Interactions between tectal radial cells in the red-eared turtle, Pseudemys scripta elegans: an analysis of tectal modules.

The optic tectum is a major subdivision of the visual system in reptiles. Previous studies have characterized the laminar pattern, the neuronal populations, and the afferent and efferent connections of the optic tectum in a variety of reptiles. However, little is known about the interactions that occur between neurons within the tectum. This study describes two kinds of interactions that occur between one major class of neurons, the radial cells, in the optic tectum of Pseudemys using Nissl, Golgi and electron microscopic preparations. Radial cells have somata which bear long, radially oriented apical dendrites from their upper poles and short, basal dendrites from their lower poles. They are divided into two populations on the basis of the distribution of their somata in the tectum. Deep radial cells have somata densely packed in the stratum griseum periventriculare. Their plasma membranes form casual appositions. Middle radial cells have somata scattered throughout the stratum griseum centrale and stratum fibrosum et griseum superficiale and do not contact each other. The apical dendrites of both populations of radial cells participate in vertically oriented, dendritic bundles. The plasma membranes of the dendrites in these bundles form casual appositions in the deeper tectal layers and chemical, dendrodenritic synapses within the stratum fibrosum et griseum superficiale. The synapses have clear, round synaptic vesicles and slightly asymmetric membrane densities. Thus, radial cells interact via both casual appositions and chemical synapses. These interactions suggest that radial cells may form a basic framework in the tectum. Because both populations of radial cells extend into the stratum fibrosum et griseum superficiale and stratum opticum, they may receive input from some of the same tectal afferent systems. Because the deep radial cells alone have somata and dendrites in the deep tectal layers, they may receive additional inputs that the middle radial cells do not. Neurons in the two populations interact via chemical dendrodentritic synapses, thereby forming vertically oriented modules in the tectum.     

The synaptic distribution of the retinal input in the superficial layers of the guinea-pig superior colliculus

An evoked potential consisting of four postsynaptic components was recorded in the guinea-pig superior colliculus following electrical stimulation of the contralateral optic nerve. This potential was generated in response to the activation of four populations of optic nerve fibres with different conduction velocities. Current source-density analysis revealed that the two slower conducting fibre populations synapse in the upper third of the stratum griseum superficiale on dendrites whose cell bodies appear to be found in the lower part of this layer and in the stratum opticum. The two faster conducting populations synapse deeper, near the border of the stratum griseum superficiale and stratum opticum, on neurons with cell bodies that may lie towards the upper part of the stratum griseum superficiale. The locations of these postsynaptic sites correspond to the layers in which the optic nerve terminates as revealed by neuroanatomical tracing techniques. Furthermore, neurons of the shape and orientation predicted by the current source-density analysis were found in the superficial layers by using the Golgi-Cox technique.     

Retinal projections in the chain pickerel (Esox niger Lesueur)

Summary The retinal projections inEsox niger, as determined with the aid of a modified cobalt-lysine method, are considerably more extensive in the diencephalon and pretectum than in other teleost fishes so far examined. Although most retinal axons terminate contralaterally, rare fibers can be traced to the same aggregates ipsilaterally. The retinohypothalamic projection appears larger than hitherto reported in teleosts, and the dorsomedial optic tract issues fibers to a series of cell clusters extending from the rostral thalamus to mid-torus levels. A retinal projection to a presumed ventrolateral optic nucleus (VLO) is described for the first time in a teleost. Other targets of retinal fibers include the nucleus geniculatus lateralis ipse of Meader (GLI), the pretectal nucleus (P), the cortical nucleus and a well-developed ventromedial optic nucleus (VMO). The projection to the optic tectum is principally to the stratum fibrosum et griseum superficiale (SFGS) and stratum marginale (SM), but a considerable number of axons also course through the stratum album centrale (SAC) before terminating there or piercing the stratum griseum centrale (SGC) and terminating in SFGS. Rare terminal arborizations of retinal fibers were also observed in stratum griseum centrale (SGS) and in the stratum griseum periventriculare (SGC) in restricted portions of the tectum. Because of the relatively large size of the visual structures inE. niger it is a potentially useful model for future experimental studies on the visual system.     

The synaptic relationships between the primary afferent terminals and the cuneo-thalamic relay neurons in the rat cuneate nucleus

In order to establish the synaptic relationship between the primary afferent terminals and the cuneothalamic relay neurons in the cuneate nucleus, the combined retrograde transport of horseradish peroxidase (HRP) and experimental degeneration have been applied in the young adult albino rats. 10 to 30% HRP was injected contralaterally (0.5 microliter) in the ventrobasal thalamic nucleus and multiple dorsal rhizotomies (C5 to T1) in the cervicothoracic dorsal roots were performed on the side ipsilateral to the cuneate nucleus. The results showed that: The cuneo-thalamic relay (CTN) neurons were the major neuronal type of the nucleus. More than 55% of neurons have been labelled. These neurons were 18-30 micron X 15-25 micron in sizes. They distributed in the whole rostrocaudal extent of the nucleus, particularly dense in the middle portion. The cells varied from round, oval, spindle to multipolar in shapes. They were rich in cytoplasmic organelles and had well-developed roughed endoplasmic reticulum. Their nucleus was either centrally or eccentrically located and was rather regular. The HRP-positive granules were randomly distribute in the perikaryon, dendrites and initial segment of the axons; At least three types of the experimental degeneration of the primary afferent terminals (PAT) were observed in the cuneate nucleus two to three days after dorsal rhizotomy, namely, electron-dense, granular and neurofilamentous. These PAT were mostly large and contained round vesicles. They were commonly found within synaptic complex, in which they were presynaptic to dendrites of various sizes, and were themselves postsynaptic to smaller axon terminals containing flattened vesicles. Degenerating PAT forming isolated synapses were less commonly seen; The PAT in the synaptic complex were directly presynaptic to the dendrites originating from the CTN neurons. The dendrites forming PAT-CTN synases were of large and medium-sized. The PAT did not form direct axo-somatic synapses with the somata of CTN or of any other cell types in the cuneate nucleus.     

Ultrastructural changes in the superficial layers of the superior colliculus in Galago crassicaudatus (primates) after eye enucleation

Summary Several types of terminals were found in the three superficial collicular layers of Galago. At least two axon terminals with round vesicles (R1 and R2) could be distinguished on the basis of vesicle packing and electron density of the cytoplasmic and mitochondrial matrices. R1 axon terminals were characterized by aggregations of vesicles in an electron lucent cytoplasm and mitochondria with a relatively dark matrix, while in R2 axon terminals the vesicles were more evenly distributed in an electron dense cytoplasm and the mitochondrial matrix was pale. R2 endings occurred in clusters in the stratum griseum superficiale; they were absent in the stratum zonale. R1 endings were found in all three superficial collicular layers. Both types of R terminals made asymmetrical contacts with small dendrites, dendritic spines and F profiles. Profiles containing flattened vesicles and establishing symmetrical contacts were numerous, and many could be identified as dendrites by accepting as criteria for dendrites evenly spaced microtubules, clusters of ribosomes and the fact that these F profiles were postsynaptic to other terminals. F terminals were presynaptic to other F profiles, dendrites and somata; they were postsynaptic to R terminals and took part in serial synapses. Dendrodendritic contacts were frequent, somatodendritic contacts rare. After eye enucleation most R2 axon terminals underwent the electron dense degenerative reaction. The degeneration process was a lengthy one; many degenerating boutons were found 30 days after axotomy and some persisted up to 180 days postoperatively. There was strong indication that the superior colliculus received more crossed than uncrossed retinofugal fibers. The crossed and uncrossed retinocollicular axons terminated in two different substrata of the stratum griseum superficiale.This study was supported by N.I.H. Grant RR-00165 to Yerkes Regional Primate Research Center and N.I.H Grant EY 00638-03 to J. Tigges. — The opportunity to use the electron microscopic facilities of the Fernbank Science Center for the initial stage of this work is gratefully acknowledged.     

Ultrastructural localization of nitric oxide synthase immunoreactivity in the cat ventrobasal complex

This study describes the ultrastructural localization of nitric oxide synthase (NOS) immunoreactivity in the cat ventrobasal complex. NOS immunoreactivity was found in the cell bodies and dendrites of local circuit neurons and in vesicle-containing profiles. The vesicle-containing profiles could be divided into two classes, those of dendritic origin (presynaptic dendrite boutons) and those of axonal origin. The NOS labelled axon terminals varied in size and packing density and were principally located in the extra-glomerular neuropil. These boutons presented a range of morphologies and it was not possible to determine the probable source based on morphological criteria. The NOS immunoreactive presynaptic dendrite boutons were found both within and outside glomeruli and established both pre- and post-synaptic relationships with other elements. Post-embedding GABA immunocytochemistry showed that some NOS immunoreactive axonal boutons and presynaptic dendrites were also immunopositive for GABA. This finding suggests that some of the NOS labelled axonal boutons are of local circuit neuron origin. These results suggest that local circuit neurons in the cat ventrobasal complex might be involved in specific, short range interactions using GABA and longer, more global interactions using nitric oxide.     

Retinal projections in the catfish,Mystus vittatus (Bloch) as revealed by tracer studies with horseradish peroxidase

Summary The retinal efferents of the catfish, Mystus vittatus, were investigated with the use of the horseradish peroxidase (HRP) technique. Most retinal fibres extended contralateral to the eye that had received HRP label, while a few fascicles projected to the ipsilateral side without decussation in the optic chiasma. The contralateral fibres projected to the suprachiasmatic nucleus, the nucleus opticus dorsolateralis, the nucleus of the posterior commissure, the nucleus geniculatus lateralis, pretectal nuclear complex, and to two layers of the optic tectum, i.e., stratum fibrosum et griseum superficiale and stratum griseum centrale. The accessory optic tract arose from the inner area of the optic tract and extended ventromedially to the accessory optic nucleus. The ipsilateral fascicles projected to almost all the above mentioned nuclei, but these projections were comparatively sparse. The ipsilateral retinal projection was restricted to the rostral tectum.     

Neurophysiological properties of magnetic cells in the pigeon's visual system

Summary Single unit electrical activity was recorded extracellularly in the nucleus of the basal optic root (nBOR) and in the optic tectum under earth-strength magnetic stimulation. Units in the nBOR which were stimulated while the eyes were illuminated by light of different wavelengths exhibited peaks of magnetic responsiveness at 503 nm and 582 nm.Magnetically directional selective cells were found in the stratum griseum et fibrosum superficiale of the optic tectum. They also showed directional selectivity to dynamic photic stimuli. Response peaks varied with the orientation of the pigeon in the horizontal plane. This confirmed that the magnetic responses contained directional information. The results suggest that the receptor and neural organisation of the pigeon's visual system provides an adequate substrate for the detection and elaboration of magnetic compass information.     

Tectal and Related Target Areas of Spinal and Dorsal Column Nuclear Projections in Hedgehog Tenrecs

The terminal distributions of spinal and dorsal column nuclear projections to tectum, pretectum, and central gray of hedgehog tenrecs (Echinops telfairi and Setifer setosus) were investigated using anterograde axonal flow and various tracer substances. In the inferior colliculus, the densest and most extensive mesencephalic projections were found within the pericentral regions. One target area, referred to as the external portion of the inferior colliculus, was represented as a semicircle of grain patches lateral and caudal to the central nucleus. This region received somesthetic afferents from the dorsal column nuclei and from spinal segments at various levels. In contrast, after high cervical injections, the pericentral portion dorsomedial to the rostral half of the central nucleus was labeled almost exclusively. This area of labeling was distinct from the labeling in the central gray and might be best compared with the intercollicular zone in other species. The superior colliculus received projections predominantly from the high cervical cord; minor projections also arose from lumbar spinal segments and the dorsal column nuclei. The terminal field covered roughly the caudal half of the colliculus and involved the stratum griseum intermediale in a patch-like fashion. Some labeling was also found in the stratum griseum profundum and in the stratum griseum superficiale. Other than in the colliculi, weak pretectal projections were observed following dorsal column nuclear injections, while the nucleus of Darkschewitsch was labeled best following lumbosacral injections. All mesencephalic target areas were labeled consistently on the contralateral side, while their ipsilateral side was involved to a varying degree: The relatively most prominent ipsilateral labeling was seen in the central gray, being roughly similar on both sides; scarcely any labeling was noted in the ipsilateral superior colliculus. Tectal injections of retrograde tracer, in addition, revealed a considerable number of labeled neurons in a relatively cell-poor region immediately ventral to the high cervial dorsal horn. This region might correspond to the lateral cervical nucleus, an aggregation of neurons that so far has only been demonstrated in higher mammals.     

Ultrastructural characteristics of vasopressin-containing neurons in the paraventricular nucleus of the hypothalamus

Summary Vasopressin-containing neurons, identified by immunocytochemistry, are located predominantly in the posterior magnocellular division of the paraventricular nucleus of the rat hypothalamus. By electron microscopy, the immunoreaction product is seen within the cell bodies and neuronal processes. In the perikarya and dendritic processes, the immunoreactive material is associated primarily with neurosecretory granules. Axonal processes, identified by their content of microtubules and accumulation of neurosecretory granules, show the immunoreaction product in association with both of these organelles. Afferent axo-dendritic, axo-somatic and putative axo-axonic synapses with immunostained vasopressinergic neurons can be identified. The presynaptic profiles do not contain immunoreactive material. This study contributes to the ultrastructural characterization of vasopressinergic neurons in the paraventricular nucleus and of their afferent synaptic input.Supported by NIH Grants HD-12956 and 2SO7RR05403     

Regional effect of monosodium-L-glutamate on the superficial layers of superior colliculus in rat

Summary Systemic administration of monosodium-1-gluta-mate by single injections of 4 mg/g body weight in infant rats (2–10 days of age) results in acute swelling of cytoplasm and nuclear pyknosis of neurons in the stratum zonale and stratum griseum superficiale of the superior colliculus. Multiple daily doses of 4 mg/g body weight monosodium-1-glutamate result in an almost complete loss of neurons in these two superficial layers. The deeper layers appear not to be affected. No pathological effects were observed in the lateral geniculate body or pretectal complex.Light-and electron-microscopic studies reveal that the optic nerves are remarkably shrunken and many myelinated as well as unmyelinated axons are lost. Injection of ³Hproline into the vitreous body of one eye results in limited transport to the suprachiasmatic nucleus, lateral geniculate body and to lateral portions of the superior colliculus.The small percentage of intact axons in the optic nerve, as well as the limited proline transport from the eye, suggest that administration of monosodium-1-glutamate leaves intact some optic fibers, a portion of which belongs to the retinohypothalamic tract.     

The control of dendrite development

Dendrite development is an important and unsolved problem in neuroscience. The nervous system is composed of a vast number of neurons with strikingly different morphology. Neurons are highly polarized cells with distinct subcellular compartments, including one or multiple dendritic processes arising from the cell body, and a single, extended axon. Communications between neurons involve synapses formed between axons of the presynaptic neurons and dendrites of the postsynaptic neurons. Extensive studies over the past decade have identified many molecules underlying axonal outgrowth and pathfinding. In contrast, the control of dendrite development is still much less well understood. However, recent progress has begun to shed light on the molecular mechanisms that orchestrate dendrite growth, arborization, and guidance.     

Gap junctions between dendrites and somata of neurons in the primate sensori-motor cortex.

Gap junctions have been found infrequently between two dendrites or a dendrite and a cell soma in the deep layers of both the motor and somatic sensory cortices of the primate. At these junctions the outer leaflets of the plasma membranes of both profiles are intimately apposed with a gap of 2 nm between them which shows a structure of hexagonal subunits in tangential sections. These gap junctions occur mainly between the dendrites or dendrites and somata of large stellate cells but are also associated in some examples with a dendro-dendritic synapse and thus occur between large stellate dendrites and presynaptic dendrites; a desmosome may also occur in association with a gap junction and dendro-dendritic synapse. Gap junctions have been identified as sites of electrical transmission between cells in a number of sites and it is therefore suggested that some neurons in the sensori-motor cortex are electrotonically couples.     

Vasoactive intestinal polypeptide (VIP)-containing neurons: distribution throughout the brain of the chick (Gallus domesticus) with focus upon the lateral septal organ

The distribution of VIP-like perikarya and fibers was determined throughout the chick brain. The most rostral immunoreactive perikarya were found to be cerebrospinal fluid-contacting neurons in the pars medialis of the lateral septal organ. Additional data were presented supporting the idea that the lateral septal organ is another circumventricular organ within the brain of birds (Kuenzel and van Tienhoven 1982). A large group of immunoreactive perikarya was found in the lateral hypothalamic area and appeared continuous with immunoreactive neurons in the anterior medial and ventromedial hypothalamic nuclei (n). A few perikarya were located in the paraventricular hypothalamic n. A number of immunoreactive neurons were found within and about the infundibular and inferior hypothalamic n., none however was immunoreactive cerebrospinal fluid-contacting neurons. Immunoreactive perikarya were found predominantly in laminae 10–11 of the stratum griseum et fibrosum superficiale. A few scattered perikarya were found ventromedial to the n. tegmenti pedunculo-pontinus pars compacta and locus ceruleus. Some of the immunoreactivity was unusual, being very homogeneous within the cell body with little evidence of the material in the axon or dendrites. Perikarya were found in the central gray, n. intercollicularis, and area ventralis of Tsai. The most caudal structure showing immunoreactive neurons was the n. reticularis paragigantocellularis lateralis. Brain areas containing the most abundant immunoreactive fibers, listed from the rostral-most location, were found in the ventromedial region of the lobus parolfactorius and the lateral septal n. Continuing caudally, there were immunoreactive fibers within the periventricular hypothalamic n.; some of the fibers were found to travel for some distance parallel to the third ventricle. Dense immunoreactive fibers were found in the tractus cortico-habenularis et cortico-septalis, medial habenular n. and posterior and dorsal n. of the archistriatum. A number of areas had what appeared to be baskets of immunoreactive fibers (perhaps immunoreactive terminals) surrounding non-reactive perikarya. Brain areas containing terminals included the piriform cortex, area ventralis of Tsai, interpeduncular n., and specific regions of the stratum griseum et fibrosum superficiale. A very dense immunoreactivity occurred within the external zone of the median eminence, the dorsolateral parabrachial n., and n. tractus solitarii. Vasoactive intestinal polypeptide appears to be a useful peptide for defining the neuroanatomical constituents of the visceral forebrain in birds.     

Vasoactive intestinal polypeptide (VIP)-containing neurons: distribution throughout the brain of the chick (Gallus domesticus) with focus upon the lateral septal organ

The distribution of VIP-like perikarya and fibers was determined throughout the chick brain. The most rostral immunoreactive perikarya were found to be cerebrospinal fluid-contacting neurons in the pars medialis of the lateral septal organ. Additional data were presented supporting the idea that the lateral septal organ is another circumventricular organ within the brain of birds (Kuenzel and van Tienhoven 1982). A large group of immunoreactive perikarya was found in the lateral hypothalamic area and appeared continuous with immunoreactive neurons in the anterior medial and ventromedial hypothalamic nuclei (n). A few perikarya were located in the paraventricular hypothalamic n. A number of immunoreactive neurons were found within and about the infundibular and inferior hypothalamic n., none however was immunoreactive cerebrospinal fluid-contacting neurons. Immunoreactive perikarya were found predominantly in laminae 10–11 of the stratum griseum et fibrosum superficiale. A few scattered perikarya were found ventromedial to the n. tegmenti pedunculo-pontinus pars compacta and locus ceruleus. Some of the immunoreactivity was unusual, being very homogeneous within the cell body with little evidence of the material in the axon or dendrites. Perikarya were found in the central gray, n. intercollicularis, and area ventralis of Tsai. The most caudal structure showing immunoreactive neurons was the n. reticularis paragigantocellularis lateralis. Brain areas containing the most abundant immunoreactive fibers, listed from the rostral-most location, were found in the ventromedial region of the lobus parolfactorius and the lateral septal n. Continuing caudally, there were immunoreactive fibers within the periventricular hypothalamic n.; some of the fibers were found to travel for some distance parallel to the third ventricle. Dense immunoreactive fibers were found in the tractus cortico-habenularis et cortico-septalis, medial habenular n. and posterior and dorsal n. of the archistriatum. A number of areas had what appeared to be baskets of immunoreactive fibers (perhaps immunoreactive terminals) surrounding non-reactive perikarya. Brain areas containing terminals included the piriform cortex, area ventralis of Tsai, interpeduncular n., and specific regions of the stratum griseum et fibrosum superficiale. A very dense immunoreactivity occurred within the external zone of the median eminence, the dorsolateral parabrachial n., and n. tractus solitarii. Vasoactive intestinal polypeptide appears to be a useful peptide for defining the neuroanatomical constituents of the visceral forebrain in birds.     

Method of modelling intracellular transport in branching neurites: application to axons and dendrites of Drosophila sensory neurons

This paper develops a method of calculating the transport of intracellular organelles in neurons with branching neurites which is based on the Smith–Simmons equations of motor-assisted transport. The method is aimed at understanding the effects of microtubule (MT) polarity orientation in branching neurites on transport of organelles at the fundamental level. The method is applied to calculating the organelle transport in axons and dendrites of Drosophila neurons, using the map of MT orientation in such neurons developed by Stone et al. (Mol Biol Cell 19:4122–4129, 2008). The proximal dendrite is assumed to branch and form two distal dendrites. Two different MT polarity arrangements in a proximal dendrite are considered, and implications of these MT arrangements on organelle transport are analysed. It is demonstrated that the MT arrangement found in Drosophila dendrites (MTs have their minus ends out in a proximal dendrite) results in much more efficient motor-driven transport than the structure with a mixed MT orientation in proximal dendrites.     

Tiling of the Drosophila epidermis by multidendritic sensory neurons

Insect dendritic arborization (da) neurons provide an opportunity to examine how diverse dendrite morphologies and dendritic territories are established during development. We have examined the morphologies of Drosophila da neurons by using the MARCM (mosaic analysis with a repressible cell marker) system. We show that each of the 15 neurons per abdominal hemisegment spread dendrites to characteristic regions of the epidermis. We place these neurons into four distinct morphological classes distinguished primarily by their dendrite branching complexities. Some class assignments correlate with known proneural gene requirements as well as with central axonal projections. Our data indicate that cells within two morphological classes partition the body wall into distinct, non-overlapping territorial domains and thus are organized as separate tiled sensory systems. The dendritic domains of cells in different classes, by contrast, can overlap extensively. We have examined the cell-autonomous roles of starry night (stan) (also known as flamingo (fmi)) and sequoia (seq) in tiling. Neurons with these genes mutated generally terminate their dendritic fields at normal locations at the lateral margin and segment border, where they meet or approach the like dendrites of adjacent neurons. However, stan mutant neurons occasionally send sparsely branched processes beyond these territories that could potentially mix with adjacent like dendrites. Together, our data suggest that widespread tiling of the larval body wall involves interactions between growing dendritic processes and as yet unidentified signals that allow avoidance by like dendrites.     

Neurons with complex receptive fields in the stratum griseum centrale of the zebra finch (Taeniopygia guttata castanotis Gould) optic tectum

The electrophysiological and morphological features of visually driven neurons of the stratum griseum centrale of the zebra finch optic tectum were studied by extracellular recording and staining techniques. Stratum griseum centrale neuron responses are sustained in most cases. Receptive fields are big, up to 150 degrees of the visual field. The excitatory center (hot spot) varies in size from 1 degrees to 15 degrees. It can be mapped by small static stimuli, adapts slower than the surround, and has a shape comparable to the excitatory fields of upper-layer neurons. In contrast, the big surround shows responses only to small moving objects which elicit a typical pattern of alternating bursts and silent periods. Alternatively, the same stimuli elicit long-lasting bursts followed by strong adaption. Anatomically, stratum griseum centrale neurons are characterized by far reaching dendrites which terminate with "bottlebrush"-like endings in the upper retinorecipient layers. In addition, they are connected with retinorecipient structures by an interneuron located between layers 10 and 11. The role of the structure of inputs for the organization of the receptive fields is discussed.     

On the fine structure of the small,heavily pigmented non-pyramidal cells in lamina II and upper lamina III of the human isocortex

Summary With the aid of a newly developed technique for the successive examination of both the Golgi and pigment picture of individual neurons (Braak, 1974a) Braak (1974b) demonstrated that within lamina II and upper lamina III of the human isocortex, heavily pigmented non-pyramidal cells are distributed irregularly and sparsely. The lipofuscin pigment granules serve as excellent internal markers to identify these non-pyramidal cells in ultrathin sections. This favourable circumstance facilitates the study of these interneurons in the electron microscope.The heavily pigmented non-pyramidal cells are small, spherical to ovoid with diameters of about 12–15 m. One pole of the cell comprising a large cytoplasmic area gives rise to a few dendrites, while the other pole is occupied by the nucleus and in some cases is in close apposition to another nerve cell body. The nucleus is deeply invaginated by the large cytoplasmic area and occasionally displays nuclear inclusions. Among the usual organelles distributed within the large cytoplasmic area the mitochondria with a moderately electron dense matrix are abundant and the coarse lipofuscin pigment granules are the most striking elements. The latter contain densely packed filamentous or tubular material and a single vacuole. The perikaryon rarely receives more than 3 type I and type II synapses per section per cell, whereas the dendrites receive numerous synapses of both type I and type II. Within the apposition zone to another nerve cell body (which in no case is a heavily pigmented non-pyramidal cell) puncta adhaerentia occur and also contacts in which the cleft of 8 nm is intersected by a dense stratum.Some of the ultrastructural findings are summarized in the schematic drawing of Figure 15.     

The Vascularization of the Anuran Brain: The Mesencephalon

The angioarchitecture of the toad's mesencephalon is studied by scanning electron microscopy of vascular corrosion casts. The arterial supply is performed by the superior mesencephalic artery and by an artery arising from the ramus posterior of the arteria carotis cerebralis just in front of the retroin-fundibular communicating artery. The venous drainage is exerted by the vena diencephalica, the vena lateralis mesencephalis and by anterior and posterior branches of the encephaloposthypophysial portal vein. Characteristics of the mesencephalic angioarchitecture are the centrifugal direction of the blood flow, the intensive capillarization, and the formation of vascular meshworks in the region of the stratum griseum periventriculare tecti and of the stratum griseum superficiale tecti.