Ossification
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What are the 2 types of ossification?
• Intramembranous 2. Endochondral
What is ossification?
Formation of bones by the replacement of pre-existing CT
What bones are formed from intramembranous ossification?
Flat bones e.g. cranial bones
How many stages are there in intramembranous ossification?
4
What occurs at stage 1 of intramembranous ossification?
• Mesenchymal cells differential into osteoblasts - Cluster of osteoblasts form ossification centre
What is the function of an ossification centre?
Lay down osteoid
What occurs at stage 2 of intramembranous ossification?
• Mineralisation of osteoid to produce island of bone - Trapped osteoblasts become osteocytes
What occurs at stage 3 of intramembranous ossification?
• Islands of bone join to form trabeculae - Mesenchyme cells at edges condense to form periosteum
What occurs at stage 4 of intramembranous ossification?
• Red marrow develops from mesenchymal cells in trabecular spaces
What bones are formed from endochondral ossification?
Long bones of the body
How many stages are involved in endochondral ossification?
5
Why is endochondral ossification called so?
Involves the ossification of a hyaline cartilage template
What makes up the hyaline cartilage template involved in endochondral ossification?
Hyaline deposited in the shape of the bone within a mass of mesenchymal tissue
What occurs at stage 1 of endochondral ossification?
• Cartilage cells in central diaphysis die to leave cavities - Perichondrium produces osteoblasts which lay down bone collar by IM ossification
What occurs at stage 2 of endochondral ossification?
• Cartilage matrix in centre continues to deteriorate • Diaphysis diameter increases by bone deposition on outside and resorption on inside • Chondrocytes in bone multiply, increasing length and breadth
What occurs at stage 3 of endochondral ossification?
• Capillaries penetrate diaphysis carrying in osteoprogenitor cells • Osteoprogenitor cells become osteoblasts and set up primary ossification centre • Bone laid down to replace lost cartilage in centre
What occurs at stage 4 of endochondral ossification?
• Capillaries penetrate epiphyses of bone • Osteoprogenitor cells pulled into epiphyses and produce secondary ossification centres • Formation of medullary cavity
When does stage 4 of endochondral ossification occur?
Approx. at birth
What occurs at stage 5 of endochondral ossification?
• Ossification of epiphysis is complete - Cartilage only remains in epiphyseal growth plate and articular cartilage
What are the 5 zones of the epiphyseal growth plate?
• Resting • Proliferate • Hypertrophic • Ossifying Cartilage • Bone Deposition
What occurs in the hypertrophic zone?
Osteoblasts get bigger and begin to calcify
What is the weakest part of the skeleton?
Joints