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.