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bone, hard tissue that forms the skeleton of the body in vertebrate animals. In the very young, the skeleton is composed largely of cartilage and is therefore pliable, reducing the incidence of bone fracture and breakage in childhood. The inorganic, or mineral, content of bone is mainly calcium, phosphate and carbonate minerals. The organic content is a gelatinous material called collagen. As the body grows older, decreases in bone mass may lead to an increased vulnerability to fractures. Bone fractures heal naturally, although they are often aided through restriction of movement in the affected area. Bones assume a variety of sizes and shapes; however, all bone tissue has a three-layered composition. A spongy layer forms the interior. Long bones (such as those in the arms and legs) are hollow, the inner spaces being filled with marrow (see bone marrow), important in the formation of blood cells. Surrounding the spongy, inner layer is a hard, compact layer that functions as the basic supportive tissue of the body. The outer layer is a tough membrane called the periosteum, which sheaths most bones. Although bone appears solid, it contains numerous microscopic canals permitting the passage of blood vessels and nerve fibers. Two types of bone are present in most bones: compact, which constitutes the shaft, and cancellous, an extremely strong variety which makes up the enlarged ends of the bone. See also osteoporosis.
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 Connective tissue
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 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 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 Thyroid gland
the principal element of the vertebrate skeleton.
Bones, the joints and ligaments joining the skeletal bones, and the muscles attached to the bones by tendons together make up the locomotor apparatus. Bones are classified as long, or tubular (for example, the humerus and the femur), flat (for example, the bones of the skull), or short (for example, the vertebrae). The middle section of the long bones is called the diaphysis. The two ends are called the epiphyses. The articulations are either immobile (synarthroses; for example, cranial sutures) or mobile (joints, or diarthroses; for example, the articulations of the limbs).
Bones consist of bony tissue, periosteum, marrow, blood and lymphatic vessels, nerves, and, in many cases, cartilage. Bony tissue, the main constituent, forms lamellae; the bone is considered compact or cancellous (spongy) according to the density of these lamellae. In long bones, the shaft is predominantly of the compact type of bony tissue, where the arrangement of lamellae depends chiefly on the distribution of the bone-feeding blood vessels in the haversian canals. In short bones and in the epiphyses of long bones, cancellous tissue is predominant; here,
|Table 1. Classification of boiler units according to parameters and output|
|Parameters of superheated steam||Rated steam output (tons/hr)|
|Pressure (MN/m2 [kgf/cm2])||Temperature (°C)|
|Primary superheated steam||Secondary superheated steam|
|Unit with natural circulation, with and without superheating. . . . . . . . . .||4 (40)||440||—||6.5, 10, 15, 20, 25, 35, 50, 75|
|10 (100)||540||—||60, 90, 120, 160, 220|
|14 (140)||570||—||160, 210, 320, 420, 480|
|Unit with natural circulation, with superheating and intermediate superheating of steam. . . . . . . . . .||14 (140)||570||570||320, 500, 640|
|Flow-through unit with superheating and intermediate superheating of steam. . . . . . . . . .||25.5 (255)||585-565||570||950; 1,600; 2,500|
there are honeycomblike cavities, filled with marrow, between the lamellae or trabeculae. The trabeculae are arranged in the direction of greatest pressure and tension, ensuring maximum tensile strength with a minimum of material. Bones are covered with periosteum, which contains blood vessels and nerves. Bone is a variety of connective tissue. Insoluble salts (chiefly hydroxylapatite) constitute about 50 percent of its bulk.
Bone cells, or osteocytes, lie embedded in the bone cavities (lacunae). They are linked to one another by thin processes in the canaliculi, through which they are supplied with nutrients. The intercellular substance of bony tissues consists of tightly packed collagen fibers (on the surface of which are hydroxylapatite crystals), polysaccharides, and proteins. The formation and calcification of the intercellular substance are brought about by osteoblasts, which become embedded in the intercellular substance during the course of osteogenesis (to become the osteocytes).
Bony tissue is the body’s main calcium depot, and it is active in calcium metabolism. Calcium is released by the resorption and bound by the formation of bony tissue. These processes occur during the reconstruction of bony tissue, which occurs constantly and throughout life.
The shape of bone changes with changing mechanical loads. Bony tissue in the human skeleton is almost completely reconstructed every ten years; multinuclear cells called osteoclasts are involved in the resorption.
Bone is classified as coarse-fibrous or fine-fibrous (lamellate) according to the arrangement of the collagen fiber in the ground substance. In coarse-fibrous bone the fibers are arranged randomly, but in fine-fibrous bone they form plates, or lamellae, in which most of the fibers are arranged in the same direction.
Bones develop either from the embryonic connective tissue, mesenchyma, or directly (secondary, or cover, bone, such as the frontal and parietal bones), or by passing through a cartilaginous stage (primary, or substitution, bone, such as the humerus and the femur). Secondary bone, in terms of vertebrate evolution, developed from dermal scales that sank beneath the skin; primary bone originated as an ossification of cartilaginous endo-skeleton. The development of secondary bone involves the formation of a skeletogenous rudiment, a collection of mesenchymal cells that eventually become osteoblasts and form bone. In the development of primary bone, the initial formation in the skeletogenous rudiment is a cartilaginous model of the future bone. The model is replaced subsequently by bony tissue, and the cartilage disintegrates. The coarse-fibrous bone formed in the rudiment is replaced by fine-fibrous bone in some amphibians and reptiles, most birds, and mammals.
The process of bone formation usually intensifies dramatically when a tubular bone is fractured. A chondro-osseous callus forms to reunite the fragments. The shape of the bone is restored in the course of further reconstruction. Bone can form in adult vertebrates, including man, not only as part of the skeleton but also in any connective tissue (ectopic osteogenesis).
REFERENCESZavarzin, A. A., and A. V. Rumiantsev. Kurs gistologii, 6th ed., chapter 6. Moscow, 1946.
Ivanov, G. F. Osnovy normal’noi anatomii cheloveka, vols. 1–2. Moscow, 1949.
Fridenshtein, A. la. Experimental’noe vneskeletnoe kosteoobrazovanie. Moscow, 1963.
A. IA. FRIDENSHTEIN
What does it mean when you dream about bones?
Bones can obviously represent death, either literal or metaphorical. They can also symbolize a state of reduction or deprivation (as in being “stripped to the bare bones” and being left with a “skeleton crew”). Less ominously, bones may simply refer to the structure of something.