Dictionary:

bone

  (bōn)

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1. 1. The dense, semirigid, porous, calcified connective tissue forming the major portion of the skeleton of most vertebrates. It consists of a dense organic matrix and an inorganic, mineral component.
   2. Any of numerous anatomically distinct structures making up the skeleton of a vertebrate animal. There are more than 200 different bones in the human body.
   3. A piece of bone.
2. bones
   1. The skeleton.
   2. The body.
   3. Mortal remains.
3. An animal structure or material, such as ivory, resembling bone.
4. Something made of bone or of material resembling bone, especially:
   1. A piece of whalebone or similar material used as a corset stay.
   2. bones Informal. Dice.
5. bones The fundamental plan or design, as of the plot of a book.
6. 1. bones Flat clappers made of bone or wood originally used by the end man in a minstrel show.
   2. Bones (used with a sing. verb) The end man in a minstrel show.

1. To remove the bones from.
2. To stiffen (a piece of clothing) with stays, as of whalebone.

bone up

1. Informal. To study intensely, usually at the last minute: boned up for the final exam.

bone of contention

1. The subject of a dispute.

1. Grounds for a complaint or dispute.

[Middle English bon, from Old English bān.]

The hard connective tissue that, together with cartilage, forms the skeleton of humans and other vertebrates. It is made of calcium phosphate crystals arranged on a protein scaffold. Bone performs a variety of functions: it has a structural and mechanical role; it protects vital organs; it provides a site for the production of blood cells; it serves as a reserve of calcium. See also Connective tissue; Skeletal system.

There are two types of bone in the skeleton: the flat bones (for example, the bones of the skull and ribs) and the long bones (for example, the femur and the bones of the hand and feet). Both types are characterized by an outer layer of dense, compact bone, known as cortical bone, and an inner spongy bone material made up of thin trabeculae, known as cancellous bone. Cortical bone consists of layers of bone (lamellae) in an orderly concentric cylindrical arrangement around tiny Haversian canals. These interconnecting canals carry the blood vessels, lymph vessels, and nerves through the bone and communicate with the periosteum and the marrow cavity. The periosteum is a thin membrane covering the outer surface of bone and consisting of layers of cells that participate in the remodeling and repair of bone. The cancellous bone is in contact with the bone marrow, in which much of the production of blood cells takes place. The interface between the cancellous bone and the marrow is called the endosteum, and it is largely at this site that bone is removed in response to a need for increased calcium elsewhere in the body.

Bone is formed by the laying down of an osteoid matrix by osteoblasts, the bone-forming cells, and the mineralization of the osteoid by the development and deposition of crystals of calcium phosphate (in the form of hydroxyapatite) within it. It is the mineral, organized in a regular pattern on a collagen scaffold, that gives bone its stiffness. Osteoid contains largely fibers of type I collagen and lesser amounts of numerous noncollagenous proteins. Although the role of these proteins in bone is not well understood, it is thought that their particular combination in bone gives this tissue the unique ability to mineralize. It is clear that these proteins interact with each other and that collagen and several of the noncollagenous proteins can bind to specialized receptors on the surface of bone cells. This binding is important for the adhesion of the cells to the bone matrix, and also delivers behavioral signals to the cells. See also Apatite; Collagen.

The primary cell types in bone are those that result in its formation and maintenance (osteoblasts and osteocytes) and those that are responsible for its removal (osteoclasts). Osteoblasts form from the differentiation of multipotential stromal cells that reside in the periosteum and the bone marrow. Under the appropriate stimuli, these primitive stromal cells mature to bone-forming cells at targeted sites in the skeleton. Under different stimuli, they are also capable of developing into adipocytes (fat cells), muscle cells, and chondrocytes (cartilage cells). Osteocytes, which are osteoblasts that become incorporated within the bone tissue itself, are the most numerous cell type in bone. They reside in spaces (lacunae) within the mineralized bone, forming numerous extensions through tiny channels (cannaliculi) in the bone that connect with other osteocytes and with the cells on the endosteal surface. Osteocytes are therefore ideally placed to sense stresses and loads placed on the bone and to convey this information to the osteoblasts on the bone surface, thus enabling bone to adapt to altered mechanical loading by the formation of new bone. Osteocytes are also thought to be the cells that detect and direct the repair of microscopic damage that frequently occurs in the bone matrix due to wear and tear. Failure to repair the cracks and microfractures that occur in bone, or when this microdamage accumulates at a rate exceeding its repair, can cause the structural failure of the bone, such as in stress fractures. A large number of molecules that regulate the formation and function of osteoblastic cells have been identified. Circulating hormones, such as insulin, growth hormone, and insulinlike growth factors, combine with growth factors within the bone itself, such as transforming growth factor beta (TGFβ) and bone morphogenetic proteins (BMPs), to influence the differentiation of osteoblasts.

Osteoclasts are typically large, multinucleated cells, rich in the intracellular machinery required for bone resorption. This is accomplished when the cells form a tight sealing zone by attachment of the cell membrane against the bone matrix, creating a bone-resorbing compartment. Into this space, the cell secretes acid to dissolve the bone mineral, and enzymes to digest the collagen and other proteins in the bone matrix. The removal of bone by osteoclasts is necessary to enable the repair of microscopic damage and changes in bone shape during growth and tooth eruption. Osteoclast-mediated bone resorption is also the mechanism for releasing calcium stored in bone for the maintenance of calcium levels in the blood. Most agents that promote bone resorption act on osteoblastic cells, which in turn convey signals to osteoclast precursors to differentiate into mature osteoclasts. These agents include the active form of vitamin D, parathyroid hormone, interleukin-1, interleukin-6, and interleukin-11, and prostaglandins such as prostaglandin E₂. Differentiation to fully functional osteoclasts also requires close contact between osteoclast precursors and osteoblastic cells. This is due to a molecule called osteoclast differentiation factor (ODF) which is located on the surface of osteoblasts, binds to receptors on the surface of osteoclast precursor cells, and induces their progression to osteoclasts.

Flat bones and long bones are formed by different embryological means. Formation of flat bones occurs by intramembranous ossification, in which primitive mesenchymal cells differentiate directly into osteoblasts and produce bony trabeculae within a periosteal membrane. The initial nature of this bone is relatively disorganized and is termed woven bone. Later, this woven bone is remodeled and replaced by the much stronger mature lamella bone, consisting of layers of calcified matrix arranged in orderly fashion. Long bones are formed by intracartilaginous development in which the future bone begins as cartilage. The cartilage template is gradually replaced by bone in an orderly sequence of events starting at the center of the growing bone. Cartilage remains at the ends of long bones during growth, forming a structure at each end termed the growth plate. Cartilage cells (chondrocytes) that arise in the growth plates proliferate and add to the length of the bone. This occurs during a complex series of events, with expansion both away from and toward the center of the bone. When the bone achieves its final length in maturity, expansion from the growth plate ceases. Cartilage persists at the ends of the long bones in a specific form called articular cartilage, which provides the smooth bearing surfaces for the joints.

Bone is a dynamic tissue and is constantly being remodeled by the actions of osteoclasts and osteoblasts. After bone removal, the osteoclasts either move on to new resorption sites or die; this is followed by a reversal phase where osteoblasts are attracted to the resorption site. It is thought that growth factors that are sequestered in an inactive form in the bone matrix are released and activated by the osteoclast activity and that these in turn promote fresh osteoid production by the recruited osteoblasts. The new osteoid eventually calcifies, and in this way the bone is formed and replaced in layers (lamellae), which are the result of these repeated cycles. In growing bone, the activities of bone cells is skewed toward a net increase in bone. However, in healthy mature bone there is an equilibrium between bone resorption and bone formation. When the equilibrium between these two cell types breaks down, skeletal pathology results.

The most common bone disease is osteoporosis, in which there is a net loss of bone due to osteoclastic bone resorption that is not completely matched by new bone formation. The best-understood cause of osteoporosis is that which occurs in women due to the loss of circulating estrogen after menopause. Another cause of osteoporotic bone loss is seen in disuse osteoporosis. Just as bone can respond to increased loading with the production of additional bone, bone is also dependent on regular loading for its maintenance. Significant bone loss can occur during prolonged bed rest or, for example, in paraplegia and quadriplegia. Likewise, an unloading of the skeleton (due to a lack of gravitational pull) in space flight results in severe bone loss in astronauts unless the effects of gravity are simulated by special exercises and devices. See also Osteoporosis.

Many metabolic and genetic diseases can affect the amount and quality of bone. Metabolic diseases such as diabetes, kidney disease, oversecretion of parathyroid hormone by the parathyroid glands, anorexia nervosa, and vitamin D-dependent rickets may cause osteopenias (the reduction in bone volume and bone structural quality). Immunosuppressive therapy in organ transplant patients can lead to reduced bone mass, as can tumors of bone and other sites. Tumors can produce substances that cause the activation of osteoclastic bone resorption. In the genetically based disease osteogenesis imperfecta, mutations in the gene for type I collagen result in the production of reduced amounts of collagen or altered collagen molecules by osteoblasts. Other common diseases of the skeleton are diseases of the joints, such as rheumatoid arthritis and osteoarthritis. See also Arthritis; Calcium metabolism.

Fairly early in the evolution of multicellular organisms it became an advantage to have a hard body component which could provide protection for soft tissues and a firm base against which contractile elements such as muscle could perform precise movements like those involved in locomotion or grasping. The hard component was often formed from calcium carbonate, as found in shellfish, but other durable defences were provided by chitin (a complex carbohydrate), in crustaceans and insects, and even silica, in the glass sponges.

Man and most vertebrates are characterized by an internal rather than an external skeleton. With the exception of the young animal and the cartilaginous fishes, the hardness is provided by calcium phosphate, laid down as crystalline hydroxyapatite on a template of collagen (a fibrous protein), forming bone. The collagen confers some elasticity. In man, bone also acts as the major reservoir for several elements such as calcium, phosphorus, magnesium, zinc, and sodium. The storage and release of at least the first three of these into the extracellular space is modified by hormones such as parathyroid hormone, calcitonin, and 1, 25-dihydroxyvitamin D.

The longest bone is the femur (thigh bone), which accounts for a quarter of one's stature; the smallest bone is the stapes — one of the tiny ossicles in the middle ear which transmits the vibrations from the ear drum to the inner ear.

There are over 200 bones in the adult skeleton. They can be divided into two principal types: the long bones such as the femur, tibia, and humerus, which develop principally from within a cartilaginous framework, and the flat bones such as the skull, bones of the pelvis, and scapula, which develop within membranes of fibrous tissue.

Bone development and growth

X-rays can detect primary centres of bone formation (ossification) in the mid shaft of long bones from the end of the second month of intrauterine life. Secondary centres of ossification appear at the ends of these bones mostly at various times after birth and always earlier in females than males. Some secondary centres, however, such as in the lower end of the femur, occur before birth and this was used in the past as a indication of the maturity of a fetus; this had crucial forensic implications as it could determine whether or not a mother would be charged for concealing the birth of a viable infant.

Towards the ends of the long bones there are specialized discs of cartilage (epiphyseal plates) stretching across the entire bone. The cells in this area have a high rate of multiplication, and it is the major site of longitudinal growth in the juvenile bone. The rate of growth is greatest in infancy and around puberty, and growth ceases when the epiphyseal plate itself finally ossifies. It is over 200 years since the anatomist, John Hunter, showed that the mass and width of bone is increased by surface accretion from the periosteum — a tough fibrous layer covering the bone — rather than by internal expansion.

The ultimate bone length and mass is largely genetically determined and the average racial differences in bone mass exemplify this, with black people having a higher peak bone mass than white Caucasians and higher still than Asians. However this may be modified by general nutritional status — particularly calcium and protein intake — and by physical load bearing. There is evidence that the impact of such environmental influences may be greatest around the time of puberty, and unfortunately the lifestyle of the average teenager in present day Western society does not favour optimal bone development.

Premature arrest of growth at the epiphyseal plate will result in dwarfism. The cause may be genetic (as in achondroplasia), or environmental (as in severe illness or starvation).

Epiphyseal growth is most rapid at the wrists and shoulders in the upper limbs and at the knees in the lower limbs. The increased output of sex hormones at puberty provides a strong stimulus to accelerated bone growth for two or three years and then leads to epiphyseal closure — fusion with the shaft of the bone. Children with precocious puberty end up with stunted growth, whereas in eunuchs the epiphyses remain open and they become tall in their later teens. Pituitary growth hormone is the other hormone involved in bone growth.

All bone surfaces, with the exception of cartilaginous articular surfaces which form a joint with a neighbouring bone, are invested with the fibrous periosteum, which has osteogenic (bone-forming) potential. The flat bones ossify directly from such fibrous tissue rather than from intermediary cartilage. The skull is made up of several bones separated by very irregular interdigitating seams called sutures. This arrangement permits the necessary flexibility of the head during the birth process and after ossification is completed the sutures seal up progressively throughout adult life. Examination of the extent of suture union provides a means for assessing the age of an adult skeleton after death, whereas in a child this can be judged by which ossification centres are present and which epiphyses are fused.

Bone structure

About 80% of the skeleton consists of compact or ‘cortical’ bone which is extremely dense and resistant to trauma, and whose degree of hardness is exceeded in the body only by the enamel of the teeth. Such material forms the thick shafts of the long bones and the surface of all bones. It is perforated by microscopic channels; the Haversian canals (described by Havers, an English physician in the seventeenth century). Blood vessels pass through these canals, and bone cells are arranged concentrically around them. These cells, the osteocytes, have long extensions which pass down an interlocking network of canaliculi in the bone. This same network also allows nutrients, gases, and solutes to permeate the bone from the Haversian blood vessels.

The other 20% of bone forms a delicate, lacy honeycomb with a high surface-to-volume ratio. Cellular activity in this component (called trabecular or cancellous bone) is greater than in compact bone, and a variety of metabolic, hormonal, or physical stimuli on the cells renders it more labile. Trabecular bone is found principally in the bodies of the vertebrae, the ribs, the pelvis, and at the ends of the long bones. It contains the red bone marrow where the cellular components of the blood are manufactured. The remainder of the interior of bones — the medullary cavity — contains fat, and the proportion of fat to red marrow increases with age.

Metabolism and remodelling

Live adult bone is not a rigid inorganic framework. If it were, then like other crystalline structures it would be subject to frequent fatigue fractures as a result of the repetitive strains to which it is subjected. At millions of microscopic sites throughout the skeleton, bone is constantly being broken down and then remade in a cellular process first detailed in the mid twentieth century by an American orthopaedic surgeon, Harold Frost, and termed remodelling. At any site and in response to signals which are, as yet, poorly understood, the osteocytes permit access to the underlying bone by osteoclasts; these are specialized bone-resorbing cells derived from primitive cells in the marrow which also generate other types of phagocytes. These large, multinucleated cells dig small pits in the bone over a period of several days and are then replaced by bone-forming cells, the osteoblasts — which are derived from precursors of the fibrous-tissue-forming series. Osteoblasts synthesize fibres of the protein collagen and dispose them in a regular pattern determined in part by the direction of local strain forces. They also synthesize matrix mucopolysaccharide and direct the later mineralization of collagen with crystalline calcium phosphate. Active metabolites of vitamin D are required to allow adequate provision of both calcium and phosphate at those sites. The osteoblasts are ultimately trapped in the calcified matrix which they themselves have created, and become osteocytes. These cells appear to be able to communicate with each other throughout the bone; their elongated processes form close junctions with each other rather than being joined together.

Whatever the details, one end of a bone can interpret strain and chemical signals from the other. In healthy young adults the remodelling process is in balance, with as much new bone being synthesized as old bone removed. It also permits some adaptation of distribution of bone within a bone (or even within the skeleton) in response to changing physical or biochemical stimuli. Thus the disposition of bony trabeculae or cortical thickness is not haphazard but determined by mechanical and growth signals. Throughout the vertebrates there is a fairly constant ratio between the amount of bone required to cope with the largest forces normally encountered, and that required to deal with average gravitational demands. It is about three to one — for mice through to elephants.

Bone mass and ageing

A minimum regular stress is required simply to maintain your skeletal mass — you either use it or lose it — but the strains required to increase your bone mass significantly have to be substantially more than is customary for the individual concerned. Men have more bone than women at all ages because they are larger, but in both sexes bone mass (as measured conveniently by ‘dual energy X-ray densitometers’) increases into the third decade and plateaus from then till about the end of the fifth decade. It then declines, very slowly in men, but more rapidly in women for some years following the menopause. The average woman has lost around 20% of her peak bone mass by the time she is 70 years old (though estimates from different studies vary). This increases her risk of fractures and broken bones. The decline in bone mass in postmenopausal women can be reduced by taking oestrogen, and load-bearing exercise can increase bone mass to a modest extent in both men and women.

Any bone will fracture if it is subjected to sufficient force, but orthopaedic statistics reveal that over the past fifty years in the Western world there has been a striking rise in the incidence of certain fractures in older people, usually associated with only modest trauma. Fractures of the femoral neck (hip fracture), vertebral bodies (spine fracture), and wrist predominate, and appear to relate to populations having less bone at these sites. This condition, where reduced bone mass increases the liability to fracture, is called osteoporosis and represents one of our principal medical challenges today. In osteoporosis the remodelling process is not in equilibrium; less new bone is being formed than is being removed. It probably relates to several modern lifestyle changes affecting bone metabolism — notably diet and physical labour. The fact that Asian populations have smaller skeletons in any event and are adopting many of the same lifestyle features has led to predictions that hip fracture in that part of the world will become one of the greatest health care problems of the twenty-first century.

Several drugs are now available which have powerful actions on bone metabolism: examples are the bisphosphonates and the calcitonins, both of which inhibit bone resorption and can be used in the treatment of osteoporosis. The drug most commonly taken with such an effect is oestrogen, as hormone replacement therapy (HRT). It may also enhance bone formation.

The other common bone pathology is osteomalacia (called ‘rickets’ in children). In this disorder the bone is poorly mineralized, permitting softening and deformity such as bow legs. The usual cause is lack of vitamin D in the diet and/or lack of exposure to sunlight, and supplements of vitamin D are the appropriate treatment.

Ancient bones

Of all the body tissues, bone — apart from the teeth — is usually the only one to survive significantly beyond our mortal span. Ignoring palaeological niceties, some ‘human’ skeletons have been dated at around two million years old and provide us with much of the scanty evidence we have concerning the evolution of our species. Depending on the bones available, careful examination can allow reasonable inference concerning cranial development, height, and nutrition, in addition to the presence of diseases such as tuberculosis and leprosy and the medico-social practice of skull trepanning. The persistence of minute quantities of DNA within ancient bones may, by the application of sophisticated Polymerase Chain Reaction (PCR) augmentation and analysis, permit conclusions on the evolutionary/racial classification of some prehistoric skeletal remains. Look after your bones — they may tell your story long after you are gone!

— Iain Boyle

Bibliography

• Frost, H. M. (1964). Laws of bone structure. Thomas Springfield, Illinois

See also calcium; hormone replacement therapy; joints; parathyroid glands; skeleton.

Bones consist of an organic matrix composed of collagen and other proteins and crystalline mineral, mainly hydroxyapatite (calcium phosphate and calcium hydroxide), together with magnesium phosphate, fluorides, and sulphates. See also calcium.

Hard tissue consisting of a calcified matrix (mainly calcium phosphate) and fibres of protein. About 200 bones make up the human skeleton. It is living tissue with its own blood supply. Bones have a number of functions: they support and protect soft tissues; they act as levers for muscle movement; and the central cavities of long bones store minerals and produce blood cells.

The long bones and some flat bones contain a central cavity filled with a very active tissue called bone marrow. In adults, the marrow in certain bones (e.g. those of the sternum, ribs, and limbs) produce new red blood cells and destroy those which are defunct. Marrow is also an important source of white blood cells which play a vital role in the body's immunity against disease.

Bone elongation stops in adults, but bones may change their density and strength at any age (figure 18). Bones need to be mechanically stressed if they are to remain strong and healthy. They tend to become thicker and denser when exercise intensity increases, but they get less dense and weaker if a person becomes inactive. They also weaken if there is a deficiency of calcium or vitamin D in the diet, or if calcium absorption is impaired. See also osteoporosis.

Figure 18 Bone density decreases with age, increasing the risk of osteoporosis (brittle bone disease)

To remove the bones from meat, fish or fowl.

Idioms beginning with bone:
bone up

See also bare bones; chilled to the bone; cut to the bone; feel in one's bones; funny bone; make no bones about; pull a boner; roll the bones; skin and bones; work one's fingers to the bone.

1. the material of the skeletons of most vertebrate animals; the tissue composing bones. n 2. dense, hard, and slightly elastic connective tissue in which the fibers are impregnated with a form of calcium phosphate similar to hydroxyapatite. Bone tissue makes up the 206 bones of the human skeleton. n 3. any single element of the skeleton such as a rib or femur.

(click to enlarge)
Internal structure of a human long bone, with a magnified cross section of the interior. The … (credit: © Merriam-Webster Inc.)

Rigid connective tissue of vertebrates, consisting of cells embedded in a hard matrix. Bones serve as the body's supporting framework, provide muscle-attachment points for movement, protect the internal organs, house the blood-cell formation system (red bone marrow), and hold about 99% of the calcium vital to many body processes. Bone consists of a matrix of crystals of calcium, chiefly the phosphate and carbonate, embedded among collagen fibres, providing strength and elasticity, and bone cells (less than 5% of its volume). An external layer of compact bone surrounds a central area of spongy bone, except at the marrow cavity. Bone does not grow by cell division; instead, different types of bone cells generate bone matrix, break it down, and maintain it. Bone is remodeled by this process, which strengthens it in areas under greatest stress, permits healing of fractures, and helps regulate calcium levels in body fluid (see calcium deficiency). The process also causes underutilized bone, as in an immobilized limb, to atrophy. Bone disorders include rheumatoid arthritis, osteoarthritis, rickets, osteoporosis, and tumours. Bone can fracture suddenly or over time, as in stress fractures.

For more information on bone, visit Britannica.com.

A belief reported in England and Scotland from the mid-19th century onwards was that it is unwise to burn bones, usually with the reason that your own bones will ache if you do so, and a much earlier notion links burnt bones with toothache (Gospelles of Dystaues (1507), part 2, p. xiii, quoted in Opie and Tatem, 1989: 15). Aubrey (1686: 165) reported that women wear a tooth taken from a skull to prevent toothache, and that ‘cunning alewives putt the ashes of … bones in their Ale to make it intoxicating. Dr. Goddard bought bones of the Sextons to make his drops with. Some make a playster for the Gowte with the earth or musilage newly scraped from the shin-bone.’ Many other recorded cures mention the use of skulls.

See also BLADE-BONE, WISHBONE, SKULL, LUCKY BONE, CRAMP BONE.

Bibliography
The full bibliography list is available here.

• Opie and Tatem, 1989: 35

The hardest connective tissue in the body. Bone consists of hard calcified matrix (mainly calcium phosphate) and collagen fibres. Over 200 bones make up the human skeleton. Bone is living material and is very well vascularized (see Haversian system). Bones support and protect soft body parts; they act as levers for muscles during locomotion; they store calcium and fats; and they are involved in the production of blood cells. Bones fall into four main categories according to their size and shape (see flat bones, irregular bones, long bones, short bones). Bone may also be classified according to its density and porosity as either smooth, dense compact bone, or less dense, porous spongy bone.

The preparation of meat for human consumption by removing the bones from the carcass.

• hot b. — the boning process is carried out immediately, before the carcass has had time to cool and the meat to set.