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