A comparative study of sporta perimedullaris musculosa in the renicule of six species of cetaceans

Organs and physiological internal systems of diving mammals display diverse and peculiar features. The kidney of cetaceans has been extensively studied and characterized. This organ is subdivided in small functional units called "renicules" displaying a peculiar structure, the sporta perimedullaris musculosa. A comparative study of renicules was made in six species belonging to mysticetes and odontocetes. in all six species, the sporta, a connective tissue basket, was located between the medullary and the cortical regions of renicules. The sporta is mainly composed of a connective stroma and, in several species, the connective tissue (histochemically studied) was interposed among muscle fibres. Within the six studied species, the sporta varied in thickness, which appears to be related to the number, thickness and discontinuity of collagenous fibre bundles. The presence of smooth muscle fibres, evidenced by immunostaining and by electron microscopy analysis, was variable: whereas fibres were numerous in examined toothed whales (Stenella, Delpbinus, Grampus, Tursiops), they were practically absent in baleen whales such as the fin whale Balaenoptera physalus. The data in the present study shed light on the morpho-functional characterization of diving mammal anatomy and support the need to re-examine the general structure of cetacean phylogeny, in line with the hypothesis of Milinkovitch et al. (1994) and Hasegawa et al. (1997).