The opisthonephric kidney of the sea lamprey of the great lakes,Petromyzon marinus L. II. Neck and proximal segments of the tubular nephron

The spatial arrangement of the five basic segments of the tubular nephron of the sea lamprey is reversed between ammocoetes and adults due to differences in the position of the glomus. The initial segment, the ciliated neck segment, resembles the pronephric nephrostome. The proximal segment is divided into two distinct regions, the pars convoluta followed by the pars recta, and has a brush border which reacts positively with the periodic acid-Schiff stain. Ultrastructurally the cells of the proximal segment resemble the corresponding cells of other vertebrates except that they lack the usual infoldings of the basal plasma membrane and they often show intercellular spaces. The mitochondria of these cells often contain filamentous and crystalline material and are usually associated with “flocculent bodies” resembling “non-nucleoid” renal microbodies. The structure of the cells of the proximal segment suggest that they are involved in endocytosis.