intramembranous except for base of skull which is endochondral
Axial skeleton
derived from sclerotome of somites
largely endochondral
Appendicular skeleton
derived from lateral plate mesoderm that contributes to the formation of limb buds
largely endochondral
Intramembranous
Directly from mesenchymal condensations
Begins at end of embryonic period
Mesenchymal cells differentiate into osteoblasts & lay down osteoid
eg. Cranial vault, facial bones, clavicle
Endochondral
mesenchymal (blastemal) condensation changes into a cartilaginous miniature precursor at end of embryonic period
mesenchymal elements begin to chondrify at ~ 6w of embryonic development
at ~ 7 weeks the cartilage cells which are all the round cell type initially, become hypertrophic & local matrix begins to calcify
simultaneously a periosteal sleeveof bone forms circumferentially at the midshaft of each cartilage anlage
by direct ossification of collagenous material intramembranous bone formation occurs
at 8w blood vessels extend thru the periosteal sleeve into the central portion of the hypertrophied & calcified cartilaginous anlage expanding the primary centre of ossification
happens first in humeri but rapidly throughout body
this marks the point between embryo & fetus
blood supply brings lineage of bone cells that differentiate into osteoblasts & osteoclasts
osteoclasts are responsible for removing the primary trabeculae & forming the medullary canal
primary centre is enlarged by the process of advancing enchondral ossification & trailing osteoclastic resorption
cartilaginous regions at either end of the 1° centre of ossification become the growth regions or physes
at time of morphogenesis the joint clefts are formed by a process of joint cavitation within
