<P> Bone is a dynamic tissue that is constantly being reshaped by osteoblasts, which produce and secrete matrix proteins and transport mineral into the matrix, and osteoclasts, which break down the tissues . </P> <P> 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 . </P> <P> 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 . During bone formation, the surface layer of osteoblasts consists of cuboidal cells, called active osteoblasts . When the bone - forming unit is not actively synthesizing bone, the surface osteoblasts are flattened and are called inactive osteoblasts . Osteocytes remain alive and are connected by cell processes to a surface layer of osteoblasts . Osteocytes have important functions in skeletal maintenance . </P> <P> Osteoclasts break down bone tissue, and along with osteoblasts and osteocytes form the structural components of bone . In the hollow within bones are many other cell types of the bone marrow . Components that are essential for osteoblast bone formation include mesenchymal stem cells (osteoblast precursor) and blood vessels that supply oxygen and nutrients for bone formation . Bone is a highly vascular tissue, and active formation of blood vessel cells, also from mesenchymal stem cells, is essential to support the metabolic activity of bone . The balance of bone formation and bone resorption tends to be negative with age, particularly in post-menopausal women, often leading to a loss of bone serious enough to cause fractures, which is called osteoporosis . </P>

The center of a group of osteoblasts is a tube called the