Early development of the vertebrate limb: an introduction to morphogenetic tissue interactions using scanning electron microscopy.

The developing limbs of most vertebrates serve as a model system for studies of morphogenesis, pattern formation, cell and tissue interactions and cell differentiation. Mesoderm in the flank of the embryo induces overlying ectoderm to form a thickened, stratified or pseudo-stratified epithelium which becomes the highly specialized apical ectodermal ridge. In turn, the apical ridge specifies individual limb parts (first from structures proximal to the body axis, then to more distal components) and is required for those elements to form. If the ridge is removed, subsequent limb development ceases and no further limb parts appear. The series of ectodermal-mesodermal interactions is poorly understood at the molecular level, but scanning electron microscopy permits the visualization of tissues and cells which participate in this remarkable process of morphogenesis and differentiation. This paper is intended to serve as an introduction for the student beginning an investigation into the multiple, integrated biological processes which culminate in the establishment of a normal vertebrate limb.