Bone

Fishing hook made of bone, from Swedish Stone Age, found in Sk�ne, Sweden

Bone is a connective tissue, found in many animals. Bones are used to support body structures, protect internal organs, and (in conjunction with muscles) facilitate movement; they 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 shellss 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 of bone

Bone can be either cortical ("compact") or cancellous ("spongy"). 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, 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, and the hollow middle is filled with yellow marrow. While the majority of long bones are formed of cortical material; at the ends are the epiphysis, which are generally composed of cancellous material and red marrow.

Cells of Bone

Bone is typically divided in to cells and matrix. 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 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 howships. They are long lived but not always active.

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 the exported. It then associates with 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, osteorectin and Cell Attachment Factor.

Bone development

Development of bone (known as ossification) proceeds by the laying down of dense connective tissue and its replacement by a trabecular network forming a primary spongiosum. The two main forms of growth are intramembrous and endochondral. Intramembrous formation occurs in the embryo, and is concerned with flat 'membranous' bones . Here, undifferentiated mesenchyme condenses to form vascularised connective tissue. Cells deposit extracellular material and simultaneously become osteoblasts, which further continue bone development.

Endochondral formation occurs to bones including the limbs and veterbrae. In this model a 'primary centre of ossification' develops within a precursor to bone made of cartilage. A new periosteal bone collar develops. Blood vessels invade which increases the oxygen availability. Osteoprogenitors invade and differentiate in to osteoblasts; these deposit bone on calcified cartilage. The centre grows and extends towards the ends of the bones. However it never reaches the ends but stops short at 'epiphyseal growth plates'. Beyond these, secondary centres of ossification develop and a similar process happens. Secondary centres allow the bone to continue to grow in length throughout childhood. Plate closure denotes the end of bone growth and occurs in adults (though at different times for different bones). If these plates close too early, achondroplasia results - with severely underdeveloped bones. This can be corrected at diagnosis with the distribution of growth hormones.

'Remodeling' is the process of resorption followed by replacement of bone with little change in shape and occurs throughout a persons life. Its purpose is the release of calcium and the repair of micro-damaged bones (from everyday stress).

Bones can be connected to muscles via 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.