MORPHOLOGY AND ELEMENT DISTRIBUTION OF MAGNESIUM-SECRETING EPITHELIUM- THE PROXIMAL TUBULE SEGMENT PII OF DOGFISH, SCYLIORHINUS-CANICULUS (L)

Proximal tubule segment PII cells of marine elasmobranch fish were stu died by transmission electron microscopy of thin sections, and X-ray m icroanalysis was performed, with freeze-dried cryosections. Epithelial cells of PII are characterized by high and dense brush border at the epical side, elaborate folding of the lateral cell membrane and large basal extrarellular labyrinth confined by a system of meandering cell extensions. Basal cytoplasmic zone, apical cytoplasmic zone, nuclei, m itochondria and epical small vacuoles were accessible for, x-ray micro analysis. Concentrations of Na, Mg, P, S, Cl and K were different in t he cytoplasmic zones along the basal-apical axis of the cell and in th e organelles. PD[ cells lacked an epical tubulovesicular apparatus, in stead they displayed an apical zone of smooth clear vesicles and small epical vacuoles. After freeze-drying, the small apical vacuoles and t he smooth clear vesicles contained flocculent mass-dense material. Sma ll epical vacuoles showed high concentrations of Mg (229 mmol/kg water ), Na (132 mmol/kg water) and CI (248 mmol/ kg water). Sequestration o f Mg in vesicles and small epical vacuoles and subsequent exocytosis b etween the microvilli of the brush border are supposed to be important steps in the transepithelial transport (tubular secretion) of magnesi um by PII cells of marine fish.