Part of “CHAPTER 50 – PHYSIOLOGY OF BONE“

The formation of the skeleton begins with condensation and differentiation of mesenchymal cells into cartilage. These provide the template for subsequent bone formation in two ways. Bone may begin to form by differentiation of osteoblasts adjacent to the cartilage rudiments. This occurs in the membranous bones, such as the skull and the periosteum of long bones. Here, the mesenchyme condenses, and osteoblasts differentiate on the surface of the cartilaginous template, which then degenerates. Endochondral bone formation occurs at the cartilage growth plate (Fig. 50-1). Here, the osteoblasts differentiate directly on calcified cartilage and form spicules of bone with a cartilaginous core. In either case, the first bone formed is of a loose, woven structure, and it is then replaced by lamellar bone.3