Ch 6.4-Bone Formation

When does bone formation occur? During embryonic development During infancy, childhood, and adolescence Remodeling of bone Repair of fractures

Embryonic Bone Formation Two Patterns Intramembranous Ossification Endochondral Ossification Forms directly within mesenchyme (Simple; few # of bones) Forms within hyaline cartilage that develops from mesenchyme (Complex; most bones)

Intramembranous Ossification STEP 1: Development of ossification center Blood capillary Ossification center Mesenchymal cell Osteoblast Collagen fiber Mesenchymal cells cluster Form osteogenic cells → osteoblasts Secrete organic bone EC matrix

Intramembranous Ossification STEP 2: Calcification Osteocyte Osteoblast Newly calcified bone EC matrix Osteocytes become fully developed Extend cytoplasmic projections Surrounding osteoblasts deposit calcium and mineral salts

Intramembranous Ossification STEP 3: Formation of trabeculae Mesenchyme condenses Blood vessel Spongy bone trabeculae Osteoblast Ossification centers fuse to form trabeculae Blood vessels grow in between

Compact bone replaces surface layer of spongy bone Intramembranous Ossification STEP 4: Development of the periosteum Periosteum Spongy bone tissue Compact bone tissue Mesenchyme condenses Forms periosteum Compact bone replaces surface layer of spongy bone

Endochondral Ossification STEP 1: Development of cartilage model Proximal epiphysis Diaphysis Distal epiphysis Hyaline cartilage Perichondrium Mesenchymal cells form shape of future bone Develop into chondroblasts Secrete cartilage EC Matrix

Endochondral Ossification STEP 2: Growth of cartilage model Perichondrium Uncalcified extracellular matrix Calcified extracellular matrix Chondroblasts deeply buried in cartilage Now called chondrocytes Mid-region chondrocytes grow; Surrounding EC matrix calcifies Chrondrocytes begin to die, leaving small cavities

Endochondral Ossification STEP 3: Development of primary ossification center Nutrient artery Periosteum Primary ossification center Spongy bone Nutrient artery penetrates perichondrium Osteogenic cells become osteoblasts Blood vessels induce growth of primary ossification center Osteoblasts turn cartilage into bone

Osteoclasts breakdown newly formed spongy bone Endochondral Ossification STEP 4: Development of the medullary cavity Uncalcified extracellular matrix Calcified extracellular matrix Periosteum Medullary cavity Osteoclasts breakdown newly formed spongy bone

Blood vessels enter epiphyses Secondary ossification centers develop Endochondral Ossification STEP 5: Development of secondary ossification centers Secondary ossification center Uncalcified extracellular matrix Nutrient artery and vein Epiphyseal artery and vein Blood vessels enter epiphyses Secondary ossification centers develop Proceeds outward

Endochondral Ossification STEP 6: Formation of articular cartilage and epiphyseal plate Articular cartilage Spongy bone Epiphyseal plate Outer cartilage becomes articular cartilage Cartilage remains between diaphysis and epiphysis Forms epiphyseal plate