Bone Structure and Composition
Bone is an organ of the vertebrates that consists of two main types of tissue: cortical bone and cancellous bone. Both bone tissues are composed of mineralized tissue that includes both an organic component, mainly collagen, and an inorganic component, calcium and phosphate.
Function of Bones
Bones are complex, specialized connective tissues that provide rigidity and strength to the skeletal system. They are made of a mixture of a flexible matrix (about 30%) and bound minerals, including an organic substance that is called ossein, and an inorganic one, calcium hydroxylapatite, which is a form of calcium phosphate. Both components are remodeled continuously by a group of specialized cells, the osteoblasts and the osteoclasts.
Composition and Stiffness of Bone
The bone matrix is composed of elastic collagen fibers (ossein) and of noncollagenous proteins, which serve as a scaffold for the formation of hydroxyapatite crystals. The combination of these components is responsible for the stiffness and rigidity of bone.
Cell Types in Bone Tissue
There are three main cell types involved in the formation and resorption of bone tissue: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are the primary bone-forming cells; they produce collagen and a protein called osteocalcin. They also secrete factors that influence the activities of other bone cells.Osteocytes are specialized bone cells that make up about 80 percent of the body's bone mass. They are primarily located in the trabeculae, or rod-like, structures of bones. They produce a range of factors that influence the activities of other bone cells and are able to sense mechanical pressures and loads on the skeleton.
Communication and Adaptation of Bone
A key feature of osteocytes is their large surface area and the ability to communicate with other bone cells. This communication is facilitated by a network of canaliculi, which are in contact with the osteocytes' processes and with bone lining cells on the bone surface. The osteocytes also have the ability to communicate with other tissues via interstitial fluid that flows between their canaliculi.These functions are important for the adaptation of bone to daily forces. The osteocyte's large surface area allows it to detect the smallest changes in mechanical forces, which help the bone adapt to daily stresses by reducing the amount of resorption and remodeling that occurs.The osteocytes' ability to communicate with other cells in the body also helps with the adaptation of bone to stress and injury. It has been hypothesized that this cell-to-cell communication is a form of mechanosensory signaling that aids in the adaptation of bone to various forces.
Osteocyte Structure and Presence
Electron micrographs of alveolar bone from rats demonstrate that osteocytes are surrounded by a layer of bundles of collagen fibrils and the calcified bone surface constitutes the osteoid. A thin cytoplasmic layer is situated on the osteoid's surface and extends some cytoplasmic projections towards the adjacent bone lining cells.The osteocyte's presence is essential for the adaptation of bone to daily stresses and injuries. It has been hypothesized that this osteocyte's presence is a form of mechanosensory communication that aids in the adaptation of bone to daily stresses by reducing the amount of bone resorption and remodeling that occurs.
