The Role of Retinoic Acid in Vertebrate Limb Morphogenesis

Abstract

How an organism develops from a fertilized egg is one of the most challenging questions in biology, and accordingly, has interested biologists for a very long time. Recent advances in molecular and cellular techniques have provided useful tools that are now applied to many of the basic questions of embryology. For example, how do cells during embryogenesis form the intricate spatial pattern of tissues characteristic for an organism, and how does the broad spectrum of differentiated cell types arise during embryonic development? It is widely appreciated that pattern formation and cell differentiation depend on a concerted expression of specific genes and in many cases also on cell-cell interactions. For example, in Caenorhabditis elegans two genes that specify cell fate (lin-12 and glp-l) encode receptor-like transmembrane proteins whose extracellular domains share structural similarities with a diffusible ligand such as epidermal growth factor (for a review see Greenwald, 1989). In Drosophila, there is also good evidence that intercellular communication is operative in pattern formation e.g. of the epidermis (reviewed by Arias, 1989) and of the retina (reviewed by Rubin, 1989).

In my possession are two little embryos in spirit, whose names I have omitted to attach, and at present I am quite unable to say to what class they belong. They may be lizards or small birds, or very young mammalia, so complete is the similarity in the mode of formation of the head and trunk in these animals. The extremities, however, are still absent in these embryos. But even if they had existed in the earliest stages of their development we should learn nothing, for the feet of lizards and mammals, the wings and feet of birds, no less than the hands and feet of man, all arise from the same fundamental form.

K. von Baer, quoted in Darwin (1859)