Cytological evidence for catecholamine-containing sensory cells bordering the ventricle of the toad hypothalamus

Precursors of neurons, neuroglia, and ependymal cells in the CNS: what are they? Where are they from? How do they get where they are going?

Neurons, neuroglia (astrocytes and oligodendrocytes), and ependymal cells are three distinct categories of neural cells in the central nervous system. In the mature brain and spinal cord, the classical histological criteria define these cells by their microscopic structure very well. During development, the precursors for all of these cells reside within the epithelium of the neural plate and its successor, the neural tube. These precursor cells are the undifferentiated, primitive neuroepithelium of the classical literature. As the cerebral vesicles enlarge and their walls thicken, the primitive neuroepithelial cells elongate, maintaining a radial orientation until they migrate. Although many, but not all, of these cells span the extent of the ventricular wall, they are the precursors of neurons, neuroglia, and ependymal cells. Thus, it is useful to retain their classical designation as primitive neuroepithelial cells and to treat them as neural precursor cells. Neural precursor cells are neither neuroglia nor neurons. It is not appropriate to call them radial glial cells anymore than it is to call them radial neuronal cells. The term "radial glia" has long been used to describe the mature, elongated astrocytes, represented by Bergmann cells in the cerebellum and Müller cells in the retina. Inevitably, during development, transitional forms between neural precursor cells and the neurons, neuroglia, and ependymal cells will occur. Such transitional cells are known as neuroblasts, glioblasts, or ependymoblasts, even though they may be postmitotic. Alternative terms are "immature neurons," "immature neuroglia," and "immature ependymal cells." The migration of many neural precursor cells is accomplished by translocation rather than free cellular locomotion. There is both direct and indirect evidence to document the translocation of the nuclear/perikaryal/somal complex through the leading process of primitive neuroepithelial cells. This is conspicuous in the neocortex, where the discrete radial arrangement of pyramidal cells may result from translocation of neuroblasts, while their leading processes still contact the pial surface. Migration by translocation occurs throughout the CNS. GLIA 43:6-18, 2003.

Paraventricular organ of the lungfish Protopterus dolloi: morphology and projections of CSF-contacting neurons

The morphology and projections of neurons in the paraventricular organ (PVO) were studied by means of silver impregnation after intraocular application of cobaltous lysine in the lungfish Protopterus dolloi. Cobalt-labeled neurons were found exclusively in the PVO in the dorsal and infundibular hypothalamus. These bipolar neurons possess one CSF-contacting process that protrudes into the ventricular lumen with a club-shape ending and a thick, ramifying process directed into the hypothalamic neuropil; the ependymofugal processes form intra- and extrahypothalamic projections. Impregnated fibers from paraventricular neurons cross in infundibular and hypothalamic commissures, the commissure of the posterior tuberculum, the postoptic, the habenular, and the anterior commissures. Projections to the infundibulum and the median eminence are relatively sparse; no fibers are labeled in the pituitary gland. Ascending projections to the forebrain are extensive. Major targets include the dorsal hypothalamus, the periventricular preoptic nuclei, the habenula, the subhabenular region, the anterodorsal thalamus, and the medial telencephalic hemisphere (septum). Most ascending fibers follow the medial forebrain bundle; others course in the fasciculus retroflexus and terminate in rostral parts of the ipsilateral habenula. Descending fibers run caudally along the ventral floor of the brainstem. They terminate in the neuropil of the mesencephalic tegmentum, ventral tectum, isthmic region, ventral portions of the reticular formation throughout the rhombencephalon, and extend into the spinal cord. Intraocular application of cobaltous lysine results in selective impregnation of neurons in the PVO and their ascending and descending projections, presumably via uptake of tracer from vascular circulation. These projections do not represent retinofugal or retinopetal projections. We provide conclusive evidence for the existence of a PVO in Protopterus. On the basis of PVO location and acetylcholinesterase histochemistry, we propose subdivisions of the infundibular hypothalamus corresponding to those in amphibians. Ascending PVO projections appear to be particularly well developed in lungfish compared with other species and may be related to specialized endocrine mechanisms in this group of vertebrates.

Changes in formaldehyde-induced fluorescence of the hypothalamus and pars intermedia in the frog, Rana temporaria, following background adaptation

Adaptation of the frog, Rana temporaria, to a white background for 12 hr has resulted in an intense formaldehyde-induced fluorescence (FIF) in the neurons of the preoptic recess organ (PRO), paraventricular organ (PVO), nucleus infundibularis dorsalis (NID) and their basal processes permitting visualization of the PRO- and PVO-hypophysial tracts that extend into the median eminence (ME) and pars intermedia (PI); the FIF is reduced in all the structures by 3 days. In frogs adapted to a black background, for 12 hr and 3 days, there was a general reduction in the FIF of the PRO neurons and PRO-hypophysial tract. By 12 hr black background adaptation, the PVO/NID neurons and only their adjacent basal processes show FIF which was sharply reduced by 3 days, making the PVO-hypophysial tract undetectable. In the PI fibers the fluorescence was more intense in black-adapted frogs than in white-adapted ones at both the intervals studied. The simultaneous changes in the FIF of the hypothalamic nuclei, tracts and PI suggest that the PRO and PVO/NID neurons participate in PI control through release of neurotransmitter(s) at the axonal ends.

The development of monamine-containing neurons in the brain and spinal cord of the salamander, Ambystoma mexicanum

The distribution of monoamine-containing neurons in the CNS of the developing and adult axolotl, Ambystoma mexicanum, has been investigated using the histochemical fluorescence technique of Falck and Hillarp combined with microspectrofluorimetry. The earliest catecholamine-containing neurons to be detected are located in the ventral ependymal zone of the spinal cord at the time of hatching (Stage 41). Between stages 43 and 46, catecholamine fluorescence can be detected in neurons in the following regions: nucleus preopticus, the hypothalamic-infundibular region, and the brain stem reticular formation. 5-HT-containing neurons are only observed in the midbrain raphe region and are first detected at stage 44. In contrast to these early monoamine fluorescing groups, catecholamine-containing neurons are not routinely detectable in the nucleus interpeduncularis until six months of age. All monoamine-containing neuronal groups detected in developing axolotls are also present in both sexes of the adult. However, the fluorescence intensity is less in monoamine-containing neurons observed in adults than in early developing subjects. All catecholamine-containing neuronal groups, with the exception of those located in the midbrain region (nucleus interpeduncularis, reticular zone) have fluorescent processes that contact the cerebrospinal fluid (CSF). The presence of CSF-contacting processes in the hypothalamic and spinal cord regions suggest that the CSF may act as a medium through which bioactive substances are transported from one brain region to another. Intense catecholamine fluorescence is observed in cells of the notochord prior to the detection of the monoamine-containing neurons in the CNS. A possible involvement of catecholamines in the inductive effects of the notochord during development is discussed.

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.

Localization of dopamine in the endocrine hypothalamus of the rat

Microspectrofluorometry, fluorescence histochemistry and light and electron microscopic autoradiography have established the presence of sub-populations of neurons in the arcuate-periventricular region of the rat hypothalamus that sequester both radiolabeled dopamine and demonstrate formaldehyde-induced fluorescence. These characteristics are consistent with a catecholaminergic function. Selective sequestration of 3H-dopamine at the light and ultrastructural level is discussed in the context of an ultrashort loop autoregulatory mechanism for this neuronal population.

The monoaminergic innervation of the telencephalon of the frog, Rana pipiens

A large number of serotonin (5-hydroxytryptamine, 5-HT)-type axon terminals has been visualized in the so-called 'striatum' (pars ventralis and dorsalis) of Rana pipiens by means of the Falck-Hillarp histochemical method. The frog striatum contains, however, a much smaller number of catecholamine (CA) type axon terminals and differs strikingly in that aspect from the neostriatum of 'higher' vertebrates which is known to receive a massive CA innervation. In the telencephalon of Rana pipiens the highest density of CA axon terminals occurs at the level of the medial wall, which appears highly hypertrophied in comparison to the thin lateral wall. The CA terminals are mainly concentrated within nucleus accumbens septi and in the ventrolateral portion of nucleus lateralis septi where they surround intimately the non-fluorescent neuronal somata of both septal nuclei. The three pallial fields (medial, dorsal and lateral) of Rana pipiens are nearly devoid of CA axon terminals. A moderate amount of 5-HT terminals is present, however, within the caudal half of both the medial and dorsal pallial fields. Finally, a few CA-type neuronal somata are found lying amongst the mitral cells of the olfactory bulb. As a whole, the telecenphalon of Rana pipiens appears more profusely innervated by axons of the 5-HT-type than by those of the CA-type. This finding has been related to the fact that in the frog brain stem the CA neuronal somata are scarce in comparison to the highly developed 5-HT neuronal systems.

The primate median eminence. I. Correlative scanning-transmission electron microscopy

A combined scanning/transmission electron microscopic (SEM/TEM) technique was used to analyze the third cerebral ventricle and underlying tissue of the median eminence of 6 mature rhesus monkeys. The same sample of the ventricular wall was subjected to both SEM and TEM. This technique demonstrates two basic subpopulations of supraependymal cells on the surface of the supraoptic, infundibular and mammillary recesses. Type 1 cells are definitely neuron-like in their surface configuration and internal fine structural organization. Type 2 cells are more similar to histiocytes and are not as numerous as type 1 cells. The functional capacity of type 1 cells is discussed in the context of their potential role as a neuronal network that may serve as a short loop autoregulatory mechanism controlling the synthesis of releasing hormones or biogenic amines.

Surface fine structure of supraependymal elements and ependyma of hypothalamic third ventricle of mouse

The ependymal surface of the hypothalamic third ventricle of the adult female and male mouse was examined with the scanning electron microscope. The material was dried with the critical point method. Supraependymal cells, exhibiting a variety of forms, lie among the cilia of the rostrodorsal two-thirds of the ventricle. The cell bodies are usually between 8 and 10 mu in diameter and have dividing pseudopodial processes which may be broad or narrow, flat or stout, smooth or varicosed. The textured surfaces of the cells seen in this material appear to have no cilia, microvilli or membraneous folds though they may have small bleb-like protrusions. Some cells and adjacent cilia bear clusters of friable-looking material that may be secretory. In several female mice, mounds of debris, that could be degenerating cells or secretory material, were seen in intimate association with clusters of cells. The cells' processes extend over and through the surface of the mound. While some supraependymal cells may be neurons or glia, it is suggested that many of the cells seen may be morphological variants of one cell line, mononuclear phagocytes. Sausage-shaped structures at the end of long cilia-like stalks are described and their possible identity discussed.