Bone growth & remodeling

Formation of the Human Skeleton  In embryos, the skeleton is primarily hyaline cartilage  During development, much of this cartilage is replaced by bone  Cartilage remains in isolated areas  Bridge of the nose  Costal cartilage of ribs  Joints (articular cartilage)

Long Bone Formation and Growth Figure 5.4a, step 1 Bone starting to replace cartilage In an embryo Bone collar Hyaline cartilage model (a)

Types of Bone Cells  Osteoblasts—bone-building cells  Osteoclasts—bone-destroying cells  Break down bone matrix for remodeling and release of calcium in response to parathyroid hormone  Bone remodeling is performed by both osteoblasts and osteoclasts  Osteocytes—mature bone cells-make matrix

Bone Growth (Ossification)  Epiphyseal plates allow for lengthwise growth of long bones during childhood  Cartilage here is continuously formed  Older cartilage is ossified by osteoblasts  Osteoclasts remodel existing bone to maintain proportions = apositional growth

Long Bone Formation and Growth Figure 5.4a, step 2 Bone starting to replace cartilage In a fetusIn an embryo Growth in bone length Blood vessels Hyaline cartilage New center of bone growth Medullary cavity Bone collar Hyaline cartilage model (a)

Long Bone Formation and Growth Figure 5.4a, step 3 Bone starting to replace cartilage Epiphyseal plate cartilage Articular cartilage Spongy bone In a childIn a fetusIn an embryo New bone forming Growth in bone width Growth in bone length Epiphyseal plate cartilage New bone forming Blood vessels Hyaline cartilage New center of bone growth Medullary cavity Bone collar Hyaline cartilage model (a)

Bone Remodeling  Bones are remodeled continuously even after growth stops  Bones are remodeled in response to two factors  Blood calcium levels (Ca+ low  increase osteoclast activity, osteoporosis?)  Pull of gravity and muscles on the skeleton (=Wolfe’s law, increase osteoblast activity)

Long Bone Formation and Growth Figure 5.4a Bone starting to replace cartilage Epiphyseal plate cartilage Articular cartilage Spongy bone In a childIn a fetusIn an embryo New bone forming Growth in bone width Growth in bone length Epiphyseal plate cartilage New bone forming Blood vessels Hyaline cartilage New center of bone growth Medullary cavity Bone collar Hyaline cartilage model (a)