Bone Composition

Like any connective tissue, bone is composed of cells and matrix. The matrix is composed of fibers, inorganic salts, and ground substance. The fibers are mostly collagen; elastic fibers are minimal. Collagen fibers are present to a greater degree than in other tissues, giving bone strength and resiliency. Organic salts give bone hardness and rigidity. Organic salts are present in crystal as hydroxyapatite crystals, a form of calcium phosphate. The ground substance consists of glycosaminoglycans (GAGs), namely, chondroitin sulfate, keratan sulfate, and hyaluronic acid.

Bone Formation

The cellular constituents and chemical reactions are the same for all bones formed in the body. Bone always forms by replacing some other preexisting material. The histogenesis of bone is dichotomized into bones that form in preexisting cartilage and bones that form from preexisting embryonic mesenchyme. The two types of bone production are not mutually exclusive. Both processes often occur within the same bone. For instance, the femoral bone is lengthened by enchondral bone formation at its metaphyses, but increased width of the bone occurs from intermembranous bone in the periosteum.

Inspection of a longitudinal section of a long bone reveals two different types of bone structure: compact (substantia compacta) and spongy (substantia spongiosa), or cancellous, bone. Compact bone is found at the outer border of a bone; cancellous bone is the latticelike network within a bone. Both compact and cancellous bone are lamellar. Woven bone represents immature bone, which is replaced with lamellar bone over time.

A typical long bone is divided into a diaphysis (shaft), metaphysis, and epiphysis. The diaphysis is the central portion or shaft of the long bone, with a hollow inner medullary cavity (marrow cavity) encased by a thick, compact bony cortex. The ends of a long bone are capped by epiphyses, representing secondary centers of ossification.

In the immature skeleton, epiphyses are separated from the shaft of the long bone by a thin, cartilaginous epiphyseal plate. Cartilaginous epiphyseal plates are attached to the bony diaphysis by a transitional region, known as the metaphysis. The epiphysis and portions of the metaphysis contain a lattice of small bone spicules known as trabeculae.

In the adult, once the cartilaginous plates ossify, the medullary cavity of the diaphysis is continuous with the intertrabecular spaces of the metaphysis and epiphysis. At each end of the bone, a thin layer of articular cartilage covers the thin cortex of the epiphyses.

The periosteum represents a specialized connective tissue that covers the bone. It is composed of two layers: an inner layer that has the ability to form bone through intramembranous ossification, and an outer fibrous layer that anchors into the compact bone by Sharpey fibers. Periosteum does not cover the ends of the bone in which articular cartilage exists, nor does it extend under joint capsules. The femoral neck and talus are examples of bone structures that lack periosteum. Fractures in these areas heal more precariously because of the absence of the osteogenic capabilities of the inner layer of the periosteum. Periosteum is also absent on bones formed within tendons (e.g., sesamoids, patella) and at ligament and tendon insertions.

The medullary cavity and interspaces of the cancellous bone are lined with endosteum. Endosteum lacks the tough fibrous nature of the periosteum, but, similar to the periosteum, it possesses osteogenic properties.

In the flat bones of the skull, the cortex forms two thick layers of bone (inner and outer table), which sandwich an interposed layer of spongy bone (diploë). The periosteum is known as pericranium, and the inner surface of the bone is lined by dura mater.