The presence of a juxtaglomerular apparatus in elasmobranch fish.

UNLABELLED: Previous studies have concluded that a juxtaglomerular apparatus evolved in phylogenetic groups "higher" than elasmobranch fishes. The present study shows for the first time a distinct juxtaglomerular apparatus in four marine elasmobranchs, the spiny dogfish Squalus acanthias), the smooth dogfish (Mustelus canis), the little skate (Raja erinacea), and the cownose ray (Rhinoptera bonasus). Serial semithin sections of these fishes' kidneys reveal the four morphological components of a juxtaglomerular apparatus at the vascular pole of the renal corpuscle: (1) an afferent arteriole surrounded by smooth muscle cells which have granules containing a material exhibiting a periodic substructure comparable to that of renin granules in higher vertebrates; (2) an efferent arteriole usually devoid of smooth muscle cells but having pericyte-like cells; (3) a macula densa portion of the distal tubule juxtaposed between the afferent and efferent arterioles; and (4) elongated, fusiform cells (Goormaghtigh cells), which are in continuity with similar cells of the abundant intra-glomerular mesangium, and fill the space bordered by the distal tubule and by the afferent and efferent vessels. The distal tubule, from the site where it lies close to the afferent arteriole, moves directly toward the urinary pole, frequently indenting the renal corpuscle. Within this indentation, the tubule may be flanked by the efferent vessel, by the extraglomerular mesangium, or by Bowman's capsule only. Facing these structures the tubular epithelial cells possess basally dilated intercellular spaces. Endothelial cells of the efferent vessel(s) are fenestrated, possessing pores which are closed by a diaphragm.
CONCLUSION: marine elasmobranch fish possess the morphological components of a juxtaglomerular apparatus which suggests that these fishes, like most other vertebrates, possess a renin-angiotensin system and a glomerular-tubular feedback mechanism.