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 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 apical side, elaborate folding of the lateral cell membrane and large basal extracellular labyrinth confined by a system of meandering cell extensions. Basal cytoplasmic zone, apical cytoplasmic zone, nuclei, mitochondria and apical 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. PII cells lacked an apical tubulovesicular apparatus, instead they displayed an apical zone of smooth clear vesicles and small apical vacuoles. After freeze-drying, the small apical vacuoles and the smooth clear vesicles contained flocculent mass-dense material. Small apical vacuoles showed high concentrations of Mg (229 mmol/kg water), Na (132 mmol/kg water) and Cl (148 mmol/kg water). Sequestration of Mg in vesicles and small apical 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.