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Page 186 of 547-
[Effect of long-term suppression and long-term stimulation of ACTH secretion on the ultrastructure of tanycytes[].
The ultrastructure of the tanycyte ependyma in males of Wistar albino rats (160--180 g of weight) was studied under experimental long-term suppression of ACTH secretion and its long-term stimulation. The former effect was accomplished by daily (for 8 days) intraperitoneal administration of dexamethasone phsophate; the stimulation of ACTH secretion was achieved by bilateral adrenalectomy. The long-term suppression of the hypophyseal adrenocorticotrophic function was followed by the ultrastructural markers of an increased transport and secretory activity in the tanycytes (mainly in beta-tanycytes, i. e. the ependyma lining floor of the third ventricle): the increased amount of microvesicles in the Golgi area and in the vicinity of the particular membranous cavity system (MCS, according to the authors' definition), the appearance of some figures of exocytosis, the elevated amount of lipid inclusions. On the contrary, the long-term stimulation of ACTH secretion was followed by the ultrastructural changes of the opposite character. The data obtained are in agreement with the previously established fact that there is a negative correlation between the tanycyte activity and the hypophyseal adrenocorticotrophic activity. It is supposed that this correlation is regulated by a feedback mechanism and attest to the involvement of tanycytes in the inhibitory control of ACTH secretion. A possible substrate inhibiting ACTH secretion by the tanycytes is discussed.
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On degeneration of peptidergic neurosecretory fibres in the albino rat.
Three types of degenerating peptidergic neurosecretory fibres have been found in the posterior pituitary of chronically dehydrated albino rats. "Dark" neurosecretory fibres and their swellings contain neurosecretory granules, neurotubules, shrunken mitochondria and diffusely distributed fine dense material. Some swellings are filled with synaptic vesicles and/or conglomerations of dense membranes. The transitional forms exist between these fibres and extracellular accumulations of electron dense material. Synaptic vesicles, single neurosecretory granules, lipid-like droplets and lamellar bodies occur in the latter. Some neurosecretory fibres and swellings have numerous polymorphous inclusions arising due to degradation of secretory inclusions and organelles, mitochondria and neurotubules in particular. "Dark" neurosecretory elements and those with numerous polymorphous inclusions are enveloped by pituicyte cytoplasm. Sometimes the plasma membranes both of the pituicytes and neurosecretory fibres are destroyed or transformed into a multi-membrane complex. It is assumed that pituicytes may phagocytize degenerating neurosecretory elements. N urosecretory fibres with a locally dissolved neuroplasm and/or large lucent vacuoles seem to be due to axonal degeneration by the "light" type. These neurosecretory elements, the largest of them in particular, may transform into large cavities bordered by a membrane and containing flake-like material and single-membrane vacuoles. Degeneration of neurosecretory elements seems to occur mainly due to hyperfunction of the hypothalamo-hypophysial neurosecretory system.
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Vesicular structures in the dendrites of an insect olfactory receptor.
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Fine structure of pars neuro-intermedia of the loach, Misgurnus fossilis L.
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Ultrastructural characterization of the central component of the medial preoptic nucleus.
The fine structure of the sexually dimorphic central part of the medial preoptic nucleus was studied in male and female rats. Neurons of this subnucleus are characterized by having convoluted nuclear membranes and fibrillar nuclear inclusions. Within the perinuclear domain there is a well developed neurosecretory apparatus, i.e., Golgi complexes, assorted types of secretory granules, and lysosome-like bodies, whereas the marginal cytosol contains stacked cisterns of rough endoplasmic reticulum, nucleolus-like inclusions, and few mitochondria. The neuropil is made up of thick dendrites, fascicles of thin unmyelinated axons, and few small myelinated axons. Additionally four types of synaptic boutons were found according to the morphology of their vesicular contents. No apparent qualitative sex differences were noted. These cytologic features which suggest secretory activity, coupled with reports that have shown the presence of several neurohormones and peptides within the central part of the medial preoptic nucleus lend further support to the neuroendocrine nature of these neurons.
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Morphology of radial glia, ependymal cells, and periventricular neurons in the optic tectum of goldfish (Carassius auratus).
There are three populations of cells in the deep layers of the optic tectum in a normal adult goldfish: the periventricular neurons, the ependymal cells, and the radial glia. The characteristic morphological features which distinguish the three cell populations are examined at light and electron microscopic levels in the present work. A radial glial cell has a deeply invaginated nucleus located in a subependymal layer. Its cytoplasm contains mitochondria with 35-nm dark granules and 20-nm microtubules but no intermediate filaments. Its prominent radial process extends through the superficial tectal layers. In contrast, the processes of ependymal cell ramify and interweave within the ependymal region. The cytoplasm of an ependymal cell contains prominent bundles of intermediate filaments but not microtubules. Its soma lies at or near the ventricular surface. A periventricular neuron has a round nucleus and a smooth dendrite which extends toward the superficial tectal layers. Its cytoplasm contains microtubules and agranular mitochondria. Axosomatic and axodendritic synapses are found on periventricular neurons. The morphological characteristics of these cell types are considered in relation to previous descriptions of teleost tectal cytology and with regard to the atypical natures of the cytoskeletal elements of the ependymal and radial glial cells.
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Immuno-electron-microscopic identification of O-antigen-bearing oligodendroglial cells in vitro.
Monoclonal antibodies to cell-surface antigens of oligodendrocytes (Sommer and Schachner 1980; Schachner et al. 1980) were used to identify this cell type by immuno-electron microscopy in monolayer cultures of fetal and early postnatal mouse cerebellum. The ultrastructural features of antigen-positive cells confirm that they are immature and mature oligodendrocytes, but not neurons, astrocytes or fibroblasts or fibroblast-like cells. Type I oligodendrocytes are the immature ones with a relatively large amount of moderately electron-lucent cytoplasm, clusters of ribosomes and complex networks of rough endoplasmic reticulum. Large numbers of mitochondria and microtubules, but not intermediate-sized filaments are seen in these cells. They comprise more than 90% of all 0-antigen-positive cells. Type II cells comprise only approximately 5% of all 0-antigen-positive cells. They are characterized by a limited amount of electron-dense cytoplasm, which appears more compact and granular than in type I cells. The rough endoplasmic reticulum is distributed evenly throughout the cytoplasm. Microtubules and mitochondria are present, but more difficult to distinguish due to the compactness of the cytoplasm. Type II cells display the more mature ultrastructural features of oligodendrocytes.
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Naturally occurring neuron death in the optic layers of superior colliculus of the postnatal rat.
Application of light and electron microscopic techniques to the superior colliculus of the normal rat shows that a number of neurons die within the first week after birth. Cells in the earliest recognizable stages of degeneration are characterized by an overall increase in electron density and dilation of the intracellular cisternae, although there are only minimal changes in the chromatin pattern of the nucleus. Synapses are found on these cells. A second type of degenerating cell, with more striking changes in the nucleus, also appears in the tissue but very infrequently. In later stages of degeneration, cells are reduced to a condensed chromatin mass surrounded by disrupted fragments of the rest of the cell. This 'cellular debris' is found within glial cytoplasm. The majority of these debris profiles appear in the first postnatal week and are usually most abundant in the caudal third compared to either the rostral or middle thirds of the colliculus. The results suggest that the mechanisms controlling neuron number in the superior colliculus are operative in the postnatal rat but argue against a simple relationship between the survival of a particular neuron and the total number of optic connections that neuron has received.
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Double-walled coated vesicle formation: evidence for massive and transient conjugate internalization of plasma membranes during cerebellar development.
A massive and transient increase in the formation of double-walled coated vesicles (DWCVs) from surface membranes during late cerebellar development is reported here. These structures are characterized by an outer vesicle (65 nm in diameter), bearing a 15 nm thick spiny coat, containing an inner vesicle (30 nm in diameter). DWCVs occur free in the cytoplasm or attached to the plasma membranes. In the latter case, the membrane of the outer vesicle can be seen to be an invagination of the plasmalemma of the parent cell process while the membrane of the inner vesicle is an evagination of the plasmalemma of the adjacent cell process. DWCVs were observed in a variety of cellular elements in the granular and molecular layers of immature mouse cerebellum, including axons, dendrites, glia and cell bodies. Morphometric analysis revealed that the number of DWCVs in cerebellar mossy terminals became elevated between 16 and 37 days of age and reached a peak 45--50 times higher at 20 days than at either 10 or 70 days of age. The data suggest that a massive conjugate internalization of apposed plasma membranes occurs during late postnatal development which may serve to remodel neural membranes.
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Embryonic development and secretory differentiation in the pars tuberalis of the mouse hypophysis.
The pars tuberalis (PT) of the mouse, like that of other mammals, consists mainly of glandular cells rich in glycogen and peculiar to this lobe. In the mouse, the glandular cells are characterized by large, dense secretory vesicles (up to 300 nm in diameter), the abundance of which indicates a marked secretory activity. The PT develops from a distinct antero-ventral area of Rathke's pouch. The border between the anlagen of the PT and the pars distalis is formed by Atwell's recessus which represents the access for the vessels afferent to the pars distalis. The pedicle of Rathke's pouch is incorporated into the PT anlage, thus contributing to its formation. The entire PT anlage is characterized by glycogen accumulation from the commencement of its formation and persisting in the adult tuberal lobe. Secretory differentiation of the glandular cells of the PT occurs at day 12 of gestation, preceding that of all other adenohypophysial cell types. The secretory features of these cells (development of ergastoplasm and Golgi apparatus, abundance of dense secretory vesicles) appear at an early stage of the enbryonic life (14 days) comparable to those of mature cells. These results confirm earlier observations in the foetal rat where hypophysial secretion also begins in the PT. The existence of peculiar glandular cells speaks in favour of a specific but still unknown function of the PT during foetal and adult life.