Mesenchymal stem cells residing in the developing periosteum migrate along the penetrating blood vessels and differentiate into osteoprogenitor cells in the bone marrow cavity. Hemopoietic stem cells (HSCs) also gain access to the cavity via the new vasculature, leaving the circulation to give rise to the marrow including all the blood cell lineages. As the calcified cartilage breaks down and is partially removed, some remains as irregular spicules. When the osteoprogenitor cells come in apposition to the remaining calcified cartilage spicules, they become osteoblasts and begin to lay down bone matrix (osteoid) on the spicule framework. Thus, the bone formed in this manner may be described as endochondral bone. This first site where bone begins to form in the diaphysis of a long bone is called the primary ossification center (see illustration 5 of Fig. 8.17). The combination of bone, which is initially only a thin layer, and the underlying calcified cartilage is described as a mixed spicule.
Histologically, mixed spicules can be recognized by their staining characteristics. Calcified cartilage tends to be basophilic, whereas bone is distinctly eosinophilic. With the Mallory stain, bone stains a deep blue, and calcified cartilage stains light blue (Fig. 8.18). Also, calcified cartilage no longer contains cells, whereas the newly produced bone may reveal osteocytes in the bone matrix. Such spicules persist for a short time before the calcified cartilage component is removed. The remaining bone component of the spicule may continue to grow by appositional growth, thus becoming larger and stronger, or it may undergo resorption as new spicules are formed.
Growth of Endochondral Bone
Endochondral bone growth begins in the second trimester of fetal life and continues into early adulthood.
The events described previously represent the early stage of endochondral bone formation that occurs in the fetus, beginning at about the twelfth week of gestation. The continuing growth process that lasts into early adulthood is described in the following section.
Growth in length of long bones depends on the presence of epiphyseal cartilage.
As the diaphyseal marrow cavity enlarges (see illustration 6 of Fig. 8.17), 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
FIGURE 8.18 • Photomicrograph of a mixed bone spicule formed during endochondral bone formation. In this MalloryAzan–stained section, bone has been deposited on calcified cartilage spicules. In the center of the photomicrograph, the spicules have already grown to create an anastomosing trabecula. The initial trabecula still contains remnants of calcified cartilage, as shown by the light-blue staining of the calcified matrix compared with the dark-blue staining of the bone. In the upper part of the spicule, note a lone osteoclast (arrow) aligned near the surface of the spicule, where remodeling is about to be initiated. 275.
zones as illustrated in Figure 8.19 and Plate 14, page 250. During endochondral bone formation, the avascular cartilage is gradually replaced by vascularized bone tissue. This replacement is initiated by vascular endothelial growth factor (VEGF) and is accompanied by expression of genes responsible for production of type X collagen and matrix metalloproteases (enzymes responsible for degradation of cartilage matrix). The zones in the epiphyseal cartilage, beginning with the zone most distal to the diaphyseal center of ossification and proceeding toward that center, are the following.