Physiology 3120 Lecture Notes - Lecture 9: Hidden Place, Chondrocyte, Ossification

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Renal Physiology
Lecture 9
Hormonal Regulation of Calcium Balance
Bone serves more functions than just locomotion and protection of vital organs;
99% of body calcium is stored in bones
Parathyroid hormone regulates the level of calcium in the extracellular fluid volume
o Stop excretion of Ca2+ as well
Calcitrol (vit D) affects absorption of calcium in the GI tract
Calcium is stored as hydroxyapatite crystals (calcium and phosphate salts) in bone
to prevent large changes in plasma calcium levels
o Hidden place for Ca2+ to be
o If you really need Ca2+, can dissolve bones a bit
These crystals also provide bone strength
Types of Bone Formation
Two types of bone formation:
1. Endochondral Ossification
o Long bones (femur, tibia, etc); most bones form this way
o Forms a cartilaginous template (pre-bone)
o Replaced by bone via mineralization
o Cartilaginous template dictates length of bone
2. Intramembranous Ossification
o Flat bones, etc
Bones are categorized by the type of developmental process that forms them
Endochondral Bone: Cartilage Template Formation
** look at study guide for detail
Mesenchymal cells group together, come in contact and differentiate into
chondrocytes
Chondrocytes are very metabolically active (secretes or reduce ECM if necessary
depending on its surroundings)
1. Chondrocytes secrete an ECM containing
Type II collagen and proteoglycans
2. Chondrocytes in the centre of the template
differentiate to hypertrophic
chondrocytes
Chondrocytes Hypertrophic
chondrocytes (terminal cell type)
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3. Hypertrophic chondrocytes secrete type X
collagen and less proteoglycans (more
rigid)
4. Localized hypoxia triggers secretion of
angiogenic factors, recruitment to
hypertrophic chondrocytes
Hypoxia bc template is getting bigger and
not getting enough nutrients via diffusion
so need to vascularize
5. Hypertrophic chondrocytes mineralize the
surrounding matrix and undergo
apoptosis (even more rigid)
o Cells are gone but leave behind
ECM
6. Invading blood vessels bring in
osteoblasts and osteoclasts to this
mineralized zone (Primary Ossification)
o Osteoblasts replace it with bone
matrix
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