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.
Bone is relatively hard for its density, although it is brittle and may snap on impact, creating what is medically called a fracture. The reason for the good material characteristics of bone is that it is not solid calcium carbonate, but instead a mesh[?], the density of which may vary at different points in the bone. Thus bird bones are generally less dense than mammalian bones.
Some bones are tubular in structure, and the hollow space in the middle is filled with marrow. The marrow is where blood cells are produced and thus critically important to healthy blood. At the ends are connections to other bones at joints which are lubricated and sometimes padded by softer tissue called cartilage.
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.
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 can be cortical or cancellous (despite the name this does not denote bone cancer). Cortical bone is compact. It makes up a large fraction of the skeleton but because of its compactness it has a low surface area. Cortical bone towards the surface of bone, whereas cancellous is found towards the centre. This type of bone is spongy and trabecular, and has a high surface area but makes up much less 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.
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.
Cut and polished bone from a variety of animals is sometimes used as material for jewelry and other crafts.