Ultrastructural evidence of a functional heterogeneity among physeal chondrocytes in growing swine

Chondrocytes from the distal ulnar physis of 1- and 5-month-old boars were studied by transmission electron microscopy. Among reserve, proliferating, and early hypertrophic chondrocytes, 2 distinct cell types (dark cells and light cells) were recognized, and in the distal proliferative and early hypertrophic regions, dark cells seemed to be involved in matrix vesicle production. Late hypertrophic light cells seemed to follow 2 lines of maturation. One pathway resulted in cells with a stellate shape, typical of hypertrophic cells in several species other than swine. A 2nd pathway resulted in large oval cells with water-clear cytoplasm. These cells appeared to become encased in a lamina of fine fibrils interspersed with irregularly shaped electron-dense material. The functional significance of these 2 pathways remains unclear.