Osteoblasts are cells with a single nucleus that synthesize bone. However, in the process of bone formation, osteoblasts function in groups of connected cells. Individual cells cannot make bone. A group of organized osteoblasts together with the bone made by a unit of cells is usually called the osteon.

Osteoblasts are specialized, terminally differentiated products of mesenchymal stem cells.They synthesize dense, crosslinked collagen and specialized proteins in much smaller quantities, including osteocalcin and osteopontin, which compose the organic matrix of bone.

In organized groups of connected cells, osteoblasts produce hydroxylapatite - the bone mineral, that is deposited in a highly regulated manner, into the organic matrix forming a strong and dense mineralized tissue - the mineralized matrix. The mineralized skeleton is the main support for the bodies of air breathing vertebrates. It is an important store of minerals for physiological homeostasis including both acid-base balance and calcium or phosphate maintenance.
Osteoblasts are the major cellular component of bone. Osteoblasts arise from mesenchymal stem cells (MSC). MSC give rise to osteoblasts, adipocytes, and myocytes among other cell types. Osteoblast quantity is understood to be inversely proportional to that of marrow adipocytes which comprise marrow adipose tissue (MAT). Osteoblasts are found in large numbers in the periosteum, the thin connective tissue layer on the outside surface of bones, and in the endosteum.

Normally, almost all of the bone matrix, in the air breathing vertebrates, is mineralized by the osteoblasts. Before the organic matrix is mineralized, it is called the osteoid. Osteoblasts buried in the matrix are called osteocytes.

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