At this stage, the perichondrial cells in the midregion of the cartilage model no longer give rise to chondrocytes. Instead, bone-forming cells or osteoblasts are produced. Thus, the connective tissue surrounding this portion of the cartilage is no longer functionally a perichondrium; rather, because of its altered role, it is now called periosteum. Moreover, because the cells within this layer are differentiating into osteoblasts, an osteogenic layer can now be identified within the periosteum. Because of these changes, a layer of bone is formed around the cartilage model (Plate 13, page 248). This bone can be classified as either periosteal bone, because of its location, or intramembranous bone, because of its method of development. In the case of a long bone, a distinctive cuff of periosteal bone, the bony collar, is established around the cartilage model in the diaphyseal portion of the developing bone. The bony collar is shown in illustration 2 of Figure 8.17.
With the establishment of the periosteal bony collar, the chondrocytes in the midregion of the cartilage model become hypertrophic.
As the chondrocytes enlarge, their surrounding cartilage matrix is resorbed, forming thin irregular cartilage plates between the hypertrophic cells. The hypertrophic cells begin to synthesize alkaline phosphatase; concomitantly, the surrounding cartilage matrix undergoes calcification (see illustration 3 of Fig. 8.17). The calcification of the cartilage matrix should not be confused with mineralization that occurs in bone tissue.
FIGURE 8.17 • Schematic diagram of developing long bone. Illustrations 1 to 10 depict longitudinal sections through the long bone. The process begins with the formation of a cartilage model (1); next, a periosteal (perichondrial) collar of bone forms around the diaphysis (shaft) of the cartilage model (2); then, the cartilaginous matrix in the shaft begins to calcify (3). Blood vessels and connective tissue cells then erode and invade the calcified cartilage (4), creating a primitive marrow cavity in which remnant spicules of calcified cartilage remain at the two ends of the cavity. As a primary center of ossification develops, the endochondral bone is formed on spicules of calcified cartilage. The bone at the ends of the developing marrow cavity constitutes the metaphysis. Periosteal bone continues to form (5); the periosteal bone is formed as the result of intramembranous ossification. It can be recognized histologically because it is not accompanied by local cartilage erosion, nor is the bone deposited on spicules of calcified cartilage. Blood vessels and perivascular cells invade the proximal epiphyseal cartilage (6), and a secondary center of ossification is established in the proximal epiphysis (7). A similar epiphyseal (secondary) ossification center forms at the distal end of the bone (8), and an epiphyseal cartilage is thus formed between each epiphysis and the diaphysis. With continued growth of the long bone, the distal epiphyseal cartilage disappears (9), and finally, with cessation of growth, the proximal epiphyseal cartilage disappears (10). The metaphysis then becomes continuous with the epiphysis. Epiphyseal lines remain where the epiphyseal plate last existed.
The calcified cartilage matrix inhibits diffusion of nutrients, causing the death of the chondrocytes in the cartilage model.
With the death of the chondrocytes, much of the matrix breaks down, and neighboring lacunae become confluent, producing an increasingly large cavity. While these events are occurring, one or several blood vessels grow through the thin diaphyseal bony collar to vascularize the cavity (see illustration 4 of Fig. 8.17).
Mesenchymal stem cells migrate into the cavity along the growing blood vessels.