Bone - 3 -- VTPP 435

Bone - 3 -- VTPP 435

Parathyroid Hormone

-comes from chief cell of parathyroid gland
-stimulated by low Ca2+ in blood
-acts on bone to release RANK L
-acts on kidneys to reabsorb more calcium
-acts on kidneys to reabsorb more Ca2+
-activates enzyme in kidney to hydroloxate Vitamin D

itamin DV

-inactive form comes from skin and diet
-acts on intestine to increase Ca2+ and PO43+ absorption

Calcitonin

-comes from clear parafollicular cells in thyroid
-stimulated by high Ca2+ in blood
-acts on osteoclasts to inhibit their action
-only has a cell receptor on osteoclasts

Osteoblasts

-build bone
-derived from mesenchymal cells
-has a Parathyroid hormone receptor on it
-line up on the osteoid
-have a high developed ER
-follows behind osteoclasts

Osteoclasts

-derived from specialized monocytic macrophages
-has a ruffled border
-has a receptor for RANK L to activate it
-uses enzymes and acid to dissolve bone -- lacunae
-moves around

Osteocytes

-located in a lacunae
-most abundant and long lived cell in human body
-branched out via canaliculi
-sense and responds to mechanical force
-sends messages to osteoblasts and osteoclasts

Bone Remodeling

1) triggered by an event that osteocytes detect
2) osteoblasts cause pre-osteoclasts to differentiate into osteoclasts via RANK Ligand
3) osteoclasts cut away until they hit an osteocyte
4) osteoblasts are recruited and proliferated by way of WNT signaling from osteoclasts to trigger osteoblasts

WNTs Signaling

-from osteoclasts
-stimulates mesenchymal stem cells to differentiate into osteoblasts
-suppresses osteoclastic activity by stimulating OPG

Sclerostin

-from osteocytes
-inhibits WNT signaling
-prevents osteoblasts from forming new bone
-stimulated by Calcitonin

Indirect Healing Stages

1) Hematoma Forms
2) Granulation Tissue
3) Cartilage Callus
4) Soft Callus
5) Remodeling

Hematoma Formation

-collection of blood outside of vessels
-triggers an inflammatory response to bring in immune cells that release cytokines
-mesenchymal stem cells differentiate into osteoblasts or chondrocytes

Granulation Tissue

-fibrous connective tissue that forms at an injury site after a hematoma forms
-protects injury from infection and replaces lost tissue
-lays found for early callus formation

Soft Callus

-cartilage gradually replaced by woven bone over time -- endochondral ossification
-chondrocytes hypertrophy and differentiate into osteoblasts
-increased vascularization of cartilage
-early callus

Hard Callus & Remodeling

-woven bone gradually replaced by mature bone over time
-osteoclasts break down woven done and osteoblasts lay new done
-hard callus is continually remodeled over time

Woven Bone

-bone made form intramembranous ossification
-made from organized collagen fibers
-higher mineral density
-mechanically strong