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====Cells====
−*Osteoblasts, osteocytes, chondroblasts and chondrocytes are derived from stromal fibroblastic system ('''osteoprogenitor cells'''); osteoclasts from haematopoietic system
−*Ischaemia and hypoxia favour development of cartilage
−*High oxygen tension and good blood supply favour bone development
−*'''Osteoblasts'''
−**Mesenchymal cells
−**Arise from bone marrow stroma
−**Histologically:
−***Plump and cuboidal when active
−***Basophilic cytoplasm
−***When inactive - less cytoplasm -> flattened
−**Produce bone matrix = '''osteoid''' - uncalcified
−***Homogeneous substance
−***Stains light pink with H&E
−**Cell membranes are rich in alkaline phosphatase (ALP)
−***Possibly involved in pumping calcium across membranes
−**Promoted by growth factors
−**Have receptors for [[Bones - normal#Bone resorption|PTH]]
−***They contract in response -> space for osteoclasts to attach
−*'''Osteocytes'''
−**Osteoblasts that have become surrounded by mineralised bone matrix
−**Occupy cavities called '''lacunae'''
−**Contact osteoblasts and each other with cytoplasmic processes
−***Reach through canaliculi in mineralised bone matrix
−**Regulate composition of bone fluid
−*'''Osteoclasts'''
−**Histologically:
−***Large, often multinucleated cells
−***Acidophilic cytoplasm
−**Derived from haematopoietic stem cells
−**Responsible for bone resorption
−***Firstly dissolve mineral followed by collagen
−***Use brush border for this
−**Sit in bone surface depression - '''Howship's lacuna'''
−**Do <u>not</u> have receptors for PTH
−**Have receptors for [[Bones - normal#Bone resorption|calcitonin]]
−***Involute their brush border in response
−***Detach from bone surface
−**Respond to vitamin D by increasing their numbers and activity (parathyroid independent)
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===Normal structure===
===Normal structure===
[[Image:Bone micro structure.jpg|right|thumb|100px|<small><center>Microscopic bone (Courtesy of RVC Histology images)</center></small>]]
[[Image:Bone micro structure.jpg|right|thumb|100px|<small><center>Microscopic bone (Courtesy of RVC Histology images)</center></small>]]
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