Morphological features of the surface of the subcommissural organ and aqueduct in the red necked wallaby (Wallabia rufogrisea).

The subcommissural organ and aqueduct of the red necked wallaby were examined using scanning electron microscopy. The surface of the subcommissural organ was similar to that of the possum brain and was superficially similar to that of the rat. However, on close examination, the apical protrusions composing the eutherian subcommissural organ were not present and tight clusters of microvilli were noted instead. Caudal to the subcommissural organ and on each side of the mesocoelic recess there were ciliated paramedian folds. These have been described in the possum but have not been noted in any eutherian mammalian brain. It is believed that these structures are peculiar to marsupial brains. Zones B and C (caudal to the subcommissural organ) represent a superior convex bulge into the aqueduct and then a concavity in the roof of the aqueduct, respectively. Both zones had sparsely ciliated ependyma, the cell surfaces being covered by microvilli. The most caudal zone, C, was much wider and longer than zone B. A line of cells in the roof of the aqueduct similar to zone C has been observed in eutherian mammalian brains in which a dorsal dilation is present. Furthermore, a ridge of long thin ependymal cells has been noted in the roof of the mesencephalon in prenatal rats as early as 16 days of gestation. It is postulated that this region is a circumventricular organ present in ontogeny in all mammals, persisting into the adult only in those with a dorsal dilation in the aqueduct.