Bone Ossification 1

Ossification bone formation Formed by: - mesenchyme (embryonic connective tissue) - hyaline cartilage (supporting structures for ossification) 2

Composition of Human Embryonic Skeleton fibrous connective tissue membranes hyaline cartilage - shaped like bones Bone Formation Patterns intramembranous endochondral 3

Intramembranous Ossification (cont.) bone formed within a fibrous membrane Examples: - broad flat bones of the skull - mandible - clavicles 4

Intramembranous Ossification (cont.) mesenchymal cells congregate at site of future bone - become vascularized - cluster together - differentiate first into osteoprogenitor cells - differentiate into osteoblasts center of ossification 5

Intramembranous Ossification (cont.) osteoblasts secrete organic matrix of bone once completely surrounded, osteoblasts cease to form matrix (cells now osteocytes) osteocytes lie in lacunae with canaliculi radiating out in all directions calcium and other minerals are deposited and tissue calcifies 6

Trabeculae develops as bone matrix forms trabeculae fuse together to create the lattice-like characteristic of spongy bone - spaces between trabeculae; filled with vascularized connective tissue which differentiates into red bone marrow 7

Periosteum membrane that covers bone consists of connective tissue, osteoprogenitor cells and osteoblasts essential for bone growth, repair, and nutrition develops from vascularized mesenchyme surface layers of spongy bone are replaced by compact bone 8

Endochondral Ossification replacement of cartilage by bone most bones formed this way Steps: - development of cartilage model - growth of cartilage model - development of primary ossification center - development of diaphysis and epiphysis 9

Development of Cartilage Model mesenchymal cells congregate together in shape of future bone mesenchymal cells differentiate into chondroblasts chondroblasts produce cartilage matrix of hyaline cartilage perichondrium membrane develops around model forming periosteal bone collar 10

Growth of Cartilage Model Cartilage model grows in length by: - cell division - continued secretion of cartilage matrix - interstitial growth (growth from within) Growth in thickness by: - addition of matrix on periphery (appositional growth) 11

Growth of Cartilage Model (cont.) chondrocytes in mid region of model increase in size and burst pH change stimulates calcification of cartilage 12

Development of Primary Ossification Center blood capillaries penetrate calcified cartilage - carry bone marrow stem cells and osteoprogenitor cells into spaces stem cells - form red bone marrow osteoprogenitor cells - differentiate into osteoblasts (secrete matrix) Together they produce periosteal bud 13

Periosteal Bud Consists of: - capillaries of periosteum - red marrow cells - osteoblasts - osteoclasts 14

Development of Diaphysis and Epiphysis diaphysis is replaced by compact bone central part contains red marrow when blood vessels enter epiphyses, secondary ossification centers develop growth is radial spongy bone occupies the epiphyses 15