Fine structure of the perivascular space of the Gecko japonicus subcommissural organ

Ontogenetical study of the chick embryo subcommissural organ by lectin-histofluorescence and electronmicroscopy

The secretory activity of the subcommissural organ (SCO) was studied during embryogenesis of the chick (Gallus gallus) using two lectins labelled with fluorescein isothiocyanate, concanavalin A (Con A) and wheat-germ agglutinin (WGA). While WGA labels the apical or ventricular border of the organ, Con A labels both, the apical and vascular poles of the cells. Glycoproteinaceous secretory products, visualized by Con A appear early, at 5 days, in the ependymal epithelium and expand progressively in a rostrocaudal direction. A correlation is established between histofluorescence and the ultrastructural aspects of the ependymocytes. This throws light on the role of the endoplasmic reticulum in the synthesis, storage and transport of the material produced by the SCO, and points to the existence of two poles of exchange between the secretory cells and the extracellular medium, i.e., the ventricular and the vascular one. WGA reactivity at the apical border is linked up with the formation of Reissner's fibre and shows that the secretory product of the SCO cells undergoes at least partly modifications during its intracytoplasmic transport preceding apical discharge.

Vascular and leptomeningeal projections of the subcommissural organ in reptiles. Lectin-histochemical, immunocytochemical, and ultrastructural studies

In the snake, Natrix maura, and the turtle, Mauremys caspica, the basal processes of the ependymal cells of the subcommissural organ project toward the local blood vessels and the leptomeninges. These processes and their endings were studied using aldehyde-fuchsin (AF), periodic-acid Schiff (PAS), periodic-acid silver-methenamine (PA-SM), concanavalin A (ConA), wheat germ agglutinin (WGA), immunoperoxidase staining (employing an antiserum against bovine Reissner's fiber; AFRU), and conventional transmission electron microscopy. For the purposes of comparison, the ventricular cell pole was also analyzed. The secretory material located in the ventricular cell pole and that present in ependymal endings had only a few staining properties in common, i.e., affinity for AF, ConA, and AFRU at a dilution of 1:1000. On the other hand, PAS, PA-SM, WGA, and AFRU at a dilution of 1:200,000 stained the apical (ventricular) secretory material but not the secretory material of the ependymal processes. The histochemical features of the secretory material located in the terminals of ependymal processes, as well as the presence at these sites of numerous rough-endoplasmic-reticulum cisternae and secretory granules, suggest that secretory material may be synthesized in these terminals. The probable fate of this material, i.e., release to the perivascular and leptomeningeal spaces or transport to the ventricular cell pole, is discussed.

Neural elements associated with the subcommissural organ of the brush-tailed possum (Trichosurus vulpecula)

By means of various techniques and on the basis of a large sampling of the brush-tailed possum, Trichosurus vulpecula, a detailed description is given of the morphology of the neural elements associated with the subcommissural organ (SCO) and the leptomeninges on the dorsal surface of the midbrain. It is shown that the SCO of the possum is richly innervated, the nerves reaching the cerebral aqueduct by passing through the organ; there is, however, no evidence of a neural link between the SCO and the pineal gland. No typical axo-glandular synapses were found; however, many axo-glandular synaptoid junctions between axons and secretory cells of the SCO were demonstrable within the parenchyma of the SCO and on the aqueductal surface of the organ. The possible functional implications of the findings are: (1) The secretory activity of the SCO may be modified by nerves associated with it; this is in accord with some recently published experimental studies. (2) The nerves that innervate the SCO may also be responsible for ciliary activity on the aqueductal surface of the SCO, thus directing the apical secretory product of the SCO and debris flowing caudally in the cerebrospinal fluid into the poorly ciliated median groove where Reissner's fibre is found.

Architecture of the basal region of the subcommissural organ in the brush-tailed possum (Trichosurus vulpecula)

The architecture of the basal region of the subcommissural organ (SCO) and the subjacent neuropil was studied in the brush-tailed possum, Trichosurus vulpecula (Marsupialia). Several structural features suggest that the basal mode of SCO-secretion may be as prominent as the well-established apical secretion. Some of the features that speak in favour of basal secretion are: (1) the existence of deep processes of secretion-laden SCO cells which reach and surround the capillaries in the hypendyma and the subjacent neuropil; (2) the presence of perivascular spaces, some of which may contain a material that resembles the secretory product of the SCO; (3) positive staining by means of paraldehyde-fuchsin of secretory material in the pericapillary zone of vessels in the hypendyma and its corresponding neuropil; (4) presence of labyrinths of the basal lamina of capillaries and associated nerves. The presence of nerve and other cell processes adjacent to the perivascular space, labyrinths and capillary wall suggests discharge into the capillaries of material such as the basal secretory product of the SCO. However, the absence of fenestrae in capillary endothelium in the context of the foregoing observations is enigmatic and speaks against the possibility of a conventional release of secretion. Nevertheless, it is possible that a secretory product containing particles of low molecular weight combined with some specific features of the local capillary endothelium, which facilitate transport, may be critical factors making possible transfer of such product(s) from the SCO to the capillaries.