My Clinical Notes

• Skeletal system is composed of 206 bones – flat, tubular and cuboid
• Calcium hydroxyapatite is the organic component of bone an gives bone its hardnes and strength. It also stores 99% of the bodies calcium, 85% of its phosphorous, and 65% of the bodies sodium and magnesium
• The formation of calcium hydroxyapatite crystals is termed mineralization. There is a 12-15 day lag between formation of matrix and mineratlisation.
• Bone that is unmineralised is called osteoid

 

Osteoblasts and osteoclasts

 

• The generation and stimulation of these cells is mediated by FGF, PDGF, TGF-b and insulin-like growth factor
• Osteoprogenitor cells
  • Pluripotent mesenchymal cells that can form osteocytes when stimulated by members of the TGF-b superfamily
• Osteoblasts
  • Located on the surface of bone and synthesise, transport matric proteins. They initiate the process of mineralization and express cell surface receptors that bind to hormones, PTH, VitD and oestrogen. They have a life span of three months after which they can either die via apoptosis or form osteocytes
• Osteocytes
  • More numerous than any other bone forming cell. They are encased in bone but can communicate with each other through an intricate network or canalculi. This network is involved in controlling fluctuations in calcium and phosphorus levels
• Osteroclasts
  • Derived from the same progenitor as monocytes and macrophages. They are involved in bone reabsorption. RANK-RANKL interaction is involved in osteoclastogenesis. When mature they are multi-nucleated and have a life span of 2 weeks
• The proteins of bone include Type I collagen (which makes up 90% of the weight) and noncollagenous proteins produced by osteoblasts.
• Osteoblasts can deposit collagen in either a random weave known as woven bone or in an orderly layered manner known as lamellar bone.
• Woven bone is found in the fetus, in growth plates and during pathology such as a fracture. It can be formed quickly and resists forces equally in all directions
• Lamellar bone is found in adults, is produced more slowly and is stronger. There are 4 types of lamellar bone;
  • Circumferential
  • Concentric
  • Interstitial
  • Trabecular lamellae

 

Bone Modelling and Remodelling

 

• Osteoblasts and osteoclasts act in coordination to model bone and are referred to as the Basic multicellular unit (BMU)
• The process of bone formation and reabsorption are tightly coupled
• As the skeleton enlarges, bone formation predominates
• Once the skeleton reaches maturity, the breakdown and renewal of bone is called remodelling and is mostly experienced in areas experiencing fatigue and microdamage
• In the adult around 1 million BMU are active at any one time turning over 10% of bone every year
• Peak bone mass is determined by;
  • Type of VitD receptor inherited
  • Nutrition
  • Level of physical activity
  • Age
  • Hormonal status
• Beginning in the 4^th decade the amount of bone absorbed by the BMU starts to become greater than that formed, resulting in a steady decrease in bone density

 

Bone growth and development

 

• Blueprint for skeletal formation is encoded by Homeobox genes, the expression of which produces localised cellular condensations of primitive mesenchyme at the site of future bones. The mesenchymal cells differentiate into chondrocytes and osteoblasts
• Most bones are formed as a cartilage model and around eight weeks gestation, endochondral ossification begins

Whilst long bones undergo endochondrial ossification. Flat bones undergo intramembranous ossificication whereby mesenchymal cells differentiate directly into osteoblasts and synthesize the osteoid on which cal

This entry was posted on Thursday, November 15th, 2007 at 2:10 pm and is filed under Orthopaedics. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.