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Human bone generally comprises osseous tissue, an outer coating called a periosteum, and bone marrow. The two main structural components typically include spongy bone on the interior, with an outer layer of compact bone. Usually found in long bones of the body, it consists of units called osteons, each of which is called a haversian system with a series of canals, concentric rings, and bone cells called osteocytes. The structure of compact bone is typically dense; it protects the inner sections as well as helps long bones tolerate the weight of the body and stresses during physical activity.
Also called cortical bone, the compact variety usually features a haversian system, or cylindrical unit within the structure. A central tube called a haversian canal typically runs in the same path as the length of the bone, and contains blood vessels, nerves, and lymphatic vessels. Around the tube are layers called lamellae, often made of calcium and phosphate deposits which help to harden the bone. There are also collagen fibers that help strengthen the compact bone.
The spaces in between each of the concentric rings are called lacunae. Bone cells called osteocytes are typically found within the rings, in between which there are also small channels for nutrients to get to the bone cells. Waste products are usually filtered out through these channels as well. The structure of compact bone is also organized so minerals like calcium and phosphorus can be stored, and then released into the blood when it is needed.
To view the structure of compact bone, scientists usually have to cut the sample into very thin slices, because light does not typically travel through it well. Special equipment is often used because bone is much harder than other tissues. Sometimes the calcium salts are removed first. The canals are usually seen as lighter spots while the rings often consist of alternating light and dark sections. There are often spaces between the osteons where researchers think that new bone tissue can be produced.
Spongy osseous tissue has similar structures to compact bone, except it is a more dispersed network of thin columns. There are blood vessels that pass through and go into the marrow, where red blood cells are typically made. Some marrow is located between the small bone columns as well. The structure of compact bone protects these components as well as the body’s organs and muscles.