Bone Development during fetal development Intramembranous ossification Takes

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 6 -1

Intramembranous Ossification (8 th 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) 6 -2

Intramembranous Ossification 6 -3

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 6 -4

Endochondral Ossification 6 -5

Endochondral Ossification 6 -6

Endochondral Ossification 6 -7

Endochondral Ossification 6 -8

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 6 -9