Proximal tubule segment PII cells of marine elasmobranch fish were studied by transmission electron microscopy of thin sections, and X-ray microanalysis 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, mitochondria and epical small vacuoles were accessible for, x-ray microanalysis. Concentrations of Na, Mg, P, S, Cl and K were different in the cytoplasmic zones along the basal-apical axis of the cell and in the organelles. PD[ cells lacked an epical tubulovesicular apparatus, instead they displayed an apical zone of smooth clear vesicles and small epical vacuoles. After freeze-drying, the small apical vacuoles and the smooth clear vesicles contained flocculent mass-dense material. Small epical vacuoles showed high concentrations of Mg (229 mmol/kg water), Na (132 mmol/kg water) and CI (248 mmol/ kg water). Sequestration of Mg in vesicles and small epical vacuoles and subsequent exocytosis between the microvilli of the brush border are supposed to be important steps in the transepithelial transport (tubular secretion) of magnesium by PII cells of marine fish.
