Electroporation into the limb: Beyond misexpression

Limb development has been studied for over 100 years by several generations of developmental biologists. The developing limb is one of the best models with which to study pattern formation in vertebrates. We have used chick limb development to answer a simple but basic question, namely, why heterogeneous tissues are formed at correct positions and times from a homogeneous population of cells (Pearse & Tabin, 1998). Limb development starts as two pairs of tissue bulges in the lateral plate meso-derm (LPM). These are called the forelimb and hindlimb fields (Fig. 9.1). After limb initiation, one can clearly identify three-dimensional axes in the limb buds: the proximal-distal (PD; from shoulder to fingers), dorso-ventral (DV; from back to palm), and antero-posterior (AP; from thumb to little fingers) axes. Morphological changes and differences along these three axes are determined by pattern formation during limb bud stages. Following establishment of these axes, one can visually recognize condensation of cartilages. Muscles, tendons, and neurons migrate and differentiate after cartilage formation. Because the stages and events are easily recognized morphologically and in detail, it is therefore the limb bud is an excellent model with which to study the molecular mechanisms of embryonic patterning and tissue differentiation in vertebrates.