MHS 203 Bone

Endochondral Bone Formation

Example of a developing bone before replacement of woven immature (or primary) bone with mature (or secondary) bone.

• Periosteum - layer of dense irregular connective tissue that covers the outside of bone. It is divided into two layers:
  • Outer Fibrous Layer - contains fibroblasts that produce the type I collagen on the outer surface of the periosteum.
  • Inner Osteogenic Layer - contains progenitor cells that can differentiate into osteoblasts. These cells are responsible for increasing the width of bones.
• Spongy (or cancellous) Bone - trabeculae of woven immature (or primary) bone. The osteocytes and collagen fibers are randomly arranged in the mineralized osteoid.
  • Osteoblasts - found on the surface of some bone spicules.
  • Mesenchymal and Osteoprogenitor Cells - found in tissue between bone spicules.
  • Osteoclasts - large, multinucleated cells that remove bone tissue. They form a resorption bay when attached to bone.
  • Haversian System - little evidence of osteons characteristic of mature (or secondary) bone. A few developing osteons can be found. Osteons are arranged parallel with the lines of stress in mature bone.
• Marrow Cavity - filled with developing blood cells.