An electron microscopic study of the choroid plexuses of necturus maculosus

The choroid plexuses of the neotenous salamander, Necturus maculosus , were examined by light and electron microscopy. Their epithelium was found to consist of a single layer of flattened to cuboidal cells resting upon a well‐developed, continuous basement membrane. The cytoplasm of a typical epithelial cell contained, besides a large, centrally‐placed nucleus, numerous mitochondria, an extensive agranular endoplasinic reticulum, prominent Golgi complexes and a variety of dense bodies. In addition, occasional clusters of ribosomes, lipid droplets, fascicles of fine filaments and numerous glycogen granules were present within the cytoplasm. Specializations of the apical (ventricular) surfaces of the epithelial cells included an irregular brush border, cilia, and minute, possibly micropinocytotic, invaginations of the plasma membrane. Surfaces of contacts between epithelial cells were specialized by “tight junctions” desmosomes, and complex inter dictations of the apposed plasma membranes. The interstitial space separating the epithelial layer from the thin‐walled, non‐fenestrated plexus blood vessels contained a mesh‐work of collagen fibrils and a variety of connective tissue cell types. Electron dense Thorotrast particles were found to traverse the endothelial walls of the blood vessels and accumulate within the interstitial spaces within 30 minutes after being injected into the blood stream. The epithelial basement membrane, however, apparently prevented the tracer particles from reaching the epithelial cells, and the ventricular spaces beyond, as long as five hours after injection.