Potassium concentration in membrane-associated particles in the epiphyseal growth plate

Electron-dense particles with a diameter of 50-200 nm have been observed at the cell membrane of chondrocytes in the zone of the initiation and advance of mineralization, using the dark field STEM mode. Electron-probe x-ray microanalysis and laser microprobe mass analysis indicate that these particles contain predominantly K and Na. They appear only in dry thin sections of shock-frozen, freeze-dried embedded tissue and not in sections of water-treated samples; hence they contain water-extractable potassium and sodium. The function of the two elements at these special sites is not yet clear. On the one hand, they might reflect exocytotic processes connected with a Na-K-ATPase; on the other hand, they might exist as a transitory state before being replaced by Ca and phosphate in the mineralizing matrix and later transported elsewhere by the blood vessels.