Pineal N-acetyltransferase and hydroxyindole-O-methyltransferase: control by the retinohypothalamic tract and the suprachiasmatic nucleus.

The limits of this subject are not easily tIefined since all cells can be regarded as absorptive at least to the extent that they take UI) substances from their environment to meet their own inetabolic needs. As a morphologist, however, I must necessarily confine the discussion 1) to those examples of absorption in bulk that are accoml)anied by microscopically visible evidence of active movements of the cell surface and 2) to those relatively stable differentiations of the surface that are commonly interpreted as devices for amplifying the area of membrane across which absorption takes place at a molecular level by passive diffusion or active transfer.

Surface specializations of absorbing cells.

After impregnation of goldfish retina by the rapid Golgi method, two classes each of photoreceptor, bipolar, and horizontal cells were observed by light microscopy. Interconnections between these elements in the outer plexiform (first synaptic) layer were investigated by electron microscopy of ultrathin sections, in which the processes of impregnated cells are easily distinguished. Dendrites of large bipolar cells (Cajal's “bipolaires destinees aux bâtonnets”) appeared to contact the synaptic endings of both rods and cones, while those of small bipolars (Cajal's “bipolaires destinees aux cones”) appeared to contact only cones. Processes from horizontal cells of the vitread level (Cajal's “cellules horizontaux intermediaires”) appeared to contact only rods, while those from horizontal cells of the sclerad level (Cajal's “cellules horizontaux externes”) appeared to contact only cones. The structures formerly called “synaptic vacuoles” are the terminals of horizontal cell processes in the goldfish, and by analogy they should be so identified in all vertebrates. Teleostean horizontal cells are not typical of neurons or glia cells, but are morphologically intermediate between them. Their most interesting properties are their unique relationship to photoreceptor synaptic endings and their segregation into rod and cone subsystems along with the corresponding photoreceptor and bipolar cells. Although their specific function is not clear, some role in information processing therefore appears likely.

The structure and relationships of horizontal cells and photoreceptor-bipolar synaptic complexes in goldfish retina.

The Mammalian Pineal Gland: Structure and Function

Filaments and fibrils that exhibit a 100-nm axial periodicity and occur in the medium and in the deposited extracellular matrix of chicken embryo and human fibroblast cultures have been tentatively identified with type VI collagen on the basis of their similar structural characteristics (Bruns, R. R., 1984, J. Ultrastruct. Res., 89:136-145). Using indirect immunoelectron microscopy and specific monoclonal and polyclonal antibodies, we now report their positive identification with collagen VI and their distribution in fibroblast cultures and in tendon. Primary human foreskin fibroblast cultures, labeled with anti-type VI antibody and studied by fluorescence microscopy, showed a progressive increase in labeling and changes in distribution with time up to 8 d in culture. With immunoelectron microscopy and monoclonal antibodies to human type VI collagen followed by goat anti-mouse IgG coupled to colloidal gold, they showed in thin sections specific 100-nm periodic labeling on extracellular filaments and fibrils: one monoclonal antibody (3C4) attached to the band region and another (4B10) to the interband region of the filaments and fibrils. Rabbit antiserum to type VI collagen also localized on the band region, but the staining was less well defined. Control experiments with antibodies to fibronectin and to procollagen types I and III labeled other filaments and fibrils, but not those with a 100-nm period. Heavy metal-stained fibrils with the same periodic and structural characteristics also have been found in both adult rat tail tendon and embryonic chicken tendon subjected to prolonged incubation in culture medium or treatment with adenosine 5'-triphosphate at pH 4.6. We conclude that the 100-nm periodic filaments and fibrils represent the native aggregate form of type VI collagen. It is likely that banded fibrils of the same periodicity and appearance, reported by many observers over the years in a wide range of normal and pathological tissues, are at least in part, type VI collagen.

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Type VI collagen in extracellular, 100-nm periodic filaments and fibrils: identification by immunoelectron microscopy.

This chapter presents an electron-microscopic study of the epiphyseal cell and its histological disposition with an emphasis on the morphological evidence of secretion. Detection of histological design in the pineal body is complicated by divergent results of fixation by immersion and perfusion. Cytological preservation is suboptimal, following immersion fixation, and it is indicated by the vesiculation of cytoplasmic membranes, the extraction of lipid droplets, the appearance of nuclear inclusions, and the slight swelling of the cells without conspicuous dilution of their cytoplasm. Such subtle swelling generates an artifactitious histological compactness resembling the texture of a gland or epithelioid body. Fixation of the epiphysis by intravascular perfusion yields the best cytoplasmic preservation, even when accompanied by a variable widening of the clefts between epiphyseal cells and the diminution of amorphous interstitial material, a phenomenon that occurs chiefly in the central portion of the pineal body.

The epiphyseal cell: an electron-microscopic study of its intercellular relationships and intracellular morphology in the pineal body of the albino rat.

A UNIQUE intercellular material has been found beneath the subcommissural organ of brains of young adult albino rats of both sexes and of two strains. In general, the spaces around cerebral arterioles and venules are bounded by typical basement membrane and contain occasional fibrocytes, small bundles of collagen and macrophages. However, at the vascular poles of the cells of the subcommissural organ, a remarkable striated material frequently occupies the perivascular space and may also be seen intercellularly as much as 4–5µ from a vessel. The space around cerebral capillaries is ordinarily occupied solely by basement membrane, but in this very localized area the space may become widened. The basement membrane then splits into endothelial and parenchymal laminae which contain between them the striated intercellular material.

David E. Wolfe

Papers

The epiphyseal cell: an electron-microscopic study of its intercellular relationships and intracellular morphology in the pineal body of the albino rat.

A striated intercellular material in rat brain.