Bone Metabolism

Bone is not a uniformly solid material, but rather has some spaces between its hard elements.The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps and spaces.Filling the interior of the bone is the trabecular bone tissue (an open cell porous network also called cancellous or spongy bone), which is composed of a network of rod- and plate-like elements that make the overall organ lighter and allow room for blood vessels and marrow. Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface area of compact bone.

There are several types of cells constituting the bone;
* Osteoblasts are mononucleate bone-forming cells that descend from osteoprogenitor cells. They make a protein mixture known as osteoid, which mineralizes to become bone. Osteoid is primarily composed of Type I collagen. Osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself. They robustly produce alkaline phosphatase, an enzyme that has a role in the mineralisation of bone, as well as many matrix proteins. Osteoblasts are the immature bone cells, and eventually become entrapped in the bone matrix to become osteocytes- the mature bone cell
* Bone lining cells are essentially inactive osteoblasts. They cover all of the available bone surface and function as a barrier for certain ions.
* Osteocytes originate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that they themselves produce. The spaces they occupy are known as lacunae.
* Osteoclasts are the cells responsible for bone resorption, thus they break down bone.Osteoclasts are large, multinucleated cells located on bone surfaces in what are called Howship's lacunae or resorption pits.

Two types of bone can be identified microscopically according to the pattern of collagen forming the osteoid:
* Woven bone, which is characterised by haphazard organisation of collagen fibers and is mechanically weak
* Lamellar bone, which has a regular parallel alignment of collagen into sheets and is mechanically strong
Woven bone is produced when osteoblasts produce osteoid rapidly, which occurs initially in all fetal bones (but is later replaced by more resilient lamellar bone). In adults woven bone is created after fractures or in Paget's disease.

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