Skeletal System

Endochondral Ossification “inside cartilage” Form within hyaline cartilage Cartilage framework for organized ossification ~ 6 wks thru ~ 25 yrs Growth pattern of long bones

Fetal Bone Development

Infant to ~25 yrs Long Bone –Epiphyseal Plate

Endochondral Ossification

Endochondral Ossification Mesenchymal model forms during development Mesenchymal differentiate into chondroblasts at center of growing cartilage Chondroblasts grow (hypertrophy) enlarged lacunae create reduce matrix to thin struts that calcify Vascularization of cartilage Perichondrium cells convert to osteoblasts Blood vessels penetrate cartilage bringing blood supply & osteoblasts Oteoblasts produce spongy bone Remodelling of spongy bone to compact bone

Mesenchyme Cells form Bone Model

Mesenchyme Cells Differentiate to Chondroblasts Chondroblasts Convert tissue to Cartilage

Perichondrium Develops Around Cartilage Model

Cartilage Grows in Length & Girth Length Growth is Called interstitial growth Width Growth is Called appositional growth

Proliferation vs Hypertrophy Edges Center

Endochondral Ossification Cartilage framework initial step As cartilage grows (proliferation), centrally located chondrocytes grow (hypertrophy) and rupture Begins ossification of cartilage At margins cartilage model continues to grow

Cartilage Continues to Grow

Hypertrophied Cells Burst Stimulating Calcification of Matrix

Hypertrophied Cells Burst Stimulating Calcification of Matrix

Osteoblast Produce Bone Collar Vascularization Stimulates Mesenchymal Cells to Convert to Osteoblasts Osteoblast Produce Bone Collar

Perichondrium Converts to Periosteum

Vascularization of Tissue Carries Osteoblast into Matrix Forming Primary Ossification Center

Primary Ossification Center

Primary Ossification Center Continues to Grow at Edges Ossification begins in diaphysis & migrates toward the epiphysis

Cartilage Continues to Grow & Ossification Continues towards ends of Diaphysis

Cartilage Continues Proliferation & Hypertophy

Ossification of Cartilage Continues Towards ends of shaft Center

Calcified Cartilage Matrix

Forms Spongy Bone

Form Trabeculae of Spongy Bone

Osteoclast Remodel Spongy Bone to Compact

Bone Remodeling

Bone Remodeling Forms Medullary Cavity

Forms Haversian System

Endochondral Ossification Blood vessels & osteoblasts migrate into epiphysis Secondary ossification centers Bone ossification remains incomplete Epiphyseal plate Site of continued bone growth Produces VERY structured compact bone

Blood Vessels Enter into Diaphysis

Secondary Ossification Centers Form Ossification of epiphysis

Secondary Ossification Centers

Cartilage at Edges Persists as Articular Cartilage

Metaphysis Location of epiphyseal plate Site of bone elongation Growing cartilage plate persists Zone of proliferation Zone of Hypertrophy

Epiphyseal Plate

Long Bone Elongation

Bone Elongation Chondrocytes divide and produce new cartilage Diaphysis- chondrocytes replaced by invading osteoblasts During growth stage chondrocyte activity matches osteocyte activity At maturation osteocyte invasion is FASTER than chondrocyte division Puberty- Sex hormones stimulate osteoblast & chondrocyte activity aka growth “spurt”

Chondrocyte Proliferation

Osteocyte Invasion

Bone Growth Disorders Achondroplastic dwarf Results from slow epiphyseal plate growth & early plate closure Chondrocytes have slower activity Results in normal trunk growth & stunted long bone growth

Bone Growth Disorders Marfan’s syndrome Dominant genetic disorder Abnormal connective tissue growth Hyperactive chondrocytes & epiphyseal plate Symptoms- long thin limbs