BONE GROWTH

Cartilage grows continually on the side of the epiphyseal plate facing the epiphysis of a long bone, while on the opposite side of the plate facing the diaphysis, cartilage breaks down continually and is replaced by bone (see Plate 1-12). These epiphyseal growth plates persist during the entire postnatal growth period. The plates are finally resorbed and replaced by bone that joins the epiphyses permanently to the diaphysis when the skeleton has acquired its adult size. The epiphyses unite with the diaphysis sooner in females than in males, so that growth in length ceases about 2 years earlier in females. In males, most fusions of the epiphyses with the diaphyses end at about age 20. Interference with the normal growth occurring at the epiphyseal plates of the appendicular skeleton results in abnormally short limbs, such as those of an achondroplastic dwarf who may have a head and body of normal length.

Peripheral growth of a typical flat bone of membrane origin occurs at the margins that articulate via connective tissue with other flat bones. At first, these articulations are broad. At certain intervals between the growing skull bones, even wider gaps known as fonticuli, or fontanels, occur (see Plate 1-7). Of these large, soft spots, the two sphenoid fonticuli may become nearly obliterated as early as 6 months after birth, whereas the two mastoid fonticuli and the single anterior fonticulus are nearly obliterated by age 2. Obliteration of the narrow intervals between the bones of the calvaria, the sutures, does not begin until about age 30.

Growth in width of a flat membrane bone and a long endochondral bone is similar. The osteoblasts of the periosteum of both the outer and inner tables of a flat bone and of the surface of a long bone lay down bone in the form of subperiosteal circumferential layers, or lamellae, that are parallel to the bone surface (see Plate 1-13). To prevent an overly thick mass of compact bone from forming as the bone grows in width, bone is resorbed concomitantly at the endosteal surface bordering the marrow cavity. This laying down of bone at the surface involves a peripheral shift of osteons that retains the necessary distance between the intrinsic blood supply and the osteocytes of the bone. There is an eccentric resorption of osteons on the side facing the outer surface of the widening bone. The bone resorption is the result of progenitor cells within the central haversian canal modulating into osteoclasts, as well as osteoclastic activity of the osteocytes within the lacunae of the circular lamellae in the path of bone erosion. The dissolution of their surrounding matrix by individual osteocytes is known as osteocytic osteolysis. When these osteoclastic osteocytes are released from their lacunae, they may fuse with each other to form multinucleated osteoclasts. Plate 1-13 shows the sequence of events in this destruction of osteons and the formation of new ones.