Bone Development and Growth

Development and Growth Begins during first 2 months of prenatal life Precursor cells migrate to areas where bones will arise: head, thorax, limb buds

Ossification – secretion of new bone matrix by osteoblasts 1. Intramembranous bones – bone cells develop between sheets of connective tissue (embryonic membranes) 2. Endochondral Bones – a cartilage template forms, which is later turned into bones Ossification – secretion of new bone matrix by osteoblasts occurs in both processes

Intramembranous Bone Development begins about 5th week of life Precursor cells transform into osteoblasts, which cluster near embryonic membranes and secrete matrix around themselves, forming thin plates of bone (spongy) Osteoblasts become trapped in lacunae, becoming osteocytes; matrix production is decreased

Intramembranous Bone Periosteum (membrane surrounding bone) is formed by cells outside developing bone Osteoblasts with in periosteum produce a layer of compact bone over spongy bone Examples: flat bones of skull, mandibles (lower jaw), and clavicles (collarbones)

Endochondral Bones All other bones Development begins around 6th week of life Precursor cells transform into chondroblasts (cartilage-producing cells), which produce hyaline cartilage in areas where bone is to form (model)

Endochondral Bones -primary ossification center forms (center of diaphysis); chondroblasts die leaving spaces in cartilage model; other chondroblasts change into osteoblasts; blood vessels penetrate bringing in more osteoblasts that fill in spaces, producing spongy bone -other osteoblasts in periosteum deposit thin layer of compact bone along surface of cartilage layer -secondary ossification centers form in epiphyses

Endochondral Bones -spaces formed by cartilage destruction at center of bone expand to form the medullary cavity and walls of diaphysis thicken w/ compact bone -spongy bone fills in epiphyses -epiphyseal plate – wall of cartilage between epiphysis and diaphysis -articular cartilage remains

Bone Growth Interstitial Growth – lengthwise expansion that occurs at the epishyseal plate Chondroblasts produce new cells and matrix toward epiphysis, becomes new epiphyseal plate Older cartilage is converted to bone near diaphysis

Bone Growth Lengthwise growth continues as long as chondroblasts are active. When activity stops, epiphysis and diaphysis permantly fuse Epishyseal line – area of ossification near area of fusion

Bone Growth Appositional Growth – widthwise growth Osteoblasts in the periosteum deposit new compact bone along outer surfaces of diaphysis. Medullary cavity expands because osteoclasts secrete enzymes that dissolve bone matrix

Bone Remodeling Osteoblasts deposit new bone and osteoclasts break down old bone Frequency of bone modeling varies from bone to bone