The appearance of bone belies its structural complexity. It is neither as solid nor as static as it seems. It consists of two types of tissue -- compact bone and trabecular bone -- both of which are beehives of microscopic activity. The origins of these terms say a lot about the structure of bone.
Compact bone was originally known as lamellar or cortical bone. Lamellar derives from lamina, meaning "plate"; cortical, from cortex (or "shell"). The basic units of compact bone are tightly packed plates wound into tubular forms, called osteons, which look a little like rolled-up magazines. Each osteon has a tiny blood vessel called a capillary running through its central channel. The osteons are arranged in stacks to form a hard, shell-like membrane.
Although the second type of bone tissue is usually referred to as trabecular (meaning "like a little beam"), it is sometimes called cancellous ("lattice-like") bone. Indeed, this tissue comprises millions of tiny beams that form a lattice-like matrix.
Most bones contain both compact and trabecular tissue. Compact bone forms the dense outer casing, while trabecular bone spans the interior. However, the proportion of these two tissues varies from bone to bone. Long, regular bones, like those of the arms, legs, and ribs, consist primarily of compact bone. Irregularly shaped bones, such as the ends of the leg or arm bones, the pelvis, and the vertebrae, consist principally of trabecular bone.
Both compact and trabecular bone are made from the same fabric -- a meshwork of collagen fibers. This meshwork is inlaid with calcium and phosphate, which are mixed with water to form a hard, cement-like substance called hydroxyapatite. Sodium, magnesium, and potassium are also present in smaller amounts.
These materials are surprisingly strong. Ounce for ounce, bone bears as much weight as reinforced concrete. However, unlike concrete, it isn't inert. Bone is a living tissue. It serves as a repository of minerals for use by the body, and these elements are continuously lent out and replaced. Thus, like most other body tissues, bone is in a constant state of flux.
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