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Key Words AER, BMP, FGF, Hedgehog, limb, morphogen, pattern formation, regeneration, secreted factors, vertebrate development, WNT, ZPA
Abstract Vertebrate limb buds are embryonic structures for which much molecular and cellular data are known regarding the mechanisms that control pattern formation during development. Specialized regions of the developing limb bud, such as the zone of polarizing activity (ZPA), the apical ectodermal ridge (AER), and the non-ridge ectoderm, direct and coordinate the development of the limb bud along the anteriorposterior (AP), dorsal-ventral (DV), and proximal-distal (PD) axes, giving rise to a stereotyped pattern of elements well conserved among tetrapods. In recent years, specific gene functions have been shown to mediate the organizing and patterning activities of the ZPA, the AER, and the non-ridge ectoderm. The analysis of these gene functions has revealed the existence of complex interactions between signaling pathways operated by secreted factors of the HH, TGF-fi/betaMP, WNT, and FGF superfamilies, which interact with many other genetic networks to control limb positioning, outgrowth, and patterning. The study of limb development has helped to establish paradigms for the analysis of pattern formation in many other embryonic structures and organs.
INTRODUCTION
Embryonic development could be defined as the orderly, stereotyped process that adds complexity to the initial relative simplicity of a fertilized egg. Complexity not only in terms of the increase in the number of total cells and the number of different cell types that comprise the embryo, but also in the number of recognizable organs or structures that accessorize the basic body plan, and in the number of functions that these organs and structures perform. Thus as development proceeds, a tight spatial and temporal control of gene expression and cell behavior sculpts the developing embryo by adding specific morphological and functional features, which will actually determine the lifestyle and functionality of the adult animal.
During the development of many structures and organs there is an initial stage in which a primordium (or precursor structure) is induced in a specific location in the embryo in response to pre-existent combinatorial positional cues. The primordium is composed of a selected group of embryonic cells, which may or may not belong to the same type of embryonic layer (endoderm, mesoderm, or ectoderm). In the primordium, very often it...