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Old Saturday, July 07, 2007
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Default Topic # 9

bone



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 , 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.




bone marrow

bone marrow, soft tissue filling the spongy interiors of animal bones. Red marrow is the principal organ that forms blood cells in mammals, including humans . In children, the bones contain only red marrow. As the skeleton matures, fat-storing yellow marrow displaces red marrow in the shafts of the long bones of the limbs. In adults red marrow remains chiefly in the ribs, the vertebrae, the pelvic bones, and the skull. Erythrocytes (red blood cells), platelets, and all but one kind of leukocyte (white blood cell) are manufactured in human red marrow. The marrow releases about 10 million to 15 million new erythrocytes every second, while an equivalent number are destroyed by the spleen.

Diseases of the marrow, such as leukemia or multiple myeloma, or injury to it from metallic poisons can interfere with the production of erythrocytes, causing anemia. A bone marrow biopsy, in which a small sample of bone marrow is obtained by aspiration through a thin needle, may be used to aid in the diagnosis of leukemia, anemia, and other blood disorders, as well as to gain insight on the normal functioning of the cells of the bone marrow.


Bone marrow transplantation, is a technique that infuses healthy bone marrow into a patient whose bone marrow is defective. The transplant can be autologous, consisting of bone marrow removed from the patient, treated, and then reinserted, or it can be allogeneic, consisting of healthy bone marrow obtained from a closely related donor, such as a sibling . Bone marrow transplants are most frequently undergone for leukemia, severe forms of anemia, and disorders of the immune system. The major complications are graft-versus-host disease (as a result of allogeneic transplantation) and infections that occur before the transplanted marrow begins to produce leukocytes.



osteoporosis

osteoporosis, disorder in which the normal replenishment of old bone tissue is severely disrupted, resulting in weakened bones and increased risk of fracture; osteopenia results when bone-mass loss is significant but not as severe as in osteoporosis. Although osteoporosis can occur in anyone, it is most common in thin white women after menopause.

Bone mass is typically at its greatest during a person's mid-twenties; after that point there is a gradual reduction in bone mass as bone is not replenished as quickly as it is resorbed. In postmenopausal women the production of estrogen, a hormone that helps maintain the levels of calcium and other minerals necessary for normal bone regeneration, drops off dramatically, resulting in an accelerated loss of bone mass of up to 3% per year over a period of five to seven years. Smoking, excessive alcohol consumption, and a sedentary lifestyle increase the risk of bone-mass loss; a diet high in protein and sodium also speed calcium loss. The disorder also has a genetic component. A vitamin D receptor gene that affects calcium uptake and bone density has been identified, and the different forms of this gene appear to correlate with differences in levels of bone density among osteoporosis patients.

Osteoporosis has no early symptoms and is usually not diagnosed until a fracture occurs, typically in the hip, spine, or wrist. A diagnostic bone density test is thus recommended as a preventive measure for women at high risk. Treatment can slow the process or prevent further bone loss. Estrogen replacement therapy for postmenopausal women is effective but has potential side effects. Calcitonin, a thyroid hormone, is administered in some cases. Nonhormonal drugs for the treatment of osteoporosis include alendronate (Fosamax) and risedronate (Actonel), bisphosphonates that decrease bone resorption, and raloxifene (Evista), a selective estrogen receptor modulator that can increase bone mineral density. Teriparatide (Forteo), which consists of the biologically active region of human parathyroid hormone, stimulates the activity of osteoblasts, the specialized cells that form new bone. Dietary and supplemental calcium and vitamin D are usually recommended for people at risk, but a seven-year study of more than 36,000 women over 50 that was released in 2006 found that supplements conferred little benefit. Exercise, including weight training, has been found to strengthen bones directly and to improve muscle strength and balance and thus minimize the chance of falls.















leg




leg, one of the paired limbs of an animal used for support of the body and for locomotion. Properly, the human leg is that portion of the extremity between the foot and the thigh. This section of the human leg contains two long bones, the tibia and the fibula. The upper end of the tibia joins with the lower end of the thighbone (femur) and forms a hinged joint. The kneecap (patella), a flat triangular-shaped bone, surrounds and protects this joint. The lower end of both tibia and fibula join with the talus, a bone in the foot, to form the ankle joint. The upper end of the femur, which is the longest bone in the body, forms a ball and socket joint where it meets the hipbone. In quadrupeds, both the hind and fore limbs are referred to as legs.















teeth



teeth, hard, calcified structures embedded in the bone of the jaws of vertebrates that perform the primary function of mastication. Humans and most other mammals have a temporary set of teeth, the deciduous, or milk, teeth; in humans, they usually erupt between the 6th and 24th months. These number 20 in all: 2 central incisors, 2 lateral incisors, 2 canines, and 4 premolars in each jaw. At about six years of age, the preliminary teeth begin to be shed as the permanent set replaces them. The last of the permanent teeth (wisdom teeth) may not appear until the 25th year, and in some persons do not erupt at all. The permanent teeth generally number 32 in all: 4 incisors, 2 canines, 4 bicuspids, and 4 (or 6, if wisdom teeth develop) molars in each jaw. Human canines are the smallest found in any mammal.

Among all mammals, the tooth consists of a crown, the portion visible in the mouth, and one or more roots embedded in a gum socket. The portion of the gum surrounding the root, known as the periodontal membrane, cushions the tooth in its bony socket. The jawbone serves as a firm anchor for the root. The center of the crown is filled with soft, pulpy tissue containing blood vessels and nerves; this tissue extends to the tip of the root by means of a canal. Surrounding the pulp and making up the greater bulk of the tooth is a hard, bony substance, dentin. The root portion has an overlayer of cementum, while the crown portion has an additional layer of enamel, the hardest substance in the body. Most nonmammalian vertebrates do not have the outer layer of enamel on their teeth, but instead have a substance known as vitrodentine, similar to dentine, though much harder.

Proper diet is necessary for the development and maintenance of sound teeth, especially sufficient calcium, phosphorus, and vitamins D and C. The most common disorder that affects the teeth is dental caries (tooth decay). A widely accepted explanation of the process of tooth decay is that salivary bacteria convert carbohydrate particles in the mouth into lactic acid, which attacks the enamel, dentin, and, if left untreated, the pulp of the teeth. Regular cleansing and semiannual dental examinations are important in preventing dental caries and gum disorders. Fluoridation of public water supplies and use of fluoride toothpastes also help prevent caries. In the study of fossil remains done in paleontology and physical anthropology, teeth are the most frequently found remains, a testament to their high mineral content and resistance to deterioration over time.



fluoridation

fluoridation, process of adding a fluoride to the water supply of a community to preserve the teeth of the inhabitants. Tooth enamel ordinarily contains small amounts of fluorides and when the amount is augmented through the intake of fluoridated water, especially during the first eight years of childhood, tooth decay can be greatly reduced.

In the early 1900s, Frederick S. McKay, a Colorado dentist, discovered that an unknown substance in the local drinking water caused a mottling or staining of the teeth and that these teeth also showed fewer cavities. In 1931 the substance was identified as a fluoride. Later, in the 1930s, it was found that a fluoride level in drinking water of about one part per million was high enough to reduce tooth decay but low enough to prevent teeth from becoming mottled.

In some communities fluorides are a natural constituent of the water supply; other communities have added fluorides to their reservoirs. Such action has the support of the American Dental Association, the American Medical Association, and other scientific organizations. Although studies have proven that fluoridation at levels of one part per million is safe, attempts at fluoridation have met with resistance and controversy. Its opponents say that it constitutes compulsory medication, that the amount of fluorine taken into the body cannot be controlled, and that those who wish to prevent tooth decay through fluorides can do so individually by adding the compound to their beverages or by using toothpaste and other dental substances to which fluorides have been added. Despite such resistance, many Americans drink artificially fluoridated water, and fluoridation programs have been started in other countries as well.



dentition

dentition, kind, number, and arrangement of the teeth of humans and other animals. During the course of evolution, teeth were derived from bony body scales similar to the placoid scales on the skin of modern sharks. Tooth structures such as those found in humans are restricted to certain vertebrates, i.e., most fish, mammals, and reptiles, and some amphibians. The teeth of sharks, which are primitive vertebrates, consist of simple conelike structures, sometimes with serrated edges and sometimes flattened for crushing shelled prey. In many lower vertebrates the individual teeth are replaced throughout the animal's life; old tooth loss and new tooth growth follow wavelike patterns down the length of jaw and affect alternate teeth at any one time, so that half the teeth in a region are always functional. Fish and reptiles that have teeth have homodont dentition; that is, all teeth are identical. The mammals have heterodont dentition, or teeth of different basic types, including incisors for nipping or cutting, canines for piercing, and premolars and molars for shearing and grinding. Carnivorous animals have relatively small incisors, used for grasping rather than for cutting; long and strong canines; and relatively thin, sharp premolars and molars, used for severing muscle and other tissues. Herbivorous animals have well-developed incisors, used to cut grass and other vegetation; canines that are either smaller than those of carnivores or absent altogether; and broad, flat premolars and molars for grinding food. In some herbivores, the upper canines are absent, so they cut vegetation by the combined action of the tongue and lower incisors. Omnivorous animals such as man have less specialized dentition. Only part of the dentition of mammals is usually replaced; however, the incisors of rodents grow out at the base as fast as they wear down at the tip. Teeth, the hardest structures in the body, have been well preserved as fossils and have played an important role for paleontologists and physical anthropologists in the study of human evolution.
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Last edited by Sureshlasi; Saturday, September 01, 2007 at 12:47 AM.
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