Osteoblasts Origin derived from undifferentiated mesenchymal cells mesenchymal cells then differentiate into osteoprogenitor cells Structure contain increased amounts of endoplasmic reticulum, Golgi apparatus, and mitochondria than other cells allows for synthesis and secretion of bone matrix Function form bone by producing non-mineralized matrix alkaline phosphatase type I collagen osteonectin osteocalcin login to view 1 more bullet regulate osteoclast function Signaling osteoblastic differentiation BMP stimulates mesenchymal cells to become osteoprogenitor cells core binding factor alpha-1 (cbf alpha -1: RUNX2) stable beta-catenin plays a major role in inducing cells to form osteoblasts with resulting intramembranous bone formation platelet derived growth factor (PDGF) induces osteoblast differentiation insulin derived growth factor (IDGF) induces osteoblast differentiation osteoblast bone production PTH receptor login to view 1 more bullet 1,25 dihydroxyvitamin D receptor login to view 2 more bullets estrogen inhibits bone resorption and stimulates bone production by inhibiting adenylyl cyclase glucocorticoids inhibit collagen and bone matrix production prostaglandins stimulate bone resorption by activating adenylyl cyclase osteoclast signaling interconnected signaling allows coupling of bone resorption and formation osteoclast activation login to view 2 more bullets osteoclast inhibition login to view 3 more bullets regulation of hematopoietic cells and immune response occurs through the Jagged1-Notch pathway login to view 2 more bullets Location more metabolically active cells at the bone surface less active cells in more central bone activated by disruption of the more peripheral osteoblasts Osteoclasts Function reabsorb bone osteoblasts regulate osteoclast bone reabsorbtion (see above) steps in resorption cycle login to view 7 more bullets Origin originate from myeloid hematopoietic cells from monocyte/macrophage cell lineage monocyte progenitors fuse together to form mature multinuclear cells Cellular biology cellular anatomy multinucleated giant cells cellular physiology bone reabsorbtion occurs at ruffled border login to view 11 more bullets Molecular biology osteoclast-bone attachment osteoclast attaches to bone matrix at sealing zone attach to bone surfaces via integrins on osteoclast surface login to view 4 more bullets osteoclast polarization contain specialized membrane domains login to view 3 more bullets mineralized bone matrix degradation hydroxyapatite crystals dissolved by HCl secreted through ruffled border into resorption lacuna (RL) login to view 4 more bullets organic bone matrix degradation lysosomal cysteine proteinases matrix metaloproteinases (MMPs), esp MMP-9 cathepsin K login to view 1 more bullet removal of degradation products by transcystosis to FSD, where they are secreted into ECF tartrate-resistant acid phosphatase (TRAP) is localized in transcytotic vesicles, generates reactive O2 species that destroys collagen osteoclast-osteoblast signaling osteoblasts upregulate and downregulate osteoclast activity login to view 8 more bullets Osteocytes Origin are former osteoblasts trapped in the matrix they produced account for 90% of cells in the mature skeleton Structure high nucleus to cytoplasm ratio have long cellular processes which communicate with other cells via canalculi in the bone Function maintain bone and cellular matrix important in regulation of calcium and phosphorous concentrations in bone do not express alkaline phosphatase Signaling stimulated by calcitonin inhibited by PTH communicate with adjacent osteocytes via gap junctions in canaliculi Osteoprogenitor Cells Origin originate from mesenchymal stem cells environment will determine their function Function become osteoblasts under low strain and high oxygen tension become cartilage under intermediate strain and low oxygen tension become fibrous tissue under high strain