Mature lamellar bone has the same chemical composition and material properties throughout the skeleton, regardless of its mechanism of formation—intramembranous or endochondral—or its structural organization—cortical (compact) or trabecular bone.

Skeletal growth and development begin in utero and continue for nearly 2 decades in a series of wellorchestrated events. These events are determined genetically and regulated by central endocrine and peripheral biophysical and biochemical processes.

Normal bone forms either by intramembranous ossification from mesenchymal osteoblasts in the absence of cartilage scaffolding or by endochondral ossification using a preexisting calcified cartilage matrix. Long bones and vertebrae increase in size by a combination of these two processes. For example, ossification of the shaft of a long bone is an intramembranous process: subperiosteal deposition of new bone widens the shaft, while endosteal resorption widens the medullary canal. Long bones increase in length by cartilage proliferation at the growth plate in an elaborate process of endochondral ossification.

HISTOLOGY

The adult skeleton contains only two types of mature bone tissue, cortical (compact) bone and trabecular (cancellous or spongy) bone.

Both of these histologic types are represented in a typical long bone such as the femur (see Plate 2-21). Cortical bone forms the wall of the shaft, and trabecular bone is concentrated at each end.

The articular surface of the femur is covered with a cap of hyaline cartilage, which is better suited than bone to withstand the friction and relative motion in the joint. The cartilage cap is continuous with the synovial membrane lining the joint cavity (see Plate 2-26). The rest of the outer surface of the bone is lined with periosteum, a dense fibrous connective tissue. In a growing bone, the inner surface of the periosteum contains osteogenic cells that are actively laying down sheets of bone matrix. The cell morphology ranges from cuboidal-shaped active cells near the bone itself to inactive flattened-appearing cells embedded among dense collagen fibers in the outer edge of the periosteum. As the layers of bone are deposited, perpendicular-arranged bundles of collagen fibers become embedded in them, forming perforating, or Sharpey’s, fibers.

The inner layer of the shaft of a long bone is lined with endosteum, a much less substantial layer. Endosteal cells possess an osteogenic capacity that is expressed during bone remodeling and fracture healing.

STRUCTURE OF CORTICAL (COMPACT) BONE

The fundamental functional unit of adult human cortical bone is the osteon, or haversian system, a cylindrical structure measuring approximately 250 µm in radius and 1 to 5 cm in length. The osteon consists of concentric layers of bony lamellae, each 2 to 3 µm thick, which surround a central haversian canal (see Plates 2-21 and 2-22). The haversian canal, first described in 1691 by the English anatomist Clopton Havers, contains blood vessels, nerve supplies, and a supporting extracellular matrix. Lateral branches, called Volkmann’s canals, carry blood vessels from one osteon to another. Each cylindrical lamella within the osteon is lined with a sparse population of regularly arranged osteocytes, which communicate with one another by fine cell processes projecting into the lamellae through minute channels or canaliculi. Oxygen and nutrients reach osteocytes in the outer lamellae through these canaliculi by diffusion or convection forces resulting from mechanical motions. In addition to osteons, the compact collar of a long bone contains at its periphery subperiosteal circumferential lamellae, which are deposited by the inner layer of the periosteum.