The pineal recess of the opossum: a scanning and transmission electron microscope study.

Scanning and transmission electron microscope studies were performed in the pineal recess region of eighteen adult Didelphis albiventris opossums. With the scanning electron microscope, three concentric regions could be observed in the pineal recess: a central zone lined by cells free of cilia or microvilli, or by a meshwork of interdigitating cell processes; an intermediate zone lined by microvillous cells; and a peripheral zone, lined by ciliated cells. The transmission electron microscope revealed that the central zone is lined mainly by pinealocytes. The lining cells are frequently overlaid by a meshwork of pinealocytic and glial processes. A "subsurface labyrinth" (a meshwork of cellular processes underneath the lining cells) is constantly seen in the central zone. The intermediate zone is lined by elongated microvillous ependymal cells, and the peripheral zone by typical ependyma. The large liquor-contacting area in the pineal recess region, as well as the peculiar organization of its surface, suggest a complex interrelationship between the liquor and the pineal gland of the opossum.