Observations on ultrastructure of rabbit mesothelium

Peritoneal mesothelium of the anterior abdominal wall of the rabbit was studied by the electron microscope. Mesothelial cells are flattened with somewhat oval nuclei and rest on a thin homogeneous discontinuous basement membrane separated from the outer cell membrane by a constant space. Attached to the basement membrane is a thick heterogeneous lamina with which collagen fibrils are associated. The cytoplasm shows occasional mitochondria, very few Golgi membranes, scanty granular endoplasmic reticulum and occasional dense microbodies. The cytoplasm is heavily packed with pinocytotic vesicles which open to the inside, to the outside and in the intercellular spaces as well. On the free surface long and unevenly distributed microvilli are present which show pinocytotic vesicles inside them. The presence of side branches taking origin from the main intercellular channel and opening to the outside or to the inside is suggested. Labyrinthine and fusiform intercellular spaces are seen. Occurrence of tight junctions of cell membranes underneath desmosomes in some intercellular boundaries is proposed. It is concluded that pinocytosis dominates the metabolic activities of the cell, which seems to have very little synthetic ability.