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METABOLIS AND ENDOCRINE DISORDERS

Bone have obvious mechanical function; they support and protect the soft tissues, transmit load

and muscular force from one part of the body to another and , mediate movement and

locomotion.

Bone as a tissue has an equally important role as a mineral reservoir which help to regulate the

composition and in particular the calcium ion concentration of the extracellular fluid. For all its

solidity, bone is structure changing from moment to moment in concert with the normal

variations in mechanical function and mineral exchange. All modulation in bone structure and

composition are brought about by celluler activity, which is regulated by hormones and local

factors; these agents, in turn, are controlled by alterations in mineral ion concentration. The

metabolic bone disorder are conditions in which generalized skeletal abnormalities result from

disruption of this complex interactive system,

Bone and Bones

Bone composition

Bone consists of a largely collagenous matrix which is impregnated which mineral salts and

populated by cell.

The matrix

The matrix is composed of type 1 collagen lying in a mucopolysaccharide ground substance.

There are also small amounts of non collagenous protein, mainly in the form of proteoglycans

and the bone specific proteins osteonectin, which appears to be involved in bone

mineralozation and osteocalcin or Gla protein whose function is unknow. Gla protein is

produced only by osteoblast and its concentration in the bood is, to some extent, a measure of

osteoblastic activity. The unmineralized matrix is known as osteoid; normally it is seen only

as a thin layer on surfaces where active new bone formation is taking place, but the proportion

of osteoid to mineralized bone increases significantly in rickets and osteomalacia.

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Bone celis

There are of three types of bone cell: osteoblasts, osteo-cytes and osteoclasts.

Osteoblasts are concerned with bone formation; they are derived from localmesenchymal precursors and form rows of small (20u.m) cuboida! cells along the freesurfaces of trabeculae and haversian systems where new bone is laid down. They are

rich in alkaline phosphatase and are responsible for the production of type I collagenas well as the non-collagenous bone proteins and for the mineralization of bonematrix (Feck and Woods, 1388). They may also be involved in the initiation andcontrol of osteoclastic activity. At the end of a bone remodelling cycle the osteoblasteither remains on the newly formed surface as a quiescent lining cell or becomesenveloped in the matrix as a resting osteocyte.

Osteocytes can therefore be regarded as spent osteoblasts.' Lying in their bony lacunae,they communicate with each other and with the surface lining cells by slender cyto-plasmic processes. Their function is obscure: they may, under the influence ofparathyroid hormone, participate in bone resorption ('osteocytic ostcolysis') andcalcium ion transport (I'eck and Woods, 1')Ht)|, It has also 1KTII suggested that they ansensitive to mechanical itlrnull

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