The principal stages of long-bone growth are shown in Figure 2. Long bones begin as cartilaginous regions in the early embryo. They grow as rapidly dividing peripheral cells add new chondrocytes to the outside of the structure and as older cells in the body of the cartilage divide, enlarge, and secrete matrix (Figure 2A).
The oldest chondrocytes (located in the middle) expand, calcify their matrix, and are termed 'hypertrophic' (Figure 2B). Osteoblasts then secrete a bony layer around the midshaft of the cartilage, forming the 'primary bone collar'. This structure is extended and thickened by successive generations of osteoblasts.
Once established, the primary bone collar is penetrated at several points by osteoclasts (Figure 2C). These rapidly erode the calcified cartilage of the interior to leave only a supportive framework inside the bone collar. As the peripheral bone gains in strength, osteoclasts remove this framework to leave the marrow cavity.
Continued growth of long bones: the growth plate Long-bone growth continues at specialized 'epiphyseal growth plates' (Figure 3). Proliferating chondrocytes at the top of the growth plate add new cells, while their more mature descendants secrete matrix and enlarge, thereby producing longitudinal growth. Ultimately, the chondrocytes hypertrophy and calcify their matrix. Osteoclasts present in the marrow cavity invade this calcified cartilage, destroying the horizontal septa separating the chon-drocytes. This leaves vertical bars of calcified cartilage projecting into the marrow cavity, and these act as a framework for subsequent bone deposition.
Complete calcification of the growth plate at the end of puberty marks the end of longitudinal growth.
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