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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

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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)

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Intramembranous Ossification

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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

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Endochondral Ossification

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Endochondral Ossification

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Endochondral Ossification

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Endochondral Ossification

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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