamic area was studied by the method of Nauta and Gygax ('54). Ascending degeneration in the medial forebrain bundle enters the preoptic area and the medial and fimbrial nuclei of the septum. Less dense degeneration is found in the lateral septal nucleus and a few fibers bypass the septum and reach the cingulate cortex. Descending degeneration in the medial forebrain bundle enters the ventral tegmental area of Tsai giving off a few fibers to the mammillary bodies on the way. In the midbrain fibers turn dorsomedially to enter the raphe nuclei and the central gray and dorsolaterally to enter the nucleus mesencephalicus profundus pars lateralis. A small group of fibers leaves the anterior part of the lateral hypothalamus in a dorsomedial direction and passes through the inferior thalamic peduncle and stria medullaris to enter the lateral habenular nucleus. Another bundle leaves the posterior part of lateral hypothalamus in a dorsomedial direction and passes through the posterior midline thalamus to enter the central gray. No degeneration was found in the ventromedial hypothalamus in rats with lateral hypothalamic lesions. Our ability to stain fine degenerating fibers in this region was demonstrated in rats with medial hypothalamic lesions.

The localization, in the nucleus ambiguus of the rabbit, of the cells of origin of motor nerve fibers in the glossopharyngeal nerve and various branches of the vagus nerve by means of retrograde degeneration.

Abstract: The glossopharyngeal nerve and the vagus nerve or its branches were cut individually or in various combinations in different rabbits. In two animals the cricothyroid muscle of the larynx was dissected away. The distribution in the nucleus ambiguus of the neurons of origin of motor fibers in the various nerve branches was deduced from the retrograde degeneration observed in these cells after the appropriate branch had been cut or muscle had been removed. The part of the glossopharyngeal nerve associated with the nucleus ambiguus was found to arise from a compact mass of cells lying level with, but ventrolateral to, the rostral tip of the principal column of the nucleus, forming the rostral extremity of the medial column. The remainder of the medial column was shown to give rise to axons innervating the cricothyroid muscle. The principal column contains a group of cells innvervating the pharynx and soft palate and a group innervating the esophagus, but there was considerable rostrocaudal overlap between these two groups. Some evidence is presented showing that the cells for the cervical esophagus are not clearly separated rostrocaudally from the cells for the abdominal esophagus. The cells of the diffuse formation innervate the intrinsic muscles of the larynx excepting the cricothyroid muscle.

Lumbosacral dorsal root terminations in the nucleus gracilis of the cat. Some observations on terminal degeneration in other medullary sensory nuclei.

Abstract: Following lumbosacral dorsal rhizotomies (L1, L5, L6 and S1), fiber degeneration could be traced throughout three rostro-caudal regions of nucleus gracilis by utilizing the Nauta method. In rostral region, fiber degeneration was less dense and localized with greater intersegmental overlap than in middle region, where roots terminated in fairly specific cell clusters. Little could be stated about the caudal region because of sparsity of terminal degeneration. Fiber degeneration in rostral region appeared to be distributed primarily in the dendritic fields while that in the middle region terminated on both cell somata and dendrites. Overlap of dermatomes in periphery and centrally in nucleus parallel each other. Throughout the nucleus, greatest overlap of root distribution was encountered in more ventral regions. A root supplying a more distal dermatome (e.g. L6) compared to a more proximal one (e.g. L1), terminates more densely and dorsally in the nucleus. An occasional degenerated fiber passed into the contralateral gracile nucleus at obex level. Root degeneration proceeded, ipsilaterally, into rostro-dorsal regions of lateral cuneate, cuneate, and caudo-lateral regions of 2 nuclei. Bilateral degeneration from all roots, excepting S1, was noted in rostral pole of subnucleus caudalis and the subnucleus interpolaris of spinal trigeminal complex and nucleus reticularis parvicellularis. The pathway to the spinal trigeminal and parvicellular nuclei was undetermined. The concept of the non-homogenous organization of the gracile nucleus was supported.

Ascending connections of the anterior ventral cochlear nucleus in the rat.

Abstract: The purpose of the experiment was to determine the connections of the three regions of the anterior ventral cochlear nucleus in the rat. Lesions were placed in the regions of the nucleus in 11 rats. After 25 days each animal was sacrificed and its brain impregnated by the protargol method of Bodian. The superior olivary complex, the lateral lemniscus and the dorsal acoustic stria were examined for diminution of synaptic endings, thinning of neuropil and debris of degeneration. Complete destruction of region III of the anterior ventral cochlear nucleus produced degeneration in the ipsilateral lateral superior olive and in the medial superior olive of both sides. Destruction of other regions of the anterior ventral cochlear nucleus, region III remaining intact, had no effect upon the lateral or medial superior olive. Destruction of region II of the anterior ventral cochlear nucleus produced degeneration in the anterior halves of the ipsilateral lateral nucleus of the trapezoid body and the contralateral medial nucleus of the trapezoid body. Complete destruction of other regions of the anterior ventral cochlear nucleus, region II remaining intact, was without effect upon the lateral and medial nuclei of the trapezoid body. No degeneration was detected following destruction of region I of the anterior ventral cochlear nucleus.

The neuronal components of the optic nerve of the crayfish as studied by single unit analysis.

Abstract: Unit responses were obtained from the optic nerve of the crayfish, Procambarus clarki (Girard) with steel needles. Eighty-three repeatedly recognized elements are described. They range from bundles of primary sensory fiber to complexly reacting multimodal interneurons. Visually reacting fibers fall into two main classes, one class reacts tonically to overall illumination, the other responds to moving objects. Sustaining fibers react to increased light with increased output, dimming fibers are active in the dark and inhibited by light. Fourteen sustaining and four dimming fibers are described. The main movement fiber type, of which eleven have been found, responds to shadows. They “habituate” quickly and react best to movement of small black objects. All these fibers, and some others, are present as centrifugal elements from the other eye. All these fibers, and some others, are present as centrifugal elements from the other eye. Four fibers, two sustaining and two movement ones, change their fields with positions of the animal. These “space-constant” fibers “look” at the sky from all positions for which possible and are otherwise “blind.” Mechanoreceptive interneurons form a bundle between the first and second classes of optical fibers. Seventeen members are described. Five multimodal fibers, with combined visual and mechanoreceptor input are described. Maps of the approximate locations of the units in the cross-section of the optic nerve are presented. The number of times specific fibers were found exceeded 50 for 28.

The ultrastructure and innervation of rat vibrissae.

Abstract: The innervation of rat vibrissae has been studied by light and electron microscopy. Nerve fibers enter the external root sheath and become associated with Merkel cells. Merkel cells are specialized sheath cells containing many secretory granules apposed to the neurite. These secretory granules are PAS-positive and diastase-resistant in light microscopic preparations. The neurite at the level of the Merkel cell is expanded and contains masses of mitochondria. As contrasted to nerve fibers in glabrous skin, the neurite does not penetrate deeply into the external root sheath but appears to terminate at the level of the Merkel cells. The neurite-Merkel cell complex is interpreted as representing a mechanoreceptor.

Rubro-cerebellar connections in the cat: an experimental study with silver impregnation methods.

Abstract: The rubro-cerebellar projection has been studied in the cat with the methods of Nauta and Glees following stereotactically placed lesions in the red nucleus. A description of the intracerebellar nuclei is given in which an attempt has been made to define precisely the limits of each nucleus. The rubro-cerebellar fibers terminate in the contralateral nucleus interpositus anterior (NIA). It cannot be decided whether the nucleus interpositus posterior (NIP) receives some fibers, but no fibers pass to the medical and lateral nuclei. A somatotopical pattern appears to be present in the projection. Rubrocerebellar fibers are thin and probably terminate in the neuropil. This fiber system is the only direct link known between the red nucleus and the cerebellum. The total number of fibers is small. Probably the fibers are collaterals from the rubrospinal tract fibers and subserve a feedback function between the red nucleus and the cerebellum.

The ultrastructural characterization of macroglial cell types

Abstract: In the prelaminar region of adult cat optic nerve, silver impregnation revealed a macroglial cell population consisting solely of small fibrous astrocytes. Electron microscopically the cells were characterized by abundant processes containing closely packed filaments and occasional glycogen granules; their perikarya contained few organelles, glycogen particles were conspicuous but not abundant, and their cytoplasmic and nucleoplasmic matrices were of low density. The laminar region contained typical fibrous astrocytes. These cells were characterized by similar processes and matrices but their perikarya contained filaments and were richer in organelles whose orientation was primarily radial. The postlaminar region contained both astrocytes and oligodendrocytes. The latter were identified as cells characterized by the absence of filaments and glycogen, the presence of canaliculi, dense cytoplasmic and nucleoplasmic matrices and perikarya very rich in organelles, whose orientation was primarily circumferential. The presence of filaments and/or glycogen on the one hand, and of dense matrices on the other hand, seemed to be mutually exclusive and provided diagnostic criteria for astrocytes and oligodendrocytes respectively. Bivalent forms were not seen. Cells usually identified as microglia were not seen. The relationship between astrocytes and finely myelinated nerve fibers in the lamina cribrosa indicates a sheath-supporting role. Astrocytes are probably generally concerned with the isolation and insulation of neurons and their processes. Oligodendrocytes may be concerned with the energetic support of neurons and their processes.

The tecto-thalamic connections in the brain of the rabbit,

Abstract: We have correlated the tectal connections and cytoarchitecture of regions in the rabbit's midbrain and caudal thalamus. The inferior colliculus projects ipsilaterally to the central gray, superior colliculus, and via the brachium of the inferior colliculus to its interstitial nucleus and the parabrachial region of the midbrain tegmentum. From the brachium, fibers fan out to the principal and internal divisions of the medial geniculate. A smaller contralateral pathway sweeps into the contralateral inferior colliculus and in its brachium to the interstitial nucleus, the parabrachial region, and the internal and principal divisions of the medial geniculate. The superior collicular projection is mainly ipsilateral. Medially, fibers terminate in the central gray and pretectal area. Laterally, fibers ascend in the superior brachium to parabrachial region, suprageniculate pretectal nucleus, posterior complex, caudodorsal internal division of the medial geniculate, and to a discrete part of the ventral nucleus of later geniculate. A component of the commissure of Gudden originates in the rostral superior colliculus and terminates in the contralateral ventral lateral geniculate, posterior complex, pretectal area and midbrain tegmentum. Interconnections between the colliculi and overlap of their projections in the parabrachial region, the central gray, and the internal division of the medial geniculate are described.

Histogenesis of the nuclei griseum pontis, corporis pontobulbaris and reticularis tegmenti pontis (Bechterew) in the mouse. An autoradiographic study.

Abstract: By means of autoradiography the time, site of origin and migration pathways of neuroblasts destined for three nuclei in the hindbrain of the mouse have been determined. The cells of the nuclei griseum pontis and corporis pontobulbaris arise on gestation days 12 through 16 in the rhombic lip from the obex to the lateral recess. Most cells migrate within 48 hours via the pontobulbar body to the base of the pons to form the nucleus griseum pontis; some migrate only a short distance along the pathway and collectively form the nucleus corporis pontobulbaris. Most cells of the nucleus reticularis tegmenti pontis (Bechterew) arise in the primitive basal ependymal plate of the pons on gestation days 12 and 13 and migrate ventrally into the tegmentum. Some probably arise at the rhombic lip and migrate via the pontobulbar body to the base of the pontine flexure where they turn dorsally into the tegmentum.

Electron microscopy of the effect of gram‐negative endotoxin on the blood‐brain barrier

Abstract: Studies with the light microscope (Am. J. Path., 34: 631, '58) indicated that an injection of endotoxin into the carotid artery of a rabbit alters the blood-brain barrier allowing the distribution of subsequently injected colloidal iron throughout the cerebral cortex. The present investigation by electron microscopy determined the distribution of the colloidal iron oxide in the normal and endotoxin altered cerebral capillaries of rabbits. Four hours prior to the intracarotid injection of saccharated iron oxide the animal received an injection via the same artery of either 50 μg gram-negative endotoxin or a control dose of normal saline. The animals were sacrificed at times from 15 minutes to two hours after the injection of iron oxide and samples of the cerebral cortex were processed for electron microscopy by routine methods. In those animals that did not receive endotoxin, the colloidal iron was limited to the lumen of the cerebral capillaries and occasional large endothelial vacuoles. None of the iron was found in the endothelial pinocytotic vesicles. In the endotoxin animals the iron oxide particles were found in quantity in the capillary endothelium phagosomes, the basement membrane, and in astrocytes and their processes. The fine structure of the cerebral capillary is reviewed and evidence is presented in support of the hypothesis that the barrier mechanism for colloidal tracers lies in a unique property of the brain's endothelial pinocytotic vesicles.

The nucleus ambiguus of the rabbit.

Abstract: Each nucleus ambiguus of the rabbit contains about 1,500 cells and is approximately 4 to 5 mm long. The author confirms the previously described division of the nucleus into a rostral compact formation, 1.6 mm long, and a caudal diffuse formation occupying the remainder of the nucleus. He further subdivides the compact formation into a lateral part, the principal column, and a medial part, the medial column. These divisions and the cytological differences between the cells in different divisions are illustrated. The cells in the medial column are more similar in appearance to those of the diffuse formation, with which they probably form a continuous column, than to those of the principal column, which are smaller, and have less distinct Nissl granules.

Numerical estimates of neurons and glia in lateral geniculate body during retrograde degeneration

Abstract: Studies were made of decrease of neurons and proliferation of glial cells in the dorsal nuclei of lateral geniculate body of 32 rabbits and 12 cats who had undergone unilateral ablation of visual cortex up to 50 weeks prior to sacrifice. Results show that degeneration occurred in 80% of neurons in rabbit lateral geniculate body within three days, and 95% disappeared within four weeks. In comparison, degeneration was present in only 70% of neurons in the cat nucleus at the seventh post-operative day, and about 26% remained normal up to the fiftieth week. Glial cell reactions were differential and complex: astrocytes showed rapid proliferation with gradual return to normal, while oligodendrocytes and microglia increased more slowly, maintaining a high level over 50 weeks.

Monoamine‐containing neurons in the nerve cord and body wall of Lumbricus terrestris

Abstract: This study represents an attempt to reconcile recent quantitative findings of large amounts of monoamines in the earthworm with early histological findings of only small numbers of chromaffin cells in that animal. Using the highly sensitive and specific fluorescence method for the histochemical localization of monoamines, it has been demonstrated that the earthworm nervous system contains large numbers of monoaminergic neurons. At least one-tenth of the motor and interneuronal population in a typical ganglion of the ventral nerve cord contain either serotonin or primary catecholamines. In addition a large number of neurons containing a primary catecholamine form an extensive sensory system with their cell bodies in the epidermis and their axons terminating in the ventral nerve cord.

A peculiar mode of muscular innervation in Amphioxus. Light and electron microscopic studies of the so-called ventral roots.

Abstract: The present study demonstrates a neuromuscular junction between the so-called ventral root fibers and the surface of the spinal cord of amphioxus. The ventral root fibers, interconnected by numerous zonulae occludentes, appear to be projections from muscle fibers of the myotome and end in conical expansions at the surface of the spinal cord, separated from an extensive layer of axon endings or boutons by an extracellular cleft containing a basement membrane. Within each ventral root two kinds of ventral root fibers can be identified. Each corresponds to a compartment of the bouton layer. The so-called thick ventral root fibers are opposed to a long ventral compartment of small boutons tightly packed with vesicles about 1,000 A in diameter, but without a dense core. The so-called thin ventral root fibers are opposed to a short dorsal compartment of large boutons filled with synaptic vesicles about 600 A in diameter.

The interstitial cell and its place in the concept of the autonomic ground plexus.

Abstract: 1. The autonomic interstitial cells have been studied in the toad small and large intestine and guinea-pig taenia coli by means of both the light and electron microscopes. The guinea-pig superior cervical ganglion has also been examined. 2. Apart from muscle cells, neurons and Schwann cells, at least one other cell type has been observed. These cells have been designated connective tissue cells on the basis of their gross form, fine structure and relation to other cells. In many respects they resemble fibroblasts and macrophages. 3. These cells are in close relation to nerve fiber bundles and smooth muscle cells. They often form a complete sheath around bundles of axons and Schwann cells. Many of the processes of the connective tissue cells lie in grooves on the surface of the muscle cells; the distance between apposing cell membranes is about 200 A. 4. The processes of the connective tissue cells often interdigitate with each other in a most complex manner, the opposing cell membranes being about 200 A apart. Such a close relationship would appear with the light microscope to be a fusion forming a syncytium. 5. Observation with the light microscope on the terminal ramifications of the autonomic nervous system where small bundles of fine axons are enclosed by connective tissue cells could give rise to the impression that they contained “neurofibrils.” 6. It is suggested that the term autonomic interstitial cell covers a number of cell types which have been confused. The cells designated connective tissue cells along with Schwann cells have between them the characteristics of the autonomic interstitial cells as described by previous workers. Cajal's original description of sympathetic interstitial cells was actually a description of connective tissue cells.

Effect of vitamin A on development of the central nervous system

Abstract: Pregnant rats and mice were treated with large doses of vitamin A to examine its effect on the closure of the neural groove in the developing embryo. In the rat the most effective dose was 30,000 U.S.P. units given at day 8, 9, and 10 of pregnancy. Twenty-six per cent of the implanted embryos showed anencephaly and/or myeloschisis. In the mouse the results of vitamin A treatment were inconsistent. In 12-day rat embryos, the lateral walls of the diencephalon and mesencephalon were slightly everted. Mitotic neuro-epithelial cells were frequently piled up two or three rows thick, causing a disruption of the internal limiting membrane. Many cells were characterized by a bleb-like cytoplasmic evagination protruding into the lumen. During the following days, the everted walls of the diencephalon and mesencephalon expanded farther and farther over the sides of the head, thereby often covering the lateral hemispheres and the eyes. Although the hemispheres were displaced by the everted diencephalic plates and the eyes were frequently buried deep inside the head, histological differentiation proceeded normally and a primitive cerebral cortex as well as normal eye structures were observed. The accumulation of mitotic figures, the formation of cytoplasmic protrusions, and the presence of small neuro-epithelial rosettes throughout the brain were the most striking cytological anomalies. By the eighteenth day of development, small hemorrhages occurred in the mesenchyme surrounding the brain and degeneration became apparent. At birth a small amount of degenerated brain tissue was all that remained from the large exencephalic brain, and the newborn had the appearance of an anencephalic. It is concluded that large doses of vitamin A prevent the neural groove from closing, possibly by interfering with the Cell membrane and/or the mitotic activity of the neuro-epithelial cells.