Epiphyseal fusion in the human growth plate does not involve classical apoptosis

By the end of puberty, growth ceases and epiphyseal fusion occurs through mechanisms not yet completely understood. Human growth plate tissues were collected in various pubertal stages including a unique late pubertal growth plate, which was about to fuse. Apoptosis was studied by TUNEL staining, immunolocalization of pro- and antiapoptotic proteins, and electron microscopy (EM). Morphologic analyses of the fusing growth plate revealed disorganized, large chondrocytes surrounded by a border of dense, cortical-like bone. In the unfused growth plates, few chondrocytes were TUNEL positive. In contrast, the fusing growth plate contained no single TUNEL-positive cell. Antiapoptotic (Bcl-2 and Bcl-XL) and proapoptotic (Bax, Bad, and cleaved caspase-3) proteins were detected in all growth plate zones without change in intensity during pubertal progression. Expression of antiapoptotic proteins was found in the fusing growth plate but of the proapoptotic proteins only Bad was detected. EM revealed no typical signs of apoptosis or autophagy in any of the growth plates. In contrast, morpohological signs of hypoxia and necrosis were observed. We conclude that classical apoptosis is not likely to be involved in the process of human growth plate fusion.