10. Describe the stages of intramembranous ossification. Name a bone that forms in this way. 11. Describe how a cartilage model transforms into a long bone in endochondral ossification. 12. Describe the five zones of a metaphysis and the major distinctions between them. 13. How does Wolff's law explain some of the structural differences between the bones of a young child and the bones of a young adult?

Intramembranous ossification forms flat bones like the parietal bone. Endochondral ossification transforms cartilage models into long bones through primary and secondary ossification centers. The five zones of a metaphysis include resting, proliferation, hypertrophic, calcification, and ossification zones. Wolff's law explains structural differences due to varying mechanical stresses on bones at different ages. Explanation 1. Stages of Intramembranous Ossification Mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center. Osteoblasts secrete osteoid, which calcifies. Trabeculae form as osteoid is laid down around blood vessels. Periosteum develops on the surface. 2. Bone Formed by Intramembranous Ossification The flat bones of the skull, such as the parietal bone, form this way. 3. Endochondral Ossification Process A cartilage model forms first. Chondrocytes enlarge and die, leaving cavities. Blood vessels invade, bringing osteoblasts that replace cartilage with bone. Primary ossification centers form in the diaphysis, secondary centers in the epiphyses. 4. Five Zones of a Metaphysis 1. Resting Zone: Small, inactive cartilage cells. 2. Proliferation Zone: Rapidly dividing chondrocytes. 3. Hypertrophic Zone: Enlarged chondrocytes. 4. Calcification Zone: Matrix calcifies, chondrocytes die. 5. Ossification Zone: New bone forms as osteoblasts lay down bone matrix. 5. Wolff's Law Explanation Wolff's law states that bone adapts to the loads under which it is placed. In children, bones are less dense and more flexible due to lower mechanical stress. In adults, increased stress leads to denser, stronger bones.