Cellular processes in limb development and evolution

Evolutionary change requires that genetic variation produces morphological variation with high fidelity through the process of developmental patterning. The vertebrate limb is a classic model used to determine the cellular processes that assign positional information during development. We review cellular processes regulating limb development and provide insights that evolutionary morphologists should consider when evaluating primate anatomical characters. Limb development is initiated by cellular interactions between paraxial and lateral plate mesoderm, which determine the position and outgrowth of the limbs. Cellular signaling centers are established to maintain interactions between limb mesoderm and overlying ectoderm. These organizers maintain limb outgrowth and assign positional information in three dimensions. Cell fate and differentiation is further governed by the expression of selector genes including Pitx1, which establishes forelimb and hind limb identity, and the combinatorial Hox code, which in the limb, patterns skeletal morphology and growth. The precise regulation of apoptosis further shapes the hands and feet and individuates the digits. Consideration of these processes can inform on important primate morphological variations such as the length of the trunk and the independent evolution of the fore and hind limbs. Proper understanding of development also aids in weighing the potential for convergence in phenotypic characters.