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 growthX-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 structureAbout 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 remodellingLive 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 ageingA 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 bonesOf 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!
Frost, H. M. (1964). Laws of bone structure. Thomas Springfield, Illinois.
See also calcium; hormone replacement therapy; joints; parathyroid glands; skeleton.
Bone serves many important functions. Bones support the body, protect underlying organs, and provide a movable skeleton against which the muscles can work. In addition, bone forms all the cells of the blood, plus takes part in calcium and acid-base balance, and storage of trace elements (such as zinc) needed by cells elsewhere.
Bone is created from osseous connective tissue . Like other types of connective tissue, osseous tissue is composed of relatively sparse cells surrounded by an extracellular network, or matrix . Bone matrix is a tough, resilient mixture of protein and minerals . Osteoblasts, a type of bone cell, secrete proteins into the matrix, which provide tensile strength (resistance to stretching and twisting). The principal protein of the bone matrix is collagen, which accounts for almost one-third of the dry weight of bone. Most of the rest of the bone's weight is due to the minerals of the matrix. These are mainly calcium phosphate and calcium carbonate. Embedded in the protein network, the minerals provide hardness and compressive strength.
Bone cells remain alive and, like other cells in the body, must be nourished by blood. In order to deliver nutrients to and remove waste from the bone interior, the hard, compact surface is pierced by "canals" through which blood vessels can travel. Once inside, these canals branch, allowing blood vessels to reach cells throughout the bone. This canal system gives bone its characteristic appearance under the microscope, with bone cells embedded in concentric rings (lamellae) of calcified matrix, all surrounding a hollow canal. These units of structure, called osteons, all run parallel in compact bone, but form a looser and less-ordered network in spongy bone. Compact bone forms in the perimeter of long bone shafts, such as those of the legs and arms, where stress forces tend to be all in the same direction. In contrast, spongy bone is found in the ends of bones, where forces come from many different directions. Spongy bone also occurs where bone is not subject to significant stress.
Formation and Growth
Ossification (bone formation) occurs in one of two ways. Intramembranous ossification occurs within parts of the skull and part of the clavicles . In this process, osteoblasts deposit matrix on a membranous network within the future bone. Once their own extracellular matrix traps the osteoblasts, they become fully mature osteocytes.
By contrast, most of the body's bones form by endochondral (within cartilage) ossification. In this process, a temporary model in the shape of the future bone is made from cartilage laid down by chondrocytes (cartilageforming cells), which later die within the shaft of the future bone. The space created by the death of these cells is invaded by osteogenic (bone-forming) cells. These cells differentiate into osteoblasts and secrete the matrix. As osteoblasts build bone, another type of cell, the osteoclast, dissolves older matrix, enlarging the cavity within. Osteoclasts dissolve matrix by secreting hydrochloric acid, which attacks the mineral portion, and enzymes that digest the collagen and other proteins. Within the shafts of the long bones, the spaces created are filled with blood-forming tissue, the bone marrow.
Growth in bone length is stimulated by sex hormones and growth hormone during puberty, accounting for the pubertal growth spurt. Growth is later halted, and bones cannot grow in length during adulthood. However, bone is constantly remodeled by the combined action of osteoblasts and osteoclasts, and can grow in width in response to mechanical stresses such as weight lifting. When a bone is fractured, chondrocytes, osteoblasts, and osteoclasts go to work repairing it and cleaning up the damage.
Some forms of osteoporosis (brittle bones) are caused by overactive osteoclasts.
The interactions of three hormones—parathyroid hormone, calcitonin, and calcitriol—control bone growth and remodeling, as well as the calcium concentration in the blood serum. Calcium is necessary for a variety of critical functions outside of bone, including muscle contraction, neuron function, glandular secretion , and blood clotting. Because of this, serum calcium is kept within very narrow limits, 9.2 to 10.4 milligrams per deciliter of blood. Calcium excesses and deficiencies are prevented by using bone as a storage pool.
Calcitriol promotes calcium absorption from the gut and prevents its loss through the kidneys. Calcitriol is made from vitamin D, either supplied from the diet or manufactured by skin cells exposed to sunlight. A lack of vitamin D can lead to rickets in childhood, osteomalacia in adulthood, or osteoporosis later in life. Once calcium is absorbed by the gut, it enters the blood, and, if in high concentration, is deposited in bone by osteoblasts, stimulated by calcitonin. When serum calcium levels drop, parathyroid hormone indirectly causes osteoclasts to break down bone and release calcium into the blood. Bone, therefore, is constantly cycling between deposition and resorption, and about one-fifth of the skeleton is built and demolished each year.
see also Blood; Connective Tissue; Musculoskeletal System; Vitamins and Coenzymes
Angie Kay Huxley
Alexander, R. M. Bones: The Unity of Form and Function. New York: Macmillan, 1994.
Ross, M. H., L. J. Romrell, and G. I. Kaye. Histology: A Text and Atlas, 3rd ed. Baltimore, MD: Williams & Wilkins, 1995.
Turner, C. H. "Homeostatic Control of Bone Structure: An Application of Feedback Theory." Bone 12 (1991): 203–217.
Zaleske, D. J. "Cartilage and Bone Development." Instructional Course Lectures 47 (1998): 461–468.
Bone is the major component of the adult vertebrate skeleton. It is a hard connective tissue comprised of living material, including bone cells, fat cells, and blood vessels, and an inorganic matrix, which is made up largely of water and minerals.
All connective tissues support and connect various parts of the body, and the specific functions of bones are diverse. As the main element of the skeleton, they provide structure and support to vertebrate bodies. They also act as levers for body movement, their position controlled by the muscles attached to them. Bones also protect the delicate internal organs from external impact. For example, the skull encases and protects the brain, and the rib cage houses the lungs and heart.
As well as serving these structural and protective functions, bones play two important physiological roles. They serve as deposits for calcium, a mineral that makes the bones stronger and is essential for the operation of nerves and muscles. Red blood cells, white blood cells, and platelets are all manufactured in the core of the bone, or bone marrow.
Bones change and develop along with the rest of the body. During the early stages of embryonic development, the vertebrate skeleton consists entirely of cartilage. As the fetus grows, calcium and phosporous deposits form around the cartilage as the mineralization process begins. At birth, the skeleton still consists mostly of cartilage and experiences further changes as the child matures. For instance, the bones of an infant's skull do not fuse until several months after birth. A newborn human has over 300 bones, which over time fuse into the 206 bones of an adult. Cartilage gradually replaces bone through the process of ossification , which is achieved through the activity of osteoblasts , the bone precursor cells.
Bone is made up of osteocytes, living bone cells that are surrounded by the matrix. Osteoblasts secrete the matrix and collagen, a protein that gives bone a slightly elastic quality and prevents it from shattering when bearing weight. The osteoblasts also secrete mineral salts, which harden the bone. As the bone matures, the osteoblasts are transformed into osteocytes, and new osteoblasts are released into the system to build more bone.
Bone tissue can be categorized as compact or spongy. Compact bone, also called cancellous bone, has a honeycomb structure that is designed to withstand stress from multiple directions. Compact bone is denser and harder than spongy bone, and is present in the main bones of the arms and legs. It is made up of long, cylindrical units called osteons, which help the bone bear weight. Blood vessels and nerves run through the center of each osteon.
Many bones are composed of an outer layer of compact bone and an inner core of spongy bone. The skull, pelvis, ribs, breastbone, and vertebrae all contain spongy bone, as do the ends of the arm and leg bones. Trabeculae are the bony struts that create the criss-cross formation of spongy bone. Bone marrow fills the spaces between the trabeculae. A thin, two-layered membrane called the periosteum surrounds and protects both bone types. Nerves and blood vessels run throughout the outer layer of the periosteum into the bone. Osteoblasts are the main constituent of the inner layer.
Bones are connected to each other at junctions called joints. There are several types of joints, each with a different range and pattern of movement. The fused joints of the skull do not permit movement, the hinge joints of the elbow and knee allow movement in one direction, and the pivot joints found between certain neck vertebrae permit side-to-side twisting motions. The ball-and-socket joint in the shoulder allows a wide range of movement.
Bone is a dynamic tissue with a structure and composition that adapt to environmental stresses. It undergoes constant breakdown and rebuilding. As a calcium deposit, bone is responsible for maintaining required levels of this mineral in the blood. When calcium levels drop, cells called osteoclasts break down bone to release calcium into the blood. Through the activity of osteoblasts, bones also thicken in response to exercise and impact.
When a bone breaks, several processes contribute to its repair. First, cells from the periosteum transfer to the site of the break and create a fibrous network. Then other cells produce cartilage around this network. In the final step, osteoblasts arrive and convert the cartilage into bone. This healing process may take weeks or months, depending on the severity and location of the injury and the individual's age and general health.
As an individual ages, the rate at which bone breaks down slowly begins to exceed the rate at which it is formed. The bone is weakened, and its size reduced. Developing and maintaining proper exercise and nutrition habits at an early age ensures that bones remain healthy in old age.
see also Skeletons.
Judy P. Sheen
Parker, Sybil P., ed. Encyclopedia of Science and Technology, 8th ed. London: McGraw-Hill, 1997.
BONE (or Bona , ancient Hippo Regius , named Annaba after Algerian independence from French rule), Mediterranean port in northeastern Algeria close to the Tunisian border. Located on a gulf between capes Garde and Rosa, it became one of the Maghreb's centers for the Phoenician settlers around the 12th century b.c.e. In later periods, Bone was dominated by the Romans before achieving its independence in the wake of the Punic Wars of 264–146 b.c.e. In 393 through 430 c.e. Bone emerged as one of the most important centers of Christian learning. It then fell into ruin (431) as a result of the massive assault by the Vandals. Aside from a Christian presence that had dwindled in the wake of the Arab conquest, only to be revitalized by the French conquest, it appears that a Jewish community existed in Bone from Roman times. When it was temporarily captured by Roger ii of Sicily (1153), some of the Jews succeeded in organizing trade activity with Italian merchants from Pisa who established a trading post there. Although there is no solid evidence to suggest that Sephardi Jews arrived in Bone following their expulsion from Spain (1492), rabbinical responsa literature from the 1400s attests to a vibrant communal life. The city's synagogue, the "Ghriba," was the site of Jewish and Muslim pilgrims. Yet there are no available statistical data to determine the size of the community prior to the 19th century.
The economic and trade influence of Jews in Bone increased during the late 18th and early 19th centuries, when Algeria was part of the Ottoman Empire. Some of the most noteworthy and powerful Jewish merchants belonged to the Bensamon and Bacri families. Whereas the Bensamons catered to British trade interests at the port of Bone, the Bacris, whose influence extended to other Algerian ports, were the chief representatives of French interests.
In 1832, two years after France penetrated Algeria, Bone became a French possession. The French were instrumental in making Bone into a modern town. In the first decade of French rule the Jewish population increased due in part to an influx of several hundred migrants from Tunisia. During World War ii the Jews numbered over 3,000. They were naturalized French citizens like the rest of Algerian Jewry by virtue of the October 1870 Crémieux Decree.
There were no Jews in Bone after 1964–65, a situation attributable to the overall decolonization process, Jewish communal self-liquidation, and the exodus to France and Israel.
A.N. Chouraqui, Between East and West: A History of the Jews of North Africa (1973); C.-A. Julien, A History of North Africa: Tunisia, Algeria, Morocco from the Arab Conquest to 1830 (ed. and rev. by R. Le Tourneau; 1970); J.M. Abun-Nasr, A History of the Maghrib in the Islamic Period (1987).
[Michael M. Laskier (2nd ed.)]
In proverbial usage, a bone is the type of something hard and dry.
Bones are traditionally used in enchantment or divination, and point the bone at means (of an Australian Aboriginal) cast a spell on someone so as to cause their sickness or death. The expression refers to an Aboriginal ritual, in which a bone is pointed at a victim. In southern Africa, to throw the bones at is to use divining bones (a set of carved dice or bones used by traditional healers in divination) to foretell the future or discover the source of a difficulty by studying the pattern they form when thrown on the ground.
a bone in her mouth water foaming before a ship's bows; the expression is recorded from the early 17th century.
bone of contention a subject or issue over which there is continuing disagreement; proverbially, a bone thrown between two dogs is the type of something which causes a quarrel.
close to (or near) the bone (of a remark) penetrating and accurate to the point of causing discomfort; (of a joke or story) likely to cause offence because near the limit of decency.
make no bones about something have no hesitation in stating or dealing with something, however unpleasant, awkward, or distasteful it is. The obsolete expression find bones in suggests how the meaning of this could have evolved: finding bones in meat or soup presents a difficulty in consuming it, but making no bones means that impediments are either ignored or overcome.
near the bone variant form of close to the bone above.
See also a dog that will fetch a bone, hard words break no bones, the nearer the bone, sticks and stones may break my bones, while two dogs are fighting for a bone.