ABSTRACT
Postnatal growth in length of long bones of the appendicular skeleton precedes growth in diameter. Growth in length involves both cartilage and bone cells. Chondroblasts continue to proliferate at the growth plate adjacent to the epiphysis to maintain this cartilage plate throughout the growth period. Chondroblasts produce cartilage matrix materials but gradually differentiate into chondrocytes, which produce and maintain mature matrix. As more chondroblasts arise, and accompanying newly synthesized matrix accumulates, mature matrix and chondrocytes begin to abut the diaphysis. Chondrocytes eventually die as a result of their initiation of the ossification process, which prevents diffusion of nutrients to cartilage cells. Ossification of cartilage results in invasion of capillaries, osteoblasts, and, ultimately, osteocytes to further bone formation adjacent to the diaphysis, thus extending its length.[3,4] While a growth plate is present at both the proximal and the distal ends of long bones, the extent of growth is not the same at each end. The differential growth rate varies among bones of an animal, e.g., in some bones the proximal end predominates and in others the distal end does.
