Bone Flashcards
Describe osteoclasts
very large and multiple nuclei and motile. essential for matrix reabsorption during bone growth and remodeling. they originate from monocytes. they require the RANKL to differentiate.
How do osteoclasts remodel bone?
proton pumps acidify and promote dissolution of the hydroxyapatite and release proteinases and other hydrolytic enzymes from lysosome related secretory vesicles.
how are osteoclasts regulated?
controlled by signaling factors from other bone cells. they are activated by parathyroid hormone and produce RANKL and other factors that regulate their activity.
describe osteoblasts
originate from mesenchymal stem cells, they produce components of bone matrix including collagen fibers, proteoglycans and matricellular glycoproteins like osteonectin.
osteoblast activity
they secrete a collagen rich material called osteoid between the osteoblast layer and the preexisting bone surface. they process of growth is completed by deposition of calcium salts into the matrix.
mineralization of osteoblasts
osteoblasts release matrix vesicles, the released matrix vesicles are with collagen fibers, there is mineralization around the vesicles and then hydroxyapatite crystals form and begin calcification. eventually this produces calcified materials embedding the collagen fibers and proteoglycans
describe osteocytes
osteoblasts become surrounded by the material they secrete and then they become osteocytes enclosed within the lacunae throughout the mineralized matrix.
Mineralization of bone from osteoblasts
- Osteocalcin – binds Ca²⁺ increase local Ca²⁺
- Matrix vesicles of alkaline phosphatase – increases PO₄⁻ locally 3. Ca and PO form calcified nanocrystal
- Nanocrystals merge and grow, forming hydroxyapatite crystals → merge into solid bone matrix
Composition of the bone matrix
90% type 1 collagen, small proteoglycans, multiadhesive glycoproteins such as osteonectin which is a calcium binding protein
Periosteum
Two layers, inner stem cells, outer dense CT
mostly cellular, includes osteoblasts, bone lining cells, and mesenchymal cells (osteoprotenitor cells). potential to proliferate extensively producing osteoblasts and osteoprogenitor cells. play major role.
Endosteum
covers small trabeculae of matrix and contains osteoprogenitor cells, osteoblasts, and bone lining cells with a sparse delicate mix of collagen fibers
Epiphyses
in long bones; the bulbous ends. composed of cancellous bone covered by a thin layer of cortical bone.
Diaphysis
the cylindrical part, totally dense compact bone with a thin region of cancellous bone on the inner surface around the central marrow cavity.
Where are osteogenic stem cells?
periosteum and endostieum
Intramembranous ossification
osteoblasts differentiate from mesenchyme and begin secreting osteoid. (most bones of the skull, jaws, scapula and clavicle are formed embryonically by intramembranous ossification.
process of intramembranous ossification
1) bone formation begins in ossification centers
2) osteoprogenitor cells arise, proliferate, adn form incomplete layers of osteoblasts around a network of developing capillaries
3) osteoid secreted by the osteoblasts calcifies forming irregular areas of woven bone with osteocytes in lacunae and canniculi
bone forms gradually and is replaced by cancellous bone with marrow and larger blood vessels.
Endochondral ossification
takes place within hyaline cartilage, shaped as a small version or model of the bone to be formed. it forms most bodies especially long bones
Endochondral ossification process
1) late in first semester, bone collar develops beneath periosteum
2) capillaries invade and osteoprogenitor cells which produce a primary ossification center in diaphysis
3) osteoid is deposited by osteoblasts and under goes calcification into woven bone
4) remodeled into compact bone
5) at the time of birth, secondary ossification centers develop.
6) during childhood, primary and secondary ossification centers are separated by the epiphyseal plate which allows for continued bone elongation
7) two ossification centers do not merge until the epiphyseal plate disappears and the full stature is achieved
Bone fracture repair
1) blood vessels torn within the fracture release blood that clots to produce a large fracture hematoma
2) clot gradually removed by macrophages and replaced by a soft fibrcartilage like mass called a procallus tissue. of torn by the break, the periosteum establishes its continuity over the tissue.
3) the procallus is invaded by regenerating blood vessels and proliferating osteoblasts. over the next few weeks fibrocartilage is replaced by woven bone that forms a hard callus.
4) over time the woven bone is remodeled to produce compact and cancellous bone
spongey bone
- LINED WITH OSTEOBLASTS (simple cuboidal layer) and OSTEOCLASTS (form a multinucleated large cell on spicule)
- REMODELING HERE!
compact bone
MANY OSTEONS
- Osteocytes in lacunae
- Haversian canals (neurovascular structure)
osteopetrosis vs. ostoporosis
petrosis - heavy bones
porosis - brittle bones
Parathyroid hormone
PTH → stimulates osteoblasts → secrete RANKL → mature osteoclasts → INCREASE Calcium in blood
Calcitonin
Calcitonin → opposite → inhibits osteoclasts → Lower net reabsorption → DECREASE BLOOD CALCIUM
