Pit cells in extrahepatic organs of the rat

Electron microscopic examination of the extrahepatic distribution of pit cells, a cell type found in the liver, revealed their existence in several other organs of the rat. They were relatively frequent in lungs, spleen (red pulp), small intestine, epididymis, trachea, and peripheral blood; much fewer in bone marrow and thymus (medulla); and nonexistent in lymph nodes, spleen (white pulp), and thymus (cortex). The pit cells in these organs, as well as in the liver, contained characteristic dense granules and rod-cored vesicles in the cytoplasm. Our observations suggest that pit cells circulating in the peripheral blood adhere to the endothelium of capillaries in the various organs and migrate into the tissue, where they have some special immunological function.