Hear This Illustration: Types of cells in bone tissue
It is very easy to think of bone tissue as something that is not alive. In fact, it is very much alive and is active throughout your entire life. It is considered a connective tissue because it fits that basic characteristic of having relatively few cells and lots of intercellular matrix. In this case, the matrix is hard because it contains minerals in addition to some of the proteins we saw earlier in the matrix of other connective tissues.
The four cell types of bone tissue shown in this figure each play a particular role in the development and maintenance of healthy bone tissue. Osteogenic cells, osteoblast and osteocytes are actually developmental stages of the same cell type. Osteogenic cells are a type of stem cell derived from embryonic mesenchyme. Once they begin to undergo changes, they are called osteoblasts and begin actively depositing new bone matrix. As they continue to build matrix, they trap themselves in small spaces called lacunae and are then known as osteocytes. Osteocytes grow small processes that penetrate the matrix and form small canals that allow nearby osteocytes to exchange nutrients and waste products despite their isolation. Osteocytes maintain the bone matrix.
Osteoclasts arise from bone marrow cells, the same type that develops into blood cells known as monocytes. In this case a large multinucleate cell develops from the fusion of these monocyte-like cells. Osteoclasts are able to produce enzymes and hydrochloric acid that can dissolve bone matrix. This important process allows the release of calcium and other minerals stored in the matrix so that it can be used for other purposes including rebuilding new bone.
Bone density is a function of how much bone matrix is being produced by osteoblasts compared with how much is being reabsorbed by osteoclast activity. The remodeling process is affected by many factors including heredity, diet, exercise, age and hormone levels.