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• Cells of the Bone
• Bone contains four types of cells: Osteocytes, Osteoprogenitor cells (stem cells), Osteoblasts (osteogenesis) and Osteoclasts (osteolysis).
• Osteoprogenitor cells
• These are mesenchymal stem cells that divide and differentiate into osteoblasts.
• These maintain populations of osteoblasts and are important in the repair of a fracture.
• Osteoprogenitor cells located in the periosteum and the bone marrow (endosteum).
• Osteoblasts
• These originate from osteoprogenitor cells.
• Form new bone matrix in a process called osteogenesis. In other words, osteoblasts form the osteoid, which is then calcified into bone.
• Osteoblasts also assist in the calcification of osteoid into bone.
• As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretion and become osteocytes.
• Osteogenesis
• Osteoblasts secrete collagen molecules and ground substance (extrafibrillar matrix – gel-like substance surrounding the collagen molecules).
• Collagen molecules combine to form collagen fibres.
• The resultant tissue is called osteoid (non-calcified bone).
• Hydroxyapatite crystals form on the collagen fibres. The osteoid is now calcified and this is bone.

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