The cellular activities of osteoblasts, osteocytes, and osteoclasts are essential to the process. Osteoblasts synthesize the collagenous precursors of bone matrix and also regulate its mineralization. To meet the requirements of skeletal growth and mechanical function, bone undergoes dynamic remodeling by a coupled process of bone resorption by osteoclasts and reformation by osteoblasts. O steoblasts are derived from mesenchymal stem cells of the bone marrow stroma. They possess a single nucleus, have a shape that varies from flat to plump, reflecting their level of cellular activity, and in later stages of maturity line up along bone-forming surfaces.
Osteoblasts have receptors for parathyroid hormone and apparently for estrogen.
Hormones, growth factors, physical activity, and other stimuli act mainly through oteoblasts to bring about their effects on bone. The mineral crystals, as shown by electron microscopy, are deposited along, and in close relation to, the bone collagen fibrils. Undecalcified bone sections, such as those stained with the von Kossa stain, are best used for the histological study of bone mineral distribution. The extracellular matrix of bone is mineralized soon after its deposition, but a very thin layer of unmineralized matrix is seen on the bone surface, and this is called the osteoid layer or osteoid seam. A biopsy of normal bone will show two discrete and separated layers of fluorescent label uptake marking successive mineralization fronts.