Chapter 1 - Embryology and Cellular Biology of Bone

Abstract

Bones are derived from three embryonic structures: the neural crest, which through the branchial arches gives rise to the craniofacial bones; the sclerotomes, which give rise to the axial skeleton; and the lateral plate mesoderm, which gives rise to the appendicular skeleton (limbs). The axial (skull, vertebrae, fibs, and pelvis) and appendicular (limbs) skeleton starts embryologically with a cartilage model in which ossification centers appear. These are characterized by vascular invasion, differentiation of cells into osteoblasts, the bone-forming cells, and deposition of bone matrix and its mineralization. The mineralized cartilage matrix serves as a scaffold for the deposition of bone, following vascular invasion, and is subsequently removed (resorbed) by large cells, chondroclasts, which are morphologically similar to the bone-resorbing cells, the osteoclasts. Two processes are involved in providing the shape and architecture of bone: modeling and remodeling. Modeling is the formation of bone on surfaces where bone had not been previously removed by osteoclastic resorption as seen, for example, in periosteal bone growth. In remodeling, which accounts for most but not all bone formation in adult life, bone is deposited on surfaces from which bone was previously removed by osteoclasts.