Aspects of the ultrastructure and function of the subcommissural organ in the Mongolian gerbil (Meriones unguiculatus).

The subcommissural organ (SCO) of the Mongolian gerbil has been studied by transmission electron microscopy. The presence of extensive rough endoplasmic reticulum replete with flocculent material, well-developed Golgi complexes, condensing vesicles, electron-opaque and dense granules in the cytoplasm of SCO cells suggests that these cells are involved in protein synthesis. The secretory granules are released at the apical surface of the ependymal cells by exocytosis into the ventricular lumen, where their product condenses to form the Reissner's fiber. From the fact that the intercellular spaces of the ependymal cells of the gerbil SCO are sealed from the ventricular lumen by tight junctions, the accumulation of secretory material within the enlarged intercellular spaces provides ultrastructural evidence of basal secretion. The secretory material observed in the prominent hypendemal intracellular canaliculi, also appears to reach the intercellular spaces. The lack of perivascular spaces and external basal lamina in the gerbil SCO apparently permits basal secretions to reach vasculature and/or leptomeningeal spaces. The presence of smooth pinocytotic and coated vesicles, multivesicular bodies and lysosomes in the cycoplasm of ependymal cells suggests that the uptake of ions and macromolecules from the cerebrospinal fluid may be an additional function of the SCO.