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→Composition of Bone
***Bone-specific proteins(10%): Osteonectin, Osteopontin, Osteocalcin
***Bone-specific proteins(10%): Osteonectin, Osteopontin, Osteocalcin
**'''Mineral''': Inorganic component, provides rigidity
**'''Mineral''': Inorganic component, provides rigidity
**Crystalline lattice of calcium phosphate and calcium carbonate
***Crystalline lattice of calcium phosphate and calcium carbonate
**Also contains Mg, Mn, Zn, Cu, Na, F
***Also contains Mg, Mn, Zn, Cu, Na, F
**Accounts for 65% of bone
***Accounts for 65% of bone
*Cells:
*Cells:
**'''Osteoblasts''': single layer of mesenchymal cells which synthesize bone extracellular matrix (ECM)
**'''Osteoblasts''': single layer of mesenchymal cells which synthesize bone extracellular matrix (ECM)
***When active, appear plump and cuboidal, with basophilic cytoplasm
***When active, appear plump and cuboidal, with basophilic cytoplasm
****Have receptors for [[Bones - normal#Bone resorption|PTH]]: contract in response -> provide space for osteoclasts to attach
****Have receptors for [[Bones - normal#Bone resorption|PTH]]: contract in response -> provide space for osteoclasts to attach
***When inactive - less cytoplasm -> flattened
***When inactive - less cytoplasm -> flattened
**'''Osteocytes''': osteoblasts embedded in their own matrix; reside within '''lacunae''' and are interconnected via channels forming '''canaliculi'''
**'''Osteocytes''': osteoblasts embedded in their own matrix; reside within '''lacunae''' and are interconnected via channels forming '''canaliculi'''
***'''Canaliculi''' create connections to form a huge neural-like junctional organization
***'''Canaliculi''' create connections to form a huge neural-like junctional organization
***Contact osteoblasts and each other with cytoplasmic processes
***Reach through canaliculi in mineralised bone matrix
***Regulate composition of bone fluid
**'''Osteoclasts''': giant (multinucleate monocytes) cells which act to resorb bone ECM
**'''Osteoclasts''': giant (multinucleate monocytes) cells which act to resorb bone ECM
***Histologically:
****Large, often multinucleated cells
****Acidophilic cytoplasm
***Derived from haematopoietic stem cells
***Sit in bone surface depression - '''Howship's lacuna'''
***Respond to vitamin D by increasing their numbers and activity (parathyroid independent)
***Responsible for bone resorption
****Firstly dissolve mineral followed by collagen, using brush border
****Do <u>not</u> have receptors for PTH
****Have receptors for [[Bones - normal#Bone resorption|calcitonin]]
****Involute their brush border in response
**Osteoblasts, osteocytes, chondroblasts and chondrocytes are derived from stromal fibroblastic system ('''osteoprogenitor cells'''); osteoclasts from haematopoietic system
**Osteoblasts, osteocytes, chondroblasts and chondrocytes are derived from stromal fibroblastic system ('''osteoprogenitor cells'''); osteoclasts from haematopoietic system
All mammalian bone is organized as lamellated sheets:
All mammalian bone is organized as lamellated sheets: