Figure 818

Diagram of a bone-remodeling unit. A bone-remodeling unit consists of an advancing cutting cone and a closing cone. The cutting cone formed by osteoclasts is responsible for boring the tunnel or resorption cavity through the compact bone. Its action is initiated within the Haversian canal at the left of the diagram (in the area corresponding to section a). The cutting cone moves along the Haversian canal, in the direction indicated by the arrow, to the area corresponding to section d. Section d shows the cross section through the cutting cone. The resorption cavity is the site where the future osteon is formed by the action of the closing cone, which consists of osteoblasts. These cells begin to deposit the osteoid on the walls of the canal in successive lamellae. Gradual formation of the new bone fills the resorption cavity. Note the deposition of the osteoid deep to the osteoblasts seen in sections b and c. As successive lamellae of bone are deposited, the canal ultimately attains the relatively narrow diameter of the mature Haversian canal, like that shown in section a. The growth reversal line that appears at the outer limits of a newly formed osteon represents a border between the resorption activity of the cutting cone and the bony matrix not remodeled by this activity.

200 CHAPTER 8 I Bouc 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 deposit the organic matrix (osteoid) of bone on the walls of the canal 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.

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