Histology and Physiology of Bone Part 2 Bone Growth 1

Bone Development Bones develop in the fetus by 2

Intramembranous Ossification- – Form within sheet-like layers of connective tissue. – Primitive connective tissue cells enlarge and differentiate into osteoblasts (bone forming cells) – Osteoblasts deposit bone matrix on collagen fibers of tissue forming trabeculae of woven bone. – Osteoblasts continue to lay down matrix on the surface forming cancellous (spongy) bone. – Red bone marrow develops within the spaces and cells surrounding this developing bone specializes to form periosteium – Osteoblasts from periosteium lay down bone matrix to form an outer surface of compact bone. 3

Centers of ossification- – These centers expand to form a bone by gradually ossifying the membrane. – So, the centers of ossification have the oldest bone while the expanding edges the youngest. 4

Endochondral Ossification- – Remember…bone starts as a hyaline cartilage model! 5

Steps of endochondral ossification… 6

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Primary ossification centers appear early in fetal development (~8 or 9 weeks) and secondary ossification centers appear about 1 month before birth. 9

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Bone Growth 14

Growth in Bone Length: – Long bones and bony projections increase in length because of growth at the epiphyseal plate. – Growth at the epiphyseal plate involves formation of new cartilage by interstitial cartilage growth followed by appositional bone growth on the surface of the cartilage. 15

– The hypertrophied chondrocytes die and are replaced by osteoblasts from the endosteum. – Through appositional bone growth…osteoblasts deposit new bone matrix on the calcified cartilage matrix. 16

Growth in Bone Width… – Fast bone growth= Osteoblasts from periosteum lay down bone to form series of ridges with grooves between them. Periosteum covers bone ridges and extend to bottom of grooves and 1 or more BVs from periosteum lie in each groove. Osteoblasts continue to produce bone, ridges grow and extend toward each other till they meet…forming a tunnel that contains BVs. Osteoblasts in the periosteum lining the tunnel (endosteum) continue to lay down bone to form concentric lamella, which eventually forms osteons. 17

Factors that affect bone growth: 18

Nutrition… Vitamin D 19

Hormones… – Hormones are important for bone growth. – Growth hormone: – Sex Hormones: 20

Bone Remodeling 21

Bone Repair Step 1- Hematoma formation When a bone is injured/fractured, the BVs in the bone and surrounding periosteum are damaged = hematoma formed. 22

Step 2- Callus formation A callus The callus forms as the clot dissolves. 23

Step 3- Callus Ossification 24

Step 4- Bone remodeling 25