Figure 1.

Three phases of limb skeleton morphogenesis in the chick. In Phase I, mesenchymal condensations (arrowheads) form in the lateral plate mesoderm of the chick embryo. Limb bud initiation and outgrowth occur in Phase II and are regulated primarily by the apical ectodermal ridge, the zone of polarizing activity and the dorsal ectoderm. As the limb bud continues to elongate, the initially isotropic bud takes on a paddle shape with a bias in tissue mass toward the posterior side. The green arrows indicate local vectors in tissue movement. In Phase III, selective apoptosis delineates the digits and cartilage condensation/differentiation occurs. Subsequently, the cartilage growth plates are established, and the process of chondrocyte maturation begins. This creates a skeletal element with a central, ossified domain surrounded by two growth plates containing chondrocytes that are progressively more mature the closer they are to the ossified domain.