also contains numerous dividing cells that give rise to osteoblasts, additional pericytes, and endothelial cells. (Recall that osteoclasts are derived from mononuclear hemopoietic progenitor cells.) The osteoclasts drill a canal about 200 min diameter. This canal establishes the diameter of the future osteonal (Haversian) system. The cutting cone constitutes only a small fraction of the length of the bone-remodeling unit; thus, it is seen much less frequently than the closing cone.
After the diameter of the future Haversian system is established, osteoblasts begin to fill the canal by depositing the organic matrix of bone (osteoid) on its walls in successive lamellae. With time, the bone matrix in each of the lamellae becomes mineralized. As the successive lamellae of bone are deposited, from the periphery inward, the canal ultimately attains the relatively narrow diameter of the adult osteonal canal.
Compact adult bone contains Haversian systems of varying age and size.
Microradiographic examination of a ground section of bone reveals that younger Haversian systems are less completely mineralized than older systems (Fig. 8.22). They undergo a progressive secondary mineralization that continues (up to a point) even after the osteon has been fully formed. Figure 8.22 also illustrates the dynamic internal remodeling of compact bone. In the adult, deposition balances resorption. In the aged, resorption often exceeds deposition. If this imbalance becomes excessive, then osteoporosis develops (see Folder 8.2).
Biologic mineralization is a cell-regulated extracellular event.
Mineralization occurs in the extracellular matrix of bone, cartilage and in the dentin, cementum, and enamel of teeth. The matrices of all of these structures except enamel contain collagen fibrils and ground substance. Mineralization is initiated in the same time within the collagen fibrils and in the ground substance surrounding them. In enamel, mineralization occurs within the extracellular matrix secreted by the enamel organ. Despite the extracellular location of biologic mineralization and the fact that physicochemical factors are basic to the process, biologic mineralization is a cell-regulated event.
Mineralization involves the secretion of matrix vesicles into the bony matrix.
In places where the mineralization of bone, cartilage, dentin, and cementum is initiated, the local concentration of Ca2 and PO4 ions in the matrix must exceed the normal threshold level. Several events are responsible for this mineralization:  The binding of extracellular Ca2 by osteocalcin and other sialoproteins creates a high local concentration of this ion.  The high Ca2 concentration stimulates the osteoblasts to secrete alkaline phosphatase (ALP), which increases the local concentration of PO4 ions. The high PO4 concentra where mineralization will be initiated.