The Primary Ossification Center

The first sign of endochondral ossification is the multiplication and swelling of chondrocytes near the center of the model, forming a primary ossification center. As the lacunae enlarge, the matrix between them is reduced to thin walls and the model becomes weak at this point. It soon gets reinforcement, however. Some cells of the perichondrium become osteoblasts, which produce a bony collar around the model. This collar acts like a splint to provide temporary support for the model, and it cuts off the diffusion of nutrients to the chondrocytes, hastening their death. Once the collar has formed, the fibrous sheath around it is considered periosteum rather than perichondrium.

endo = within + chondr = cartilage

Endochondral Ossification

Figure 7.8 Stages of Endochondral Ossification. (a) Chondrocyte hypertrophy at the center of the cartilage model and formation of a supportive bony collar. (b) Invasion of the model by blood vessels and creation of a primary marrow space. (c) Typical state of a long bone at the time of birth, with blood vessels growing into the secondary marrow space and well-defined metaphyses at each end of the primary marrow space. (d) Appearance of a long bone in childhood. By adulthood, the epiphyseal plates will be depleted and the primary and secondary marrow spaces will be united.

Figure 7.8 Stages of Endochondral Ossification. (a) Chondrocyte hypertrophy at the center of the cartilage model and formation of a supportive bony collar. (b) Invasion of the model by blood vessels and creation of a primary marrow space. (c) Typical state of a long bone at the time of birth, with blood vessels growing into the secondary marrow space and well-defined metaphyses at each end of the primary marrow space. (d) Appearance of a long bone in childhood. By adulthood, the epiphyseal plates will be depleted and the primary and secondary marrow spaces will be united.

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Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

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Buds of connective tissue grow from this periosteum into the cartilage and penetrate the thin walls between the enlarged lacunae. They break down the lacunae and transform the primary ossification center into a cavity called the primary marrow space. Osteogenic cells invade the model by way of the connective tissue buds, transform into osteoblasts, and line the marrow space. The osteoblasts deposit osteoid tissue and then calcify it to form a temporary framework of bony trabeculae. As ossification progresses, osteoclasts break down these temporary trabeculae and enlarge the primary marrow space. The ends of the bone are still composed of hyaline cartilage at this stage.

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