Bone is a tissue composed of living cells (osteocytes) distributed in an intercellular matrix that contains organic and inorganic substances. The organic component, largely collagenous fibers, is responsible for the strength and resilience of bone while the inorganic salts, mostly calcium phosphate, contribute to its hardness, and rigidity. The inorganic constituents make up approximately 67% of bony matter in the adult. The radiopacity of bone depends largely upon the amount of minerals present. Lack of mineral content in the young and aged alters the radiopacity and requires compensation.

Forms of Bone Tissue

There are two forms of bone tissue, cancellous and compact. Cancellous or spongy bone consists of irregular strands of tissue, which branch and join one another, forming a loose network in which the intercommunicating spaces are filled with marrow. Compact or dense bone has a more solid, regular appearance and its intercommunicating canals are microscopic in size. The basic structure of these two types of bone is essentially the same. They differ mainly in the relative amount of solid substance and the number, size, and arrangement of the intercommunicating spaces they contain. Both cancellous and compact forms are present in most bones of the body, but the extent and distribution of each varies considerably. In adults, the exterior of all bones is compact bone while the interior is usually cancellous.

A Typical Long Bone

In a typical long bone, each end (epiphysis) is largely cancellous and is covered by a thin layer of compact bone. The reverse is true in the shaft (diaphysis), which is mostly compact bone tissue. The central medullary canal, or cavity in the shaft of a long bone, is continuous with the intercommunicating spaces in the cancellous bone located at the ends. Depending on the age of the individual and the type of bone considered, either red or yellow marrow fills these cavities. Red marrow, active in the production of blood cells, is present in all bones at birth and blood cells are produced in all locations. With advancing age, the production of blood cells decreases and red marrow is replaced by yellow marrow, which consists mostly of fat cells. In the adult, red marrow is found mainly in the skull, vertebrae, ribs, sternum, and the articular ends of some long bones.

Long Bone Structure

Each long bone, except for its articular surface, is enclosed by a thick, fibrous sheet of membranous tissue, called the periosteum, which develops when the perichondrium, the outer covering of the embryonic skeleton, becomes permeated with blood vessels. The marrow cavity, and also the canal system, are lined by a delicate layer of reticular (netlike) tissue, called the endosteum.

Some Other Bones

In flat bones, such as the ribs, one or more plates of compact bone surround the cancellous bone. In many irregular bones, such as the vertebrae, spongy bone is enclosed by a thin shell of compact bone.

Blood Supply

The living bone cells are nourished by a system of blood vessels and capillaries. In the long bones, for example, blood vessels in the shaft supply the bone marrow. Branches of blood vessels contained in the periosteum supply the compact and cancellous bone areas.

Microscopic Structure

When bone tissue is examined under a microscope, it is seen in layers either as a series of flat plates (for cancellous bone) or concentric cylinders (for compact bone). In compact bones, the series of concentric cylinders are formed in units called haversian systems. Here, living bone cells lie in minute cavities called lacunae. The lacunae communicate with each other, and indirectly with a central haversian canal, through a system of microscopic canals called canaliculi that contain protoplasmic extensions of bone cells. They are nourished by blood vessels from the periosteum that enter the compact bone through small pits on the surface. Branches of these blood vessels penetrate the matrix and enter the central haversian canal in each haversian system.

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