Osteoid is secreted and calcifies.• Osteoblasts continue to secrete osteoid, which calcifies in afew days.• Trapped osteoblasts become osteocytes.• Mesenchymal cells cluster and differentiate into osteoblasts,forming an ossification center.
Intramembranous Ossification (4 of 4) Figure 6.11 Intramembranous ossification.MesenchymalcellOsteoblastOsteoidOsteocyteNewly calcifiedbone matrixOsteoidOsteoblastMesenchymecondensingto form theperiosteumFibrousperiosteumPlate ofcompact boneDiploë (spongybone) cavitiescontain redmarrowBlood vessel• Trabeculae just deep to the periosteum are remodeled andreplaced with compact bone.• The immature spongy bone in the center is remodeled intomature spongy bone that is eventually filled with red marrow.OssificationcenterCollagenfiberOsteoblastCompact bone replaces immature spongy bone, just deep tothe periosteum. Red marrow develops.1234Ossification centers develop in the fibrous connectivetissue membrane.Osteoid is secreted and calcifies.• Osteoblasts continue to secrete osteoid, which calcifies in afew days.• Trapped osteoblasts become osteocytes.Immature spongy bone and periosteum form.•Accumulating osteoid is laid down between embryonic bloodvessels, forming a honeycomb of immature spongy bone.• Vascularized mesenchyme condenses on the external face of thebone and becomes the periosteum.• Mesenchymal cells cluster and differentiate into osteoblasts,forming an ossification center.Trabeculae ofimmature spongy bone
Postnatal Bone Growth•Interstitial (longitudinal) growth•Increase in length of long bones•Appositional growth •Increase in bone thickness
Interstitial Growth: Growth in Length of Long Bones•Requires presence of epiphyseal cartilage•Epiphyseal plate maintains constant thickness•Rate of cartilage growth on one side balanced by bone replacement on other•Concurrent remodeling of epiphyseal ends to maintain proportion•Result of five zones within cartilage•Resting (quiescent) zone•Proliferation (growth) zone•Hypertrophic zone•Calcification zone•Ossification (osteogenic) zone
Interstitial Growth:Growth in Length of Long Bones •Resting (quiescent) zone•Cartilage on epiphyseal side of epiphyseal plate•Relatively inactive•Proliferation (growth) zone•Cartilage on diaphysis side of epiphyseal plate•Rapidly divide pushing epiphysis away from diaphysis lengthening•Hypertrophic zone•Older chondrocytes closer to diaphysis and their lacunae enlarge and erode interconnecting spaces
Interstitial Growth:Growth in Length of Long Bones •Calcification zone•Surrounding cartilage matrix calcifies, chondrocytes die and deteriorate•Ossification zone•Chondrocyte deterioration leaves long spicules of calcified cartilage at epiphysis-diaphysis junction•Spicules eroded by osteoclasts •Covered with new bone by osteoblasts•Ultimately replaced with spongy bone
Interstitial Growth:Growth in Length of Long Bones •Near end of adolescence chondroblasts divide less often•Epiphyseal plate thins then is replaced by bone•Epiphyseal plate closure•Bone lengthening ceases•Requires presence of cartilage•Bone of epiphysis and diaphysis fuses•Females – about 18 years•Males – about 21 years