Bone is a type of hard endoskeletal connective tissue establish in many vertebrate animals. Bones support body structures, protect internal organs, and (in conjunction with muscles) facilitate movement; are also involved with cell formation, calcium metabolism, and mineral storeroom. The bones of an animal are, in a group, known as the skeleton. Bone has a different composition than cartilage, and both are derived from mesoderm. In common parlance, cartilage can also be called "bone", certainly when referring to animals that only have cartilage as hard connective tissue, such as cartilaginous fish (Chondrichthyes) like sharks. True bone is present in bony fish (Osteichthyes) and all tetrapods.

There are several evolutionary alternatives to bone. These evolutionary solutions are not totally functionally analogous to bone.

Exoskeletal defense is offered by shells, carapaces (consisting of calcium compounds or silica) and chitinous exoskelotons. A true endoskeleton (that is, protective tissue derived from mesoderm) is also there in Echinoderms. Porifera (sponges) possess simple endoskeletons that consist of calcareous or siliceous spicules and a sponging fiber network. Bones and skeletons are studied in osteology. Bones can be prepared for study by several methods, such as maceration.

Formation

The formation of bone occurs by two methods: intramembranous and endochondral ossification. Intramembranous ossification mostly occurs during formation of the flat bones of the skull; the bone is created from mesenchyme tissue. Endochondral ossification occurs in long bones, such as limbs; the bone is formed from cartilage.

Endochondral ossification begins with points in the cartilage called "primary ossification centers". They mostly show during fetal development, though a few short bones begin their primary ossification after birth. They are dependable for the formation of the diaphyses of long bones, short bones and certain parts of uneven bones. Secondary ossification occurs after birth, and forms the epiphyses of long bones and the extremities of uneven and flat bones. The diaphyses and the epiphyses of long bones remain separated by a growing zone of cartilage (the metaphysis) until the child reaches skeletal maturity (18 to 25 years of age), whereupon the cartilage ossifies, fusing the two jointly (epiphyseal closure).

Marrow can be found in most any bone that holds cancellous tissue. In newborns, all such bones are full exclusively with red marrow (or hemopoietic marrow), but as the child ages it is mostly replaced by yellow marrow (or fatty marrow). In adults, red core is mostly found in the flat bones of the skull, the ribs, the vertebrae and pelvic bones.

Remodeling is the process of resorption followed by substitute of bone with little change in shape and occurs throughout a person's life. Its purpose is the let go of calcium and the repair of micro-damaged bones (from everyday stress). Repeated stress results in the bone thickening at the points of utmost stress. It has been hypothesized that this is a result of bone's piezoelectric properties, which root bone to generate small electrical potentials under stress.

Bone pathologies

One of the most general bone illnesses is a bone fracture. Bones heal by natural processes, but untended and unsupported can lead to misgrown bone. Other illnesses are for example osteoporosis and bone cancer (osteosarcoma). The joints can be affect by arthritis.