Preliminary electron microscopic observations on ependymal cells in the caudal segment of the filum terminale and the ampulla caudalis of Macaca fascicularis and Cercopithecus griseoviridis (primates, Cercopithecidae).

In a previous paper, the concept of the terminal organ (TO) of the subcommissural complex was forwarded. Functionally this complex is a neuro (glio-) hemal organ which serves to discharge the Reissner's secretory material into the systemic circulation. The TO is characterized by structural specializations that make feasible the discharge and chemical decomposition of the secretory material stowed in the massa caudalis (MC). The TO is probably not only the ampulla caudalis (AC); it may comprise even parts of the filum terminale next to the AC. The boundary of the TO is uncertain as yet. It cannot be precluded that the AC, which itself varies in shape and size, is just a receptaculum massae caudalis. The material of the MC escapes from the AC either through apertures of the wall of the AC or of the filum terminale (Neuropori caudalis, slit-shaped gaps). It is also likely that the secretory material becomes chemically decomposed in the AC and is intra- (trans-) cellularly discharged. In this connexion, certain ependymal cells may be of significance. These cells exhibit large, tongue-shaped central projections (temporarily developed?) which bear a considerable number of long microvilli. The significance of these cells probably lies in the enlargement of the cell surface bathing in the CSF which contains the MC. These cells are most abundant in the area of the TO; single, isolated cells of the same type occur in other areas of the ependyma of some primates. This would indicate that the TO does not contain special types of cells not found in other parts of the ependyma, but that the TO differs from other ependymal regions in the density of peculiar cell types.