Endochondral bone formation is the principal process by which the long bones, e.g., the bones of the axial skeleton and the appendages and digits, increase in length to achieve their adult dimensions. So long as epiphyseal cartilage exists between the diaphyseal and epiphy-seal ossification centers, the bone will continue to grow. Cessation of bone growth is the result of the cessation of interstitial growth of the epiphyseal cartilages. X-ray examination of the bones of late adolescents can determine whether there is still an epiphyseal cartilage plate and, therefore, determine the potential for further growth in bone length and body height.
Endochondral bone formation, epiphysis of long bone, human, H&E ×80; inset ×380.
This is a photomicrograph of an epiphysis at higher magnification than that seen in Plate 13. Different zones of the cartilage of the epiphyseal plate reflect the progressive changes that occur in active growth of endochondral bone. These zones are not sharply delineated, and the boundaries between them are somewhat arbitrary. They lead toward the marrow cavity (M), so that the first zone is fur 250 thest from the cavity. There are five zones:  Zone of reserve cartilage (RC). The cartilage cells of this zone have not yet begun to participate in the growth of the bone; thus, they are reserve cells. These cells are small, usually only one to a lacuna, and not grouped. At some time, some of these cells will proliferate and undergo the changes outlined for the next zone.  Zone of proliferating cartilage (PC). The cells of this zone are increasing in number; they are slightly larger than the reserve cells and close to their neighbors; they begin to form rows.
 Zone of hypertrophic cartilage (HC). The cells of this zone are aligned in rows and are significantly larger than the cells in the preceding zone.
 Zone of calcified matrix (CM). In this zone the cartilage matrix is impregnated with calcium salts.
 Zone of resorption (R). This zone is represented by eroded cartilage that is in direct contact with the connective tissue of the marrow cavity. Spicules (actually a honeycomb at the level of the advancing blood vessels) of cartilage are formed because the pericapillary cells invade and resorb in spearheads rather than along a straight front. Specifically, the pericapillary cells break into the rows of hypertrophied chondrocytes, temporarily leaving the calcified cartilage (C) between the rows of cells. In this manner, spicules of calcified cartilage are formed. Endochondral bone (EB) is then deposited on the surfaces of these calcified cartilage spicules by osteoblasts (Ob), thus forming mixed spicules as seen in the inset.
Endochondral bone formation, epiphysis of long bone, human, H&E ×150; inset ×380.
This is a higher magnification of the lower middle area of upper figure. It shows calcified cartilage spicules on which bone has been deposited. In the lower portion of the figure, the spicules have already grown to create anastomosing bone trabeculae. These initial trabeculae still contain remnants of calcified cartilage, as shown by the bluish color of the cartilage matrix (compared with the red staining of the bone). Osteoblasts (Ob) are aligned on the surface of the spicules, where bone formation is active.
The upper inset shows the surface of several spicules from the left circle in lower figure, at higher magnification. Note the osteoblasts (Ob), some of which are just beginning to produce bone in apposition to the calcified cartilage (C). The lower right of the inset shows bone (EB) with an osteocyte (Oc) already embedded in the bone matrix.