Bone

For other uses of the word "bone", see bone (disambiguation).

Bone refers either to a hardened connective tissue or to one of the individual structures, or organs, into which it is formed, found in many 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 storage. The bones of an animal are, collectively, known as the skeleton.

Evolutionary alternatives to bones are shells and chitin.

General overview

It is a relatively hard and light-weight composite material, formed mostly of calcium phosphate (in the chemical arrangement termed calcium hydroxyapatite), which has relatively high compressive strength though poor tensile strength. While bone is essentially brittle, it does have a degree of significant elasticity contributed by its organic components (chiefly collagen). Bone is a mesh, the density of which may vary at different points.

Forms and structure of bone

Bone can be either compact or cancellous (spongy). Cortical (outer layer) bone is compact; the two terms are often used interchangeably. Cortical bone makes up a large portion of skeletal mass; but, because of its density, it has a low surface area. Cancellous bone is trabecular (honeycomb structure), it has a relatively high surface area, but forms a smaller portion of the skeleton.

Bone can also be either woven or lamellar. Woven bone is put down rapidly during growth or repair. It is so called because its fibres are aligned at random, and as a result has low strength. In contrast lamellar bone has parallel fibres and is much stronger. Woven bone is often replaced by lamellar bone as growth continues.

Long bones are tubular in structure (e.g. the tibia). The central shaft of a long bone is called the diaphysis, and has a hollow middle - the medullar cavity - filled with bone marrow. Surrounding the medullar cavity is a thin layer of cancellous bone that also contains marrow. The extremities of the bone are called the epiphyses , and are mostly cancellous bone covered by a relatively thin cortical of compact bone. In children, the bones are filled with red marrow, which is gradually replaced with yellow marrow as the child ages (see Bone Development below).

Short bones (e.g. finger bones) have a similar structure to long bones, except that they have no medullar cavity.

Flat bones (e.g. the skull and ribs) consist of two layers of compact bone with a zone of cancellous bone sandwiched between them.

Irregular bones are bones which do not conform to any of the previous forms (e.g. vertebrae).

Cells of bone

Bone is typically divided in to cells and matrices. Bone cells include osteoblasts, so called Bone Lining Cells, osteocytes and osteoclasts. Osteoblasts are typically viewed as bone forming cells. They are located near to the surface of bone and their functions are to make osteoid and manufacture hormones such as prostaglandin which act on bone itself. Osteoblasts are mononucleate. Active osteoblasts are situated on the surface of osteoid seams* and communicate with each other via gap-junctions. They contain alkaline phosphatase - a chemical which has a role in the mineralisation of bone.

Bone Lining Cells (BLCs) share a common lineage with osteogenesis (bone forming) cells. They function as a barrier for certain ions, induced osteogenetic cells. They are flattened, mononucleate cells which line bone

Osteocytes originate from osteoblasts which have migrated into and become trapped and surrounded by bone matrix which they themselves produce. The space which they occupy is known as a lacuna. Osteocytes have many processes which reach out to meet osteoblasts probably for the purposes of communication. Their functions include to varying degrees: formation of bone, matrix maintenance and calcium homeostasis. They possibly act as mechano-sensory receptors - regulating the bones response to stress.

If osteoblasts can be described as bone forming cells, osteoclasts role is the reverse: its destruction. These are large, multinucleated cells located on bone surface in Howship's lacunae.

Bone matrix

Bones consist of living animal cells embedded in a calcium carbonate matrix that makes up the main bone material. In the event of a broken bone, the cells are brought out of semi-stasis to repair the matrix.

Matrix comprises the other major constituent of bone. It has inorganic and organic parts. The inorganic is mainly crystalline mineral salts and calcium, which is present in the form of hydroxylapatite. The matrix is initially laid down as unmineralized osteoid (manufactured by osteoblasts). Mineralisation involves osteoblasts secreting vesicles containing alkaline phosphatase. This cleaves phosphate groups and acts as the foci for calcium and phosphate deposition. The vesicles then rupture and act as a centre for crystals to grow on.

The organic part of matrix is mainly Type I collagen. This is made intracellularly as tropocollagen and then exported. It then associates into fibrils . Also making up the organic part of matrix include various growth factors, the functions of which are not fully known. Other factors present include GAGs, osteocalcin , osteonectin and Cell Attachment Factor.

Bone development

The formation of bone occurs by two methods: intramembranous and endochondral ossification. Intramembranous ossification mainly occurs during formation of the flat bones of the skull; the bone is formed 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 appear during fetal development, though a few short bones begin their primary ossification after birth. They are responsible for the formation of the diaphyses of long bones, short bones and certain parts of irregular bones. Secondary ossification occurs after birth, and forms the epiphyses of long bones and the extremities of irregular 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 adulthood (18 to 25 years of age), whereupon the cartilage ossifies, fusing the two together.

Marrow can be found in most any bone that holds cancellous tissue. In newborns, all such bones are filled exclusively with red marrow, but as the child ages it is mostly replaced by yellow marrow. In adults, red marrow 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 replacement of bone with little change in shape and occurs throughout a person's life. Its purpose is the release of calcium and the repair of micro-damaged bones (from everyday stress). Repeated stress results in the bone thickening at the points of maximum stress. It has been hypothesized that this is a result of bone's piezoelectric properties, which cause bone to generate small electrical potentials under stress.

Bones can be connected to muscles via tendons and other bones by ligaments.

The science of the interaction of bone and muscle is called biomechanics. The science of bones is called osteology.

Some illnesses afflict human bones, for example osteoporosis and cancer. The joints can be affected by arthritis.

Cut and polished bone from a variety of animals is sometimes used as material for jewelry and other crafts.

Terminology

See also

• Fracture
• Bone healing
• List of bones of the human skeleton
• Terms for anatomical location
• Endochondral ossification
• Joint

External links

• Review (including references) of piezoelectricity and bone remodelling