When the proliferation of new cartilage ceases, the cartilage that has already been produced in the epiphyseal plate continues to undergo the changes that lead to the deposition of new bone until, finally, there is no remaining cartilage. At this point, the epiphyseal and diaphyseal marrow cavities become confluent. The elimination of the epiphyseal plate is referred to as epiphyseal closure. In illustration 9 of Figure 8.17, the lower epiphyseal cartilage is no longer present; in illustration 10, both epiphyseal cartilages are gone. Growth is now complete, and the only remaining cartilage is found on the articular surfaces of the bone. Vestigial evidence of the site of the epiphyseal plate is reflected by an epiphyseal line consisting of bone tissue (see Fig. 8.2).
Development of the Osteonal (Haversian) System
Osteons typically develop in preexisting compact bone.
Compact bone can take several different forms. Compact bone may be formed from fetal spongy bone by continued deposition of bone on the spongy bone spicules; it may be deposited directly as adult compact bone (e.g., the circumferential lamellae of an adult bone); or it might be older compact bone consisting of osteons and interstitial lamellae. The process in which new osteons are formed is referred to as internal remodeling.
During the development of new osteons, osteoclasts bore a tunnel, the resorption cavity, through compact bone.
Formation of a new osteons in compact bone initially involves the creation of a tunnel-like space, the resorption cavity, by osteoclast activity. This resorption cavity will have the dimensions of the new osteon. When osteoclasts have produced an appropriately sized cylindrical tunnel by resorption of compact bone, blood vessels and their surrounding connective tissue occupy the tunnel. As the tunnel is occupied, new bone deposition on its wall begins almost immediately. These two aspects of cellular activity—namely, osteoclast resorption and osteoblast synthesis—constitute a bone-remodeling unit. A bone-remodeling unit consists of two distinct parts: an advancing cutting cone (also called a resorption canal) and a closing cone (Fig. 8.21).
The tip of the cutting cone consists of advancing osteoclasts closely followed by an advancing capillary loop and  pericytes. It
FIGURE 8.21 • 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.