four main types of bone cells are osteoprogenitor cells, osteoblasts,
osteocytes and osteoclasts.
Osteoprogenitors are a type of stem cell which differentiates
to produce osteoblasts. They are found in the inner cell layer of the periosteum,
endosteum and the lining of osteonic canals. They are most active during
periods of growth but are also active later in adult bone during remodeling.
Osteoblasts are cuboidal cells found on the surface of
growing bone. They are immature bone cells responsible for secretion of the mineralized
matrix. Along the surface of bone, the osteoblasts are polarized to deposit osteoid
onto the existing bone surface. As the osteoblast continues to secrete bone
matrix, it eventually becomes entrapped within it. At this stage, the cell is
said to have become a mature bone cell, known as an osteocyte.
Osteocytes occupy the lacunae of mineralized bone and
function to maintain the matrix by regulating the flow of mineral ions. They
have cytoplasmic extensions which form a network within the canaliculi for
communication with other osteocytes through gap junctions. Osteocytes are
likely to function in the detection of mechanical forces that act on upon the
bone and direct the subsequent bone response.
Osteoclasts are derived from the myeloid lineage and
are different to the other three cell types. They are large multinucleate cells
present on the bone surface, predominantly within cavities. Rather than functioning in the formation of
bone, they are responsible for bone resorption via secretion of acid
phosphatases and collagenase which act to dissolve the matrix.