Long bone-Bones-Lecture Slides, Slides of Computer Architecture and Organization

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Long bone

Chemical Composition of Bone:

Organic

• Osteoblasts – bone-forming cells
• Osteocytes – mature bone cells
• Osteoclasts – large cells that resorb or break

down bone matrix

• Osteoid – unmineralized bone matrix

composed of proteoglycans, glycoproteins,

and collagen

Hormonal Regulation of Bone

Growth During Youth

• During infancy and childhood, epiphyseal plate

activity is stimulated by growth hormone

• During puberty, by testosterone and estrogens
  • Initially promote adolescent growth spurts
  • Cause masculinization and feminization of specific parts of the skeleton
  • Later induce epiphyseal plate closure, ending longitudinal bone growth

Bone Remodeling

• Remodeling units – adjacent osteoblasts and

osteoclasts deposit and resorb bone at

periosteal and endosteal surfaces

Bone Resorption

• Accomplished by osteoclasts
• Resorption bays – grooves formed by osteoclasts

as they break down bone matrix

• Resorption involves osteoclast secretion of:
  • Lysosomal enzymes that digest organic matrix
  • Acids that convert calcium salts into soluble forms
• Dissolved matrix is transcytosed across the

osteoclast’s cell where it is secreted into the interstitial fluid and then into the blood

Remodeling

Calcitonin

• Calcitonin targets the skeleton, where it:
  • Inhibits osteoclast activity and thus bone resorption and release of calcium from the bone matrix
  • Stimulates calcium uptake and incorporation into the bone matrix

Effects of Parathyroid Hormone

• PTH release increases Ca2+^ in the blood as

it:

• Stimulates osteoclasts to digest bone matrix
• Enhances the reabsorption of Ca2+^ and the secretion of phosphate by the kidneys
• Increases absorption of Ca2+^ by intestinal mucosal cells
• Rising Ca2+^ in the blood inhibits PTH

release

Response to Mechanical Stress

• Wolff’s law – a

bone grows or

remodels in

response to the

forces or

demands

placed upon it

• Observations

supporting Wolff’s law include:

• Long bones are thickest midway along the shaft (where bending stress is greatest)
• Curved bones are thickest where they are most likely to buckle
• Large, bony projections occur where heavy, active muscles attach