On this Wikipedia the language links are at the top of the page across from the article title. Left bone in filet of extinct elephant, Alaska, Ice Age Wellcome L0057714. Bertazzo S – SEM deproteined bone – wistar rat – x10k.

A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. 206 separate bones in the adult, not counting numerous small sesamoid bones. Their unique composition and design allows bones to be relatively hard and strong, while remaining lightweight. The elasticity of collagen improves fracture resistance. Bone is actively constructed and remodeled throughout life by special bone cells known as osteoblasts and osteoclasts.

Within any single bone, the tissue is woven into two main patterns, known as cortical and cancellous bone, each with a different appearance and characteristics. The hard outer layer of bones is composed of cortical bone, which is also called compact bone as it is much denser than cancellous bone. Cancellous bone, also called trabecular or spongy bone, is the internal tissue of the skeletal bone and is an open cell porous network that follows the material properties of biofoams. It was first illustrated accurately in the engravings of Crisóstomo Martinez.

Bone marrow, also known as myeloid tissue in red bone marrow, can be found in almost any bone that holds cancellous tissue. Blood enters the endosteum, flows through the marrow, and exits through small vessels in the cortex. Bone is metabolically active tissue composed of several types of cells. These cells include osteoblasts, which are involved in the creation and mineralization of bone tissue, osteocytes, and osteoclasts, which are involved in the reabsorption of bone tissue. Light micrograph of decalcified cancellous bone tissue displaying osteoblasts actively synthesizing osteoid, containing two osteocytes. They are located on the surface of osteon seams and make a protein mixture known as osteoid, which mineralizes to become bone. Osteocytes are cells of mesenchymal origin and originate from osteoblasts that have migrated into and become trapped and surrounded by a bone matrix that they themselves produced.

Osteoclasts are very large multinucleate cells that are responsible for the breakdown of bones by the process of bone resorption. New bone is then formed by the osteoblasts. Bone is constantly remodeled by the resorption of osteoclasts and created by osteoblasts. Woven bone is produced when osteoblasts produce osteoid rapidly, which occurs initially in all fetal bones, but is later replaced by more resilient lamellar bone. In adults, woven bone is created after fractures or in Paget’s disease. The extracellular matrix of bone is laid down by osteoblasts, which secrete both collagen and ground substance.