The lateral recess of the third ventricle in teleosts: an electron microscopic and Golgi study

The Cytoarchitecture of the Tectal-Related Pallium of Squirrelfish, Holocentrus sp

The squirrelfish, which live in visually complex coral reefs, have very large eyes and a special dual-system “day and night vision” retina. They also have atypical expansions of brain areas involved in processing visual information. The midbrain tectum sends information via diencephalic relay to two enlarged dorsal telencephalic regions. The latter include a superficial dorsal/lateral “cortex-like area” of small to medium-sized cells [area dorsalis telencephali, pars lateralis-dorsal region (dorsal segment); Dld1] which projects to an underlying dorsocentral region of relatively large cells (the area dorsalis telencephali, pars centralis-dorsal region; Dcd) which in turn reconnects with the tectum. Additionally, the cerebellum is also involved in this pathway. The hypertrophied pallial regions, termed the tectal-related pallium (TRP), most likely exert major influences on a variety of visually-related sensorimotor systems. This research aimed at better establishing the cellular structures and possible connections within the TRP. Nissl and rapid Golgi staining, biotinylated dextran amine tracing and cell-filling, and electron microscopy were used in this study. For gross observation of the pallial areas and plotting of the study sites, a mini-atlas of transverse and horizontal sections was constructed. This research better documented the known cellular elements of the TRP and defined two novel cell types. Species differences in the TRP may be related to possible differences in behavior and ecology.

Evidence for the c-ret protooncogene product (c-Ret) expression in the spinal tanycytes of adult rat

Expression of the protein product of c-ret (c-Ret) in the spinal cord of the adult rat was examined immunohistochemically at both electron and light microscopic levels. In the cervical, thoracic and lumbar segments of the spinal cord, a large number of c-Ret immunoreactive cells were found in both ependymal and subependymal layers of the central canal. These cells were ovoid or triangular in shape and had a well developed single cytoprocess which protruded into the central canal. None of the neuropeptides and neuronal markers examined, including substance P, CGRP, galanin, neuropeptide Y, tyrosine hydroxylase, methionine-enkephalin, choline acetyltransferase and glially fibrilally acidic protein, was present in these c-Ret immunoreactive cells in the spinal cord. Ultrastructurally, a desmosome-like structure was found between the apical part of the cytoprocess and the ependymal cell. These morphological observations indicated that c-Ret positive cells are spinal tanycytes. The present results suggest that spinal tanycytes in the rat express a trophic factor receptor and may respond to GDNF in the cerebrospinal fluid.

Periventricular morphology in the diencephalon of antarctic notothenioid teleosts

We have examined the subependymal region of the diencephalic third ventricle in notothenioid perciforms and report a pattern of neuropil expansions that appears to be phyletically derived for notothenioids and their outgroups but that is otherwise unique among vertebrates. We recognize five types of expansions based on their composition (from less dense neuropil to sacs) and width or protrusion into the third ventricle. In the species with the most elaborate morphology, Trematomus bernacchii, bilateral subependymal expansions fuse along the midline to form a single sac within the ventricular cavity. The extent of these expansions loosely corresponds with phyletic position but also (and perhaps more importantly) is correlated with the habitation of cold water (r2 = 0.48; P = 0.012). Furthermore, subependymal expansion type is positively correlated with the maximum size of the soma of neurons in two hypothalamic nuclei, the preopticus magnocellularis (r2 = 0.54; P = 0.006) and the lateralis tuberis (r2 = 0.40; P = 0.038). These nuclei project to the pituitary and contain cerebrospinal fluid-contacting neurons. In considering the functional consequences of this morphology, we cannot dismiss the possibility that these structures form a specialized enteroceptive system tied to the monitoring of cerebrospinal and extracellular fluid components, including antifreeze glycopeptides and inorganic ions.

Diencephalic gustatory connections in the channel catfish

Vertebrate gustatory systems include a tertiary ascending pathway from a secondary gustatory nucleus in the hindbrain to several forebrain nuclei. This connection is prominent in catfish, corresponding to their highly developed sense of taste. Iontophoretic injections of horseradish peroxidase were used to identify the specific target nuclei of the tertiary gustatory pathway in channel catfish and to characterize those nuclei by their respective connections. Efferents from the secondary gustatory nucleus (nGS) ascend in the tertiary gustatory tract to the caudal inferior lobe, where they terminate caudally in the nucleus lobobulbaris (nLB) and nucleus centralis (nCLI), and rostrally in the nucleus diffusus (nDLI). Secondary projections from the facial lobe (FL) also terminate in the nLB and in the nucleus subglomerulus (nSG). The nLB forms three cell groups (caudal--nLB, rostrolateral--rl nLB, parvicellular--nLBp), which project to the facial lobe, vagal lobe, and telencephalon, respectively. Cells from the nCLI project throughout the caudal inferior lobe and to the acousticolateral torus semicircularis and telencephalon, while the nDLI and nSG have intrinsic connections within the inferior lobe. The lateral thalamic nucleus projects from this region back to the nGS. Through these identified connections several mechanisms for the processing of gustatory information can be proposed. The descending projections from the nLB and nLT could provide feedback to the primary and secondary gustatory nuclei, and could modulate feeding-related motor circuits in the medulla. The connections of nCLI and nLBp with the telencephalon allow for the involvement of gustation in learning processes and other complex behaviors.

Spinal tanycytes in the adult rat: a correlative Golgi gold-toning study

In Golgi impregnated transverse sections through the cervical spinal cord of the 7-12-week-old adult rat, numerous tanycytes were observed radiating from the ependyma into the gray matter that surrounds the central canal. The tail processes of these tanycytes terminated as foot processes in association with blood vessels. Spinal tanycytes were classified into ependymal (E) and subependymal (S) types on the basis of the shape and position of the soma. The soma of the E tanycyte was shaped as a column and was entirely located within the ependyma. In contrast, the soma of the S tanycyte was flask shaped, with the widest portion of the flask located subependymally and the elongated portion extending through the ependyma ultimately reaching the luminal surface. Selected Golgi impregnated sections were gold toned and deimpregnated for direct correlative analysis at the ultrastructural level. Gold-toned tanycytes contained the fine clusters of gold particles underlying the plasma membrane of the cell body and coarse clusters of gold particles throughout the tail and foot processes. The apical surface of tanycytes was characterized by numerous microvilli and large cytoplasmic protrusions that evaginated from the apical surface into the lumen of the central canal. At the luminal surface, adjacent tanycytes were joined laterally by junctional complexes with punctate tight junctions and zonulae adhaerentes associated with fibrils and microtubules. In contrast, gap junctions, hemidesmosomes, and puncta adhaerentia were found between adjacent tail processes of tanycytes. The foot processes interdigitated with one another and abutted the basal lamina around the perivascular space of blood vessels. The basal lamina was continuous around the lateral walls of foot processes and filled the spaces between membranous infoldings of the lateral walls. These basal membrane labyrinths were continuous with the basal lamina of the blood vessel and may provide an extensive surface relation between the perivascular space and the neighboring extracellular compartment. The findings of the present study support the contention that tanycytes may modify the composition of substances moving between the perivascular and extracellular spaces.

A hypothalamic channel-system in the inferior lobes of a trigger-fish (Rhinecanthus aculeatus, Balistidae)

In the course of a general investigation of the optic system in some trigger-fishes, a hypothalamic, ventricular channel-system with pores, leading to the outside of the brain, has been discovered in the inferior lobes of Rhinecanthus aculeatus (Balistidae). A description of the morphological relations of the channel-system to the blood-vessel-supply and the cranium suggests that the organ-systems involved form a functional unit. The possibility of a hormone-producing system is discussed in the light of physiological and ecological aspects.

Morphological relations between the receptor neurons, sustentacular cells and Schwann cells in the olfactory mucosa of the salamander

The morphological characteristics of olfactory receptor neurons and their relations to the sustentacular cells and to the sheath cells of Schwann in the olfactory mucosa of the salamander (Ambystoma maculatum et tigrinum) were studied using a modification of the rapid Golgi technique. The bipolar receptor neurons had a fusiform-shaped cell body whose apical pole gave rise to a surface-reaching dendrite and a basal pole which gave rise to an axon. The length and width of the dendrite, although variable, were positively correlated with the relative depth at which the cell body was located in the sensory epithelium. Beyond the initial segment, the axon had a sinuous course prior to its entrance into the lamina propria. Within the lamina propria, the axons were associated with the sheath cells of Schwann to form the olfactory nerve fascicles. The processes of adjacent sheath cells formed an elaborate network of continuous cavities through which the axons coursed. Two morphologically distinct varieties of sustentacular cells, designated types I and II, were found in the sensory epithelium. Both types had a columnar profile consisting of an elongated cell body, a central stalk, and a basilar expansion of the stalk found at the junction of the epithelium with the lamina propria. The central stalk of type I sustentacular cells was unbranched, whereas that of type II cells gave rise to riblike processes from which cytoplasmic veils extended to envelop the cell bodies of receptor neurons. The basilar expansions were found in close apposition to the wall of capillaries or to acinar cells of Bowman's glands located in the most superficial region of the lamina propria. The morphological relationships and possible interdependencies among receptor neurons, the types of sustentacular cells, and the sheath cells are discussed.

Aqueductal tanycytes in the rabbit brain: A Golgi study

Utilizing Golgi-Cox impregnation, tanycytes were found in the ependyma of the cerebral aqueduct of the neonatal and adult rabbit. These tanycyte somas showed a variety of shapes, apical projections into the aqueduct, and basal processes (shafts) projecting into the mesencephalon, particularly into the periaqueductal gray (PAG). The shafts showed a variety of branching patterns, and sometimes abutted or terminated on capillaries or on specific neuronal elements. Other shafts coiled within the PAG or terminated within the neuropil of the mesencephalon. It is possible that these tanycytes provide a route for transport of cerebrospinal-fluid-borne substances from the aqueduct to the neuronal regions and vasculature of the mesencephalon. The presence of these tanycytes with complex branching patterns in proximity to neural and vascular structures suggests a permanent, active role for these specialized ependymal cells.

The organization of monoamine-containing neurons in the brain of the sunfish (Lepomis gibbosus) as revealed by fluorescence microscopy

The morphological organization of the monoamine-containing neurons in the brain of the sunfish (Lepomis gibbosus) was studied by means of the Falck-Hillarp histofluorescence method. No attempt was made to distinguish between norepinephrine and dopamine, both primary catecholamines (CA) yielding a similar yellow-green fluorescence after paraformaldehyde treatment. In the brain stem of this teleost fish, three groups of CA-containing neuronal somata have been found. First, there is a small collection of CA perikarya located just caudal to the obex of the fourth ventricle. The neurons of this medullo-sinal group give rise to numerous CA fibers many of which ascend within the central portion of the medulla. Intermingled with these CA fibers are some CA cells that constitute the central medullary group. The CA perikarya of this group are scattered between the levels of cranial nerves X and VIII. The tegmentum of the isthmus also contains a small group of very closely packed CA neurons. The large-sized CA cells of the isthmal group are located dorsolateral to the medial longitudinal fasciculus, partly within the periventricular gray. High densities of CA varicosities were also disclosed in various brain stem structures such as the optic tectum, the torus semicircularis and the cerebeller valvula. In addition, numerous serotonin (5-HT)-type neuronal somata were found in the raphe region of the brain stem, particularly at caudal mesencephalic, isthmal and rostral medullary levels. A large number of CA cell bodies were visualized in the sunfish hypothalamus. Most of them form two populations of small, round cells that are located along and partly within the ependymal walls of the posterior and lateral recesses of the third ventricle. These bipolar cells possess one short club-like process protruding into the ventricle and their thin ependymofugal processes contribute to the CA innervation of numerous hypothalamic regions. Large CA neurons apparently without direct CSF contact also occur in the area of nucleus posterior tuberis, at the level of the mesodiencephalic junction. Although the hypothalamic inferior lobes are devoid of CA cell bodies they are heavily innervated by CA axons. The sunfish telencephalon also receives a strikingly massive and complex monoaminergic innervation. Numerous CA fibers which are first observed at the level of the preoptic area, ascend through the central zone of the telencephalon and arborize profusely particularly within the medial zone of area dorsalis telencephali. Other CA fibers, as well as abundant fine 5-HT varicosities were found in the lateral zone of area dorsalis. Although the exact origin of the telencephalic CA afferents in Lepomis is not known, part of it may arise from the isthmal CA cell group which appears similar to the locus coeruleus of reptiles, birds and mammals.

The structure of the hypothalamic inferior lobes of the blacktip reef shark: scanning and transmission electron microscopic observations

The inferior lobes of the shark hypothalamus were examined with light, transmission and scanning electron microscopy. The cells bordering the floor of the lateral recess appear to be typical liquor-contacting neurons. With scanning electron microscopy (SEM) the apical ends of these cells are seen to bulge into the ventricular lumen. In contrast, the roof is lined by a more typical ependymal cell characterized by numerous cilia and microvilli. In addition, SEM reveals several kinds of supraependymal cells with processes that appear to penetrate the ventricular lining. A periventricular nucleus underlies the ependymal cells. Neurons of the periventricular nucleus contain numerous lipofuchsin granules. The rest of the inferior lobe consists of many neuronal fibers. The morphology of the hypothalamic inferior lobe is discussed in relation to its possible role in feeding and aggressive behavior in both elasmobranchs and teleosts.