Fine structure of the lateral areas of the rhombencephalic tela of the bullfrog, Rana catesbeiana.

The lateral areas of the rhombencephalic tela of the bullfrog contain long, irregular islands of ependymal cells that are similar in fine structure to the epithelium of the rhombencephalic choroid plexus. These cells are characterized by apical microvilli, numerous mitochondria and pinocytotic vesicles, and basal infoldings of the plasma membrane. Dorsally a basal limina and varying amounts of collagen occur. The pia mater associated with this ependyma includes two cell types. Fibroblast-like, loosely arranged cells without organized junctions line the subarchnoid space. The most abundant cells of the pia in this area, however, contain numerous intermediate filaments and frequent desmosomes. Caveolae lie along their plasma membranes. Closely organized sheets of similar filament-containing cells are also seen in the arachnoid mater of this animal. These findings demonstrate ependymal cells in the lateral areas of the rhombencephalic tela of the bullfrog that have the essential features of choroid plexus epithelium, with ultrastructural characteristics that suggest transport function. They are, however, usually separated from neighboring, non-fenestrated vessels by several layers of leptomeningeal cells joined by desmosomes. The relationship between structure and function of these cells is enigmatic.