There are three primary types of bone cells, each with its own sphere of influence on bone activities.
Osteoblasts are mononucleate cells that are responsible for bone formation. The osteoblast’s major activity is to biosynthesize the organic matrix that constitutes 40% of the weight of bone. The bone matrix is principally comprised of collagen which is the most abundant protein in our body (~30% of total body protein). Osteocalcin, which is the fifth most prevalent protein in our body, is also a significant structural component of bone matrix. Surprisingly, it was discovered by G. Karsenty (2009) that osteocalcin is also a protein hormone secreted by bone that stimulates secretion of insulin by the pancreas beta cells. Osteocalcin also binds to receptors on fat and testis cells, suggesting additional complexity to the endocrine role of osteocalcin. The osteoblast has receptors for both parathyroid hormone and for the steroid hormone 1α,25(OH)2D3.
Osteocytes are mononuclear fully mature osteoblasts (see Figure 1) and represent ~90% of all bone cells. The osteocytes in their active osteoblast phase become embedded in bone lacunae (compartments) as a con sequence of their active role in bone formation. They send bone canaliculi (slender cytoplasmic processes) that make contact with the canaliculi of other osteocytes. These canaliculi provide the osteoblasts with access to nutrients and oxygen from the bone mar row and bone periosteum (see Figure 1). Osteocytes are a primary source of the hormone fibroblast growth factor 23 (FGF-23) that is involved in phosphate homeostasis and regulation of the production of 1α,25(OH)2D3 and parathyroid hormone.
Fig1. Role of osteoblasts and osteocytes. Osteoblasts secrete the organic matrix of bone (termed osteoid). The osteoid is principally collagen fibers and two other bone proteins, osteonectin and osteopontin. As a consequence of mineralization of the osteoid, each osteoblast becomes surrounded with calcified bone matrix and then becomes converted to an osteocyte. The isolated osteocytes communicate with one another via their cell process, the tiny canaliculus.
Osteoclasts are the mediators of bone resorption. They produce small pits or trenches that are generated by the solubilization and export of calcium hydroxy apatite. Osteoclasts are multinucleate and are derived from bone marrow macrophage cells in a complex cell differentiation process that includes direct physical interaction with osteoblasts. The details of the origin of osteoclasts will be discussed later in this chapter.
