The ependyma of the saccular pineal gland in the non-eutherian mammal Trichosurus vulpecula

Summary The pineal gland in the possum is represented by a thickening in the wall of the pineal recess. A superficial pineal body and a pineal stalk are characteristically lacking.The ependyma related to the gland is specialized but differs markedly from the lining in other circumventricular organs in form and in surface morphology. Two distinct topographic zones have been recognized. In the middle is a mass of cells which form a prominent knobby-surfaced central zone. These cells are characterized by the absence of cilia, the paucity of microvilli and blebs and the presence of processes which overlap adjacent cells. A surface pattern formed of cell outlines was lacking. It is suggested that the central zone is lined by pinealocytes, supporting cells and the processes of both cell types. Most of the central zone is surrounded by an intermediate zone of variable width. The latter region has been observed to possess a circumventricular organ-type surface morphology. It is sparsely ciliated, almost totally covered by a carpet of microvilli and it exhibits a variety of surface specializations. Supraependymal cells and various transitory supraependymal cell processes are also present.Outside the specialized ependyma is the peripheral zone which like the regular ventricular lining is densely ciliated. Supraependymal processes are found among the clusters of cilia, or rarely, on the surface of the ciliary bed.Season and sex related differences in surface ultrastructure were not observed.