A Practical Approach, Second Edition=Ronald D. Ho.pdf

POSTNATAL DEVELOPMENTAL MILESTONES 971OsteonCompact bone(Osteon, Haversian system)Cement lineHaversian canalConcentric lamellaeCollagen fibersResorptioncavitiesInterstitiallamellaeInnercircumferentiallamellaePeriostealvesselLacunaCanaliculiCancellousbone(Trabeculae)OutercircumferentiallamellaeVolkmann's canalConcentric lamellaeFigure 1 (6) Schematic view of a part of the proximal shaft of a long bone containing cortical (left) and trabecular(right) bone. The cortical bone is composed mainly of Haversian systems or osteons of concentric lamellae withinner and outer circumferential lamellae at the periphery. Many completed osteons can be seen, each with itscentral Haversian canal, cement line, concentric lamellae, canaliculi, and lacunae. Note the orientation of thecollagen fibers in successive lamellae in the protruding osteon. Cross connections between osteons are establishedby Volkmann’s canals. Volkmann’s canals connect with periosteal vessels and the marrow cavity and,unlike the Haversian canals, lack a covering of concentric lamellae. Interstitial lamellae are seen betweencompleted Haversian systems. Three resorption cavities are also depicted.cancellous bone (3, 4, 5, 6). In general, cancellous bone is composed of a lattice of thin bony rods,plates, and arches that form the spaces occupied by bone marrow. Macroscopically, compact boneappears as a solid continuous mass; however, microscopic examination reveals that compact boneis made up of densely packed tubular structures called osteons (3). In the adult human skeleton,compact bone makes up about 80% of total bone mass.Mature compact and cancellous bone are both made up of lamellae, or stacks of parallel orconcentrically curved sheets (Figure 1). Each lamella is about 3 to 7 µm thick and is composed ofhighly organized, densely packed collagen fibers (6, 4). There are 4 general types of lamellar bone(trabecular lamellae, inner and outer circumferential lamellae, and lamellae of osteons), three ofwhich are found only in compact bone (4).Cancellous or spongy bone is made up of trabecular lamellae. Trabecular lamellae form thespaces in spongy bone that are filled with bone marrow (Figure 1). Also housed in trabeculae areosteocytes that are connected by tiny branching tubular passages called canaliculi. The canaliculiprovide a channel for the exchange of metabolites between osteocytes and the extracellular space.Outer and inner circumferential lamellae are found only in compact bone (Figure 1). Outercircumferential lamellae are located directly underneath the periosteum extending around the entirecircumference of the bone shaft. Inner circumferential lamellae are located on the inner surface of© 2006 by Taylor & Francis Group, LLC