ABSTRACT
The earliest forms of skeleton are the primitive mesenchymal condensations that formed when undifferentiated mesenchymal cells migrate into areas destined to become bone. In mouse, mesenchymal condensation begins to form at 9.5 days post coitum (dpc). These mesenchymal condensations can eventually become bone through two distinct processes. During intramembranous ossification, cells of mesenchymal condensations differentiate directly into osteoblasts without any intermediate step. Bones formed through this process include frontal, parietal, and parts of the temporal and occipital bones, the majority of the facial bones, and the clavicles.1 All other bones are formed through endochondral ossification, a two-step process. At 11.5 dpc of mouse embryonic development, cells in the mesenchymal condensation differentiate first into chondrocytes to create a cartilaginous anlage of the future bone. This process, also called chontrogenesis, will give rise to essentially an entire skeleton consisting of cartilaginous ele ments. In the center of each cartilaginous anlage, cells stop dividing and become hypertrophic chondrocytes, a subpopulation of chondrocytes surrounded by a calcified ECM. Vascular inva sion from this ECM will bring in osteoblast progenitors that will form ossification centers. The
E ngineered Boney edited by Herve Petite and Rodolfo Quarto. ©2005 Eurekah.com.
