Bone

Osteoprogenitors differentiate into osteoblasts.

Osteoblasts: synthesize ECM; secrete vesicles that induce mineralization of the osteoid; lots of RER

Osteocytes: osteoblasts that become trapped in the matrix; maintain bone, stimulate osteoblasts and bone lining cells; can secrete or resorb small amounts of bone

Osteoclasts: bone resorption, very big and multinucleate

Spongy: high strength to weight ratio; bone marrow is here.

Cortical: densly arranged, contains Haversian systems

woven: early growth and development, lots of cells, disorganized collagen

Lamellar: almost the entire adult skeleton; collagen fibres are organized into layers called lamellae.

• Organic (30% OF MASS):
•   -mostly collagen (90%) (tensile strength)

• Inorganic (70% of Mass)
•  -Mostly Hydroxyapatite intamitely attached to collagen (provides compressive strength)

• 1. Small ‘model’ of bone is formed from hyaline cartilage
• 2. Cartilage matrix calcifies and cells degenerate
• 3. Pericondrium transforms into a periosteum
• 4. A bony ‘collar’ forms
• 5. Blood vessels and mesenchymalcells invade
• 6. Osteoblast and bone marrow cell populations are established
• 7. Bone is formed on the remnants of calcified cartilage – to be later remodeled.
• 8. The initial ossification in the diaphysis is known as the primary centre
• 9. Secondary centres form within the epiphyses
• 10. Cartilage remains within the ‘growth plates’ (epiphyseal plates, physes) which facilitate continued longitudinal growth

• 1) protection of organs
• 2) structural support
• 3) calcium and phosphorus reservoirs
• 4) Site for blood cell formation

• 1) osteoblasts secrete vesicles mineralization inhibitors
• 2) osteoblasts also secrete collagen
• 3) ions are elevated above the level of the osteoid
• 4) vesicles burst and allow mineralization of collagen to begin

At the periosteum, the oseocytes here lay down bone that increases the diameter of long bones.

internal: the internal turnover of bone

external: remodeling the extenal morphology of bone.