Bone is formed by three primary cell types: Osteoblasts, Osteocytes and Osteoclasts.
Osteoblasts are bone-forming cells that descend from osteoprogenitor cells. They form 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 mineralization 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, which are the mature bone cells. All bone lining cells are osteoblasts.
Osteocytes are mature bone cells that originate from osteoblasts, which have migrated into and become trapped and surrounded by bone matrix, produced by themselves. The spaces they occupy are known as lacunae. Osteocytes have many processes that reach out to meet osteoblasts and other osteocytes probably for the purposes of communication. Their functions include formation of bone, maintenance of matrix and homeostasis of Calcium.
Osteoclasts are the cells responsible for bone resorption and remodelling. They are large, multinucleated cells located on bone surfaces in what are called Howship’s lacunae or resorption pits. These lacunae, or resorption pits, are left behind after the breakdown of the bone surface. Because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with phagocytic-like mechanisms similar to circulating macrophages.