Bone is living, dynamic tissue that affects three main functions of the body: skeletal homeostasis (providing structure to house the internal organs as well as a system of pulleys and levers to move the body), mineral homeostasis (storing and re- leasing ions), and hematopoiesis (accommo dating the machinery of blood cell formation). As living tissue, bone is in a state of contin- uous flux and renewal; osteoclasts resorb bone, and osteoblasts create new bone. This process allows ions to be released and stored. It also repairs areas of structural damage.

Most bone forms according to the carti- lage model, a process called endochondral ossification. The process of bone growth and development, therefore, is one of transfor- mation from a cartilage skeleton in utero to one made entirely of bone in the adult. The pediatric skeleton contains remnants of this cartilage skeleton, the cartilaginous growth plate or physis. The physis is active metabol- ically and allows the pediatric skeleton to grow longitudinally; however, it is also struc- turally weaker than bone, which makes the pediatric skeleton susceptible to a class of diseases and injuries not seen in the adult.

1. Zaleske DJ: Cartilage and bone development. Instr Course Lect 1998;47:461-468.

 

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Alkaline phosphatase An enzyme produced by osteoblasts that is believed to play a role in the mineralization of bone.

 

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Skeletal Formation:

Current investigations attempt to identify the genetic, biochemical, and biomechanical signals that cause the skeleton to be formed, take the shape it does, and stop the formation process when complete. These basic biologic studies add to our knowledge, of course, but they may also suggest why diseases of bone loss or dysfunction occur.

 

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Calcitonin is a peptide that is produced and secreted by the chief cells of the thyroid gland. This peptide has a direct effect on osteoclasts by decreasing their size and inhibiting their ability to resorb bone.

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Vitamin D is a steroid hormone; as such, it binds to the nucleus of a cell and modifies gene expression and protein synthesis.

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PTH is synthesized by the parathyroid glands and stored within the glands themselves. In response to low concentrations of serum calcium, PTH is released, and the synthesis of additional supplies is initiated.

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The primary circulating factors that affect calcium balance are,

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The three points of calcium flux, the intestine, the kidneys, and the bone, are all regulated and can modify their normal exchange rates in response to metabolic demands.

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Like calcium, phosphorus is stored in the body primarily in the bone, although nearly 100 g are present in the extracellular fluid, approximately 10 times the amount of extraskeletal calcium.

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Calcium is a key mineral that contributes to the structural strength of the bone, but it has a myriad of other functions in the body as well, including participating in enzyme activation, nerve transmission, and muscle contraction.

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