ULTRASTRUCTURAL AND HISTOCHEMICAL-STUDIES OF THE EPIPHYSEAL PLATE IN NORMAL CHICKS

Background: Chondrocytes in the epiphyseal plate undergo a series of w ell-defined stages, each stage containing a morphologically homogeneou s cell population. However, biochemical studies show that there are so me functionally heterogeneous cell types in the calcifying zone of the chick epiphyseal plate. Methods: We studied the sequence of chondrocy tic maturation in the normal chick epiphyseal plate ultrastructurally and histochemically. Chondrocytes in the calcifying zone were of three distinct types, the appearance of each cell type being closely relate d to the stage of matrix calcification. Results: Clear cells were obse rved in the upper calcifying region, stellate cells appeared in the mi ddle calcifying region, and hypertrophic clear cells appeared in the l ower calcifying region. Rough endoplasmic reticulum (RER) and lysosome -rich cells were found, these being limited to the outermost layers of the calcifying zone and containing ACPase-positive products. Osteocla sts were attached to the matrix near the RER and lysosome-rich cells i n the poorly calcified regions. Conclusion: We hypothesized that each cell type played a different role in the initiation, progression, and maintenance of cartilage calcification. RER and lysosome-rich cells ma y be responsible for the resorption of uncalcified cartilage matrix, t his resulting in induction of the osteoclastic resorption of the calci fied matrix. In addition, the fate of the chondrocytes was twofold: hy pertrophic clear cells died, while the RER and lysosome-rich cells sur vived, suggesting that these cells were transformed into osteogenic ce lls. (C) 1995 Wiley-Liss, Inc.