The fine structure of the elasmobranch renal tubule: intermediate, distal, and collecting duct segments of the little skate.

Sharks, skates, and rays (Elasmobranchii) have evolved unique osmoregulatory strategies to survive in marine habitats. These adaptations include a complex renal countercurrent system for urea retention. The fine structure of the complete renal tubular epithelium has yet to be elucidated in any species of cartilagenous fish. The present study, which is a companion to our recent paper describing the ultrastructure of the neck and proximal segments of the elasmobranch nephron, uses thin sections and freeze-fracture replicas to elucidate the fine structural organization of the intermediate, distal, and collecting duct segments of the little skate, Raja erinacea, renal tubule. The epithelium of the intermediate, distal, and collecting duct segments consists of two major cell types: nonflagellar cells, the major epithelial cell type; and flagellar cells, described elsewhere. The intermediate segment consists of six subdivisions lined by cuboidal-columnar cells with variously elaborated microvilli and interdigitations of lateral and basal cell plasma membranes, as well as some subdivisions with distinctive vesicles and granules. The distal segment consists of two subdivisions, both of which are lined by a simple epithelium, and are distinguished from each other by their distinctive contents; dense bodies and granules. The collecting duct segment also has two subdividions, the first lined by a simple columnar epithelium and the second by a stratified columnar epithelium. Both subdivisions have apical secretory granules. The present findings show a more highly specialized and diverse epithelium lining the renal tubule of these cartilagenous fish than is found in either of the "adjacent" phylogenetic taxa, Agnatha or Ostheichthyes, suggesting significant differences among these groups in transepithelial transport mechanisms and renal function.