Bone Development (during fetal development) Intramembranous ossification Takes place in connective tissue membrane Endochondral ossification Takes place in cartilage Both methods of ossification Produce woven bone that is then remodeled After remodeling, formation cannot be distinguished as one or other

Intramembranous Ossification (8th week – 2 years of development) Takes place in connective tissue membrane formed from embryonic mesenchyme Forms many skull bones, part of mandible, diaphyses of clavicles When remodeled, indistinguishable from endochondral bone. Centers of ossification: locations in membrane where ossification begins (centers of ossification expand outwards to form a bone by gradually ossifying the membrane) Fontanels: large membrane-covered spaces between developing skull bones; unossified (bones eventually grow together & all fontanels have closed by 2 years of age)

Intramembranous Ossification

Endochondral Ossification Bones of the base of the skull, part of the mandible, epiphyses of the clavicles, and most of remaining bones of skeletal system Cartilage formation begins at end of fourth week of development Some ossification beginning at about week eight; some does not begin until 18-20 years of age

Endochondral Ossification

Endochondral Ossification

Endochondral Ossification

Endochondral Ossification

Growth in Bone Length Growth in length occurs at the epiphyseal plate Involves the formation of new cartilage by Interstitial cartilage growth Appositional growth on the surface of the cartilage Closure of epiphyseal plate: epiphyseal plate is ossified becoming the epiphyseal line. Between 12 and 25 years of age Articular cartilage: does not ossify, and persists through life Appositional growth only Interstitial growth cannot occur because matrix is solid Occurs on old bone and/or on cartilage surface