FIGURE 8.3 • Diagram of a section of compact bone removed from the shaft of a long bone. The concentric lamellae and the Haversian canal that they surround constitute an osteon (Haversian system). One of the Haversian systems in this diagram is drawn as an elongated cylindrical structure rising above the plane of the bone section. It consists of several concentric lamellae that have been partially removed to show the perpendicular orientation of collagen fibers in adjacent layers. Interstitial lamellae result from bone remodeling and formation of new Haversian systems. The inner and outer surfaces of the compact bone in this diagram show additional lamellae—the outer and inner circumferential lamellae—arranged in broad layers. The inner circumferential lamella is covered by a thin layer of endosteum that faces the marrow cavity, similar to the outer surface of the bone, which is covered by periosteum. Branches of nutritional arteries accompanied by small veins are shown within the Haversian and Volkmann’s canals. These arteries also supply the periosteum, endosteum, and bone marrow.
called interstitial lamellae (see Fig. 8.3). Because of this organization, mature bone is also called lamellar bone.
The long axis of an osteon is usually parallel to the long axis of the bone. The collagen fibers in the concentric lamellae in an osteon are laid down parallel to one another in any given lamella but in different directions in adjacent lamellae. This arrangement gives the cut surface of lamellar bone the appearance of plywood and imparts great strength to the osteon.
Lamellar bone is also found at sites other than the osteon. Circumferential lamellae follow the entire inner and outer circumferences of the shaft of a long bone, appearing much like the growth rings of a tree (see Fig. 8.3). Perforating canals (Volkmann’s canals) are channels in lamellar bone through which blood vessels and nerves travel from the periosteal and endosteal surfaces to reach the osteonal canal; they also connect osteonal canals to one another (Plate 11, page 244). They usually run at approximately right angles to the long axis of the osteons and of the bone (see Fig. 8.3). Volkmann’s canals are not surrounded by concentric lamellae, a key feature in their histologic identification.
Mature spongy bone is structurally similar to mature compact bone.
Mature spongy bone is similar in structure to mature compact bone except that the tissue is arranged as trabeculae or spicules; numerous interconnecting marrow spaces of various sizes are present between the bone tissue. The matrix of the bone is lamellated.
Arteries that enter the marrow cavity through the nutrient foramina supply blood to the shaft of long bones.
Nutrient foramina are openings in the bone through which blood vessels pass to reach the marrow. The greatest numbers of nutrient foramina are found in the diaphysis and epiphysis (Fig. 8.4). Metaphyseal arteries supplement the blood supply to the bone. Veins that exit through the nutrient foramina
FIGURE 8.4 • Diagram showing the blood supply of an adult long bone. The nutrient artery and the epiphyseal arteries enter the bone through nutrient foramina. These openings in the bone arise developmentally as the pathways of the principal vessels of periosteal buds. Metaphyseal arteries arise from periosteal vessels that become incorporated into the metaphysis as the bone grows in diameter.
or through the bone tissue of the shaft and continue out through the periosteum drain the blood from bone.
The nutrient arteries that supply the diaphysis and epiphysis arise developmentally as the principal vessels of the periosteal buds. The metaphyseal arteries, in contrast, arise developmentally from periosteal vessels that become incorporated into the metaphysis during the growth process (i.e., through the widening of the bone).