The opisthonephric kidney of the sea lamprey of the great lakes,Petromyzon marinus L. IV. Intermediate,distal, and collecting segments of the adult

Both short and long intermediate segments are found in the opisthonephric kidney of the adult sea lamprey and they possess morphologically distinct cell types. Wide lateral intercellular spaces appear between both the cuboidal cells of short segments and the squamous cells of long segments. Periodically the epithelium of the long segment is reduced in thickness providing for a close relationship between the interstitium and the tubular fluid. These features indicate that the segment may be involved in urine formation. The distal segment is divisible into two distinct portions: a straight pars recta and a twisted pars convoluta. The cuboidal cells of the pars recta and the columnar cells of the pars convoluta are characterized by abundant smooth endoplasmic reticulum and mitochondria, features reminiscent of cells involved in ion transport. Multivesicular bodies, coated vesicles, and dense apical tubules suggest that cells of both regions may also be involved in endocytosis. The cells of the collecting segment have few organelles but they possess a cytoskeleton of coarse filaments and wide intercellular spaces. The parallel arrangement of the intermediate, distal, and collecting segments and of the blood vessels within the adult kidney is suggestive of a functional unit similar to that found in the medulla of the mammalian kidney.