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The primary ossification center develops in the center of the bone, and is the source of bone development. Osteoblasts found in the bone will secrete osteoid against the walls of the diaphysis. The diaphysis ,made of hyalin cartilage, is now encased in compact bone. This bone collar gives the developing bone structural support to begin hardening. Osteoblasts of the converted periosteum secrete osteoid against the hyaline cartilage diaphysis encasing it in a bone collar

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The chondrocytes (cartilage cells) go through hypertrophication or enlargement and signal the hyalin cartilage to harden into bone. This hardening into bone is called calcification. The calcified hyalin cartilage is impermeable to the diffusion of nutrients. Since the chondrocytes cant receive any nutrients they begin to die and leave small cavities. These small cavities leave room in the hardened bone for blood vessels to travel through. Chondrocytes w/I the shaft hypertrophy & signal surrounding cartilage matrix to calcify. Chondrocytes die due to lack of nutrients (impermeability of calcified matrix) Matrix deteriorates thus opening up cavities

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Periosteal bud invasion is the introduction of a nutrient highway to the bone. Previously nutrients were delivered through the cartilage by simple diffusion. To support the capacity of the hardening bone, the periosteal region is invaded by a bud containing blood vessels and nerves. This allows not only nutrients, but osteoblasts and osteoclasts cells to enter into the cavities that were previously inhabited by chondrocytes. The osteoblasts secrete osteoid into the remaining hyalin cartilage and give rise to early spongy bone. Think of the periosteal bud invasion as a group of grave robbers who invaded the graves (cavities) where fallen chondrocytes used to rest. The forming cavities are invaded by a collection of elements Periosteal bud contains a nutrient artery and vein, lymphatics, nerve fibers, red marrow elements, osteoblasts, and osteoclasts Osteoclasts erode the calcified cartilage matrix & osteoblasts secrete osteoid around the remaining hyal ine cartilage forming bone-covered cartilage trabuculae (the formation of spongy bone)

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After the nutrient source is delivered to the center of the bone, the diaphysis region has the resources to elongate. The elogated region is powered by cells dividing in the primary center of ossification. This elongated region is known as the medullary cavity. The medullary cavity is where the bone marrow is contained. Osteoclasts open up a medullary cavity by breaking down the newly formed spongy bone Cartilage is growing, bones being calcified and eroded and then replaced by bony spicules on the epiphyseal surfaces facing the medullary cavity

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Just before birth the ends of the bone or Epiphysis will develop their own centers of ossification. They cant stay made of cartilage or they would be structurally weak. These centers are known as the secondary centers of ossification. They go through the same process as the primary center of ossification: hypertrophication, calcification, cavitation, and periosteal bud invasion. Secondary ossification centers appear in one or both epiphyses. Steps 1-4 occur there except no medullary cavity forms

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