Figure 814

Schematic diagram of developing long bone. Illustrations t to 10 depict longitudinal sections; la to 4a depict cross sections through the shaft of the long bone. The process begins with the formation of a cartilage model (1 and la); next, a periosteal (perichondria!) collar of bone forms around the shaft (diaphysis) of the cartilage model (2 and 2a); then, the cartilaginous matrix in the shaft begins to calcify (3 and 3a). Blood vessels and connective tissue cells then erode and invade the calcified cartilage (4 and 4a), creating a primitive marrow cavity in which remnant spicules of calcified cartilage remain at the two ends of the cavity. Endochondral bone forms on these spicules of calcified cartilage. The bone at the ends of the developing marrow cavity constitutes the metaphysis. Periosteal bone continues to form; 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. (From Bloom W, Fawcett DW. A Textbook of Histology. Philadelphia: WB Saunders, 1975.)

period of the individual into early adulthood, is described below.

Growth of long bones depends on the presence of epiphyseal cartilage throughout the growth period

As the diaphyseal marrow cavity enlarges (see illustration 5 of Fig. 8.14), a distinct zonation can be recognized in the cartilage at both ends of the cavity. This remaining cartilage, referred to as epiphyseal cartilage, exhibits distinct zones as illustrated in Figure 8.16. The zones in the epiphyseal cartilage, beginning with that most distal to the diaphyseal center of ossification and proceeding toward that center, are

• Zone of reserve cartilage, which exhibits no cellular proliferation or active matrix production.

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