Bones in our bodies are living tissue. They have their own blood vessels and are made of living cells, which help them to grow and to repair themselves. As well, proteins, minerals, and vitamins make up the bone.
Bones are composed of two types of tissue:
A hard outer layer called cortical (compact) bone, which is strong, dense and tough. A spongy inner layer called trabecular (cancellous) bone. This network of trabeculae is lighter and less dense than compact bone.
Bone is also composed of:
Bone forming cells (osteoblasts and osteocytes) Bone resorbing cells (osteoclasts) Nonmineral matrix of collagen and noncollagenous proteins (osteoid) Inorganic mineral salts deposited within the matrix
Bone cells
Cells in our bones are responsible for bone production, maintenance, and modeling:
Osteoblasts: These cells are derived from mesenchymal stem cells and are responsible for bone matrix synthesis and its subsequent mineralization. In the adult skeleton, the majority of bone surfaces that are not undergoing formation or resorption (i.e. not being remodeled) are lined by bone lining cells. Osteocytes: These cells are osteoblasts that become incorporated within the newly formed osteoid, which eventually becomes calcified bone. Osteocytes situated deep in bone matrix maintain contact with newly incorporated osteocytes in osteoid, and with osteoblasts and bone lining cells on the bone surfaces, through an extensive network of cell processes (canaliculi). They are thought to be ideally situated to respond to changes in physical forces upon bone and to transduce messages to cells on the bone surface, directing them to initiate resorption or formation responses. Osteoclasts: These cells are large multinucleated cells, like macrophages, derived from the hematopoietic lineage. Osteoclasts function in the resorption of mineralized tissue and are found attached to the bone surface at sites of active bone resorption. Their characteristic feature is a ruffled edge where active resorption takes place with the secretion of bone-resorbing enzymes, which digest the bone matrix.
Bone matrix
Osteoid is comprised of type I collagen (~94%) and noncollagenous proteins. The hardness and rigidity of bone is due to the presence of mineral salt in the osteoid matrix, which is a crystalline complex of calcium and phosphate (hydroxyapatite). Calcified bone contains about 25% organic matrix (2-5% of which are cells), 5% water and 70% inorganic mineral (hydroxyapatite).
Types of bone
Two types of bone can be identified according to the pattern of collagen forming the osteoid:
Woven bone is characterized by a haphazard organization of collagen fibers and is mechanically weak. Lamellar bone is characterized by a regular parallel alignment of collagen into sheets (lamellae) and is mechanically strong.
Woven bone is produced when osteoblasts produce osteoid rapidly. This occurs initially in all fetal bones, but the resulting woven bone is replaced by remodeling and the deposition of more resilient lamellar bone. In adults, woven bone is formed when there is very rapid new bone formation, as occurs in the repair of a fracture. Following a fracture, woven bone is remodeled and lamellar bone is deposited. Virtually all bone in the healthy mature adult is lamellar bone.