Ultrastructure of the nephron of grey mullets (Mugil cephalus L. and Liza saliens Risso 1810).

Mugil cephalus and Liza saliens nephrons consist of renal corpuscles, neck segments, first, second and third proximal segments, collecting segments and collecting ducts. The glomerular endothelial cells are fenestrated showing a thin diaphragm. Mesangial cells are located around and between the capillaries, suggesting a role in vessel contraction; they show no phagocytic ultrastructural features. An extensive part of the podocyte surface lies flat on the glomerular basement membrane; this characteristic is probably related to the diminishing of the glomerular filtration rate. Podocytes show large multivesicular bodies, lysosomes and vesicles with a dense bar. Large globules in the podocytes point to their possible secretory activity. The first proximal segment is made up of principal cells with a well developed brush border and tightly packed basal infoldings, and scarce ciliated cells. The second proximal segment consists of principal cells, with loose-packed microvilli and numerous deep basal invaginations, ciliated cells and dark cells. The third proximal segment shows cells with irregularly arranged microvilli and a basal labyrinth, and some ciliated cell. The collecting segment and the collecting duct cells were characterized by apical mucous droplets; the latter also show a well developed basal labyrinth and are surrounded by a layer of smooth muscle cells.