Bones

What is the structure of a typical human bone?

It is easy to think of bones as nonliving. After all, most of the mass of bone is mineral salts—mainly calcium and phosphorus. However, bones are living tissue. Bones are a solid network of living cells and protein fibers that are surrounded by deposits of calcium salts.

Structure of Bones The structure of a typical long bone is shown in Figure 32–2. The bone is surrounded by a tough layer of connective tissue called periosteum (pehr ee AHS tee um). Beneath the periosteum is a thick layer of compact bone. Although compact bone is dense, it is far from solid. Nerves and blood vessels run through compact bone in channels called Haversian canals.

A less dense tissue known as spongy bone may be found under the outer layer of compact bone. Spongy bone is found in the ends of long bones such as the femur. It is also found in the middle of short, flat bones such as the bones in the skull. Despite its name, spongy bone is not soft and spongy; it is actually quite strong. Near the ends of bones where force is applied, spongy bone is organized into structures that resemble the supporting girders in a bridge. This latticework structure in spongy bone adds strength without adding excess mass.

Within many bones are cavities that contain a soft tissue called bone marrow. There are two types of bone marrow: yellow and red. Yellow marrow consists primarily of cells that store fat. Red marrow contains the stem cells that produce most types of blood cells.

In Your Notebook Use a Venn diagram to compare compact bone and spongy bone.

Development of Bones The skeleton of a human embryo is composed almost entirely of a type of connective tissue called cartilage. Cartilage-producing cells are scattered in a network of protein fibers including both tough collagen and flexible elastin.