CAPILLARY ENDOTHELIAL CELLS OF ANTERIOR PITUITARY SHOULD BE EXCLUDED FROM RETICULO-ENDOTHELIAL SYSTEM - LIGHT AND ELECTRON MICROSCOPIC OBSERVATIONS ON RABBITS AND RATS

Most capillary endothelial cells of the anterior pituitary of rats and rabbits are not vitally stained by acid dyes and india ink, while the pericapillary cells are intensively stained by acid dyes, especially by trypan blue. In electron micrographs, capillary endothelial cells have few lysosomes in their relatively scanty cytoplasm. These cells do not react markedly to repeated injections of acid dye whereas most pericapillary cells are increased in number and size after repeated injections of acid dye and show numerous lysosomal structures in the cytoplasm. These cells are considered to correspond to the acid dye-accumulating pericapillary cell observed by light microscopy. From these facts, the present authors emphasize that the capillary endothelial cell of the anterior pituitary should be excluded from the reticulo-endothelial system, as it is a pericapillary histiocyte that is stained vitally by acid dyes. The capillary endothelium has numerous fenestrae of 500 Å diameter, closed by a thin diaphragm. The diaphragm is not trilaminar but mono-layered and there is a dense ring-like structure in its central area. The overall diameter of the ring-like structure is about 200 Å and the light center is about 70 Å in diameter. It is suggested that the highly organized structures of the diaphragm may relate to the passage of substances such as acid dye from the capillary lumen into the pericapillary space. © 1969 Springer-Verlag.