Bone formation, modeling and remodeling Flashcards
two mechanisms of bone formation
endochondral ossification
intramembranous ossification
Endochondral Ossification
- -Formation of the appendicular skeleton
- -hyaline cartilage model is sequentially resorbed and then replaced with bone
- -emphasis on importance of blood supply
- -cartilage precursor
endochondral ossification: longitudinal growth
1) epiphyseal cartilage removal: secondary center of ossification
2) After mineralization angiogenesis and resorption
3) woven bone deposition (primary spongiosa), remodeling to cortical bone
4) endochondral ossification of physeal cartilage between the primary and secondary centers of ossification
Resting zone
small, inactive mesenchymal stem cells that divide and give rise to chondrocytes
Proliferation zone
Columns of chondroblasts which quickly divide, pushing epiphysis away from the diaphysis
THIS LENGTHENS THE BONE
maturation/hypertrophic zone
Chondrocytes mature and enlarge
calcification zone
cartilaginous matrix calcifies resembling trabeculae
Chondrocytes die
ossification zone
Osteoclasts degrade cartilage
osteoblasts form actual trabecular bone
Intramembranous Ossification
Key difference: No cartilage intermediate
involves condensation and differentiation of mesenchymal tissues
Circumferential bone growth
appositional bone growth occurs by direct deposition of bone at periosteal surface
–at same time bone is resorbed from the endosteal surface to cause the marrow cavity to increase in size
Basic multicellular unit (BMU)
Osteoblast + osteoclast
General bone remodeling
ARFF=activation, resorption, Reversal, Formation
Trabecular bone remodeling
1) the BMU is located on the suface
2) covered by a canopy predominantly of mesenchymal cell origin with osteoclasts resobing bone
3) resorbed surface is cleaned up by lining cells and probably macrophages
4) osteoblast precursors differentiate to fill the space that has been resorbed
cortical bone remodeling
1) BMU in cortex comprises a cutting zone led by osteoclasts
2) Cutting conce proceeds through bone followed by differentiating osteoblasts
3) resultant space is filled by blood vessels, nerves and connective tissue
4) like trabecular bone it can also occur on periosteal and endosteal surfaces
what are cement lines
- -basophilic lines between lamellae
- -represent “reversal” points (bone is removed down to this level)
- -new bone is then added on top of the cement line
- -observed in both cortical and trabecular bone
Modeling
Skeletal immaturity= formation»resorption and is longitudinal and appositional growth
involves changes in bone size and shape (formation and resorption on different surfaces)
Ex: Radial growth (increasing diameter) of growing bone
At Skeletal Maturity
Formation=resorption (remodeling)
turnover of skeleton
100%=first year of life
5-10% annually thereafter
when resorption exceeds formation
the integrity of the bone is decreased
what drives remodeling
1) mechanical loading
2) systemic and local hormones
3) Trauma
4) Microfractures-failure to remove leads to stress fractures
Skeletal response to loading
bone remodels according to the forces acting upon it (wolff’s law)
bone can sense mechanical forces and transduce the signal and respond accordingly
Bone deformation
1) piezo-electric currents
2) increase or decrease local bone formation
3) induce complementary osteoclast activity
loading stimulates..
net formation (decreased activation, increased bone formation)
Unloading stimulates..
bone resorption (increased activation frequency)
Disuse osteopenia
due to Limb disuse (paralysis or surgery)
Restoration of optimal bone
depends on anatomic alignment and a return to functional activity
