Perichondrium is vascularized & bone collar formed Perichondrium becomes periosteum with osteoblasts which secrete bone matrix (collar blocks nutrients from reaching chondrocytes) Primary ossification center produced Chondrocytes enlarge,compress matrix (into trabeculae), calcify matrix (release enzymes and osteocalcin), die & leave large empty lacunae (cavity); periosteal bud brings osteoprogenitor cells & osteoblasts which lay down bone (woven) matrix on septa of cartilage matrix Osteoclasts resorb calcified cartilage/bone complex and enlarge cavity which fills with bone marrow; bone replaces cartilage along diaphysis