In vitro induction systems for analyses of amphibian organogenesis and body patterning.

The discovery that some well-known growth factors have inducing activity in embryogenesis has accelerated our understanding of embryonic induction. Relevant receptors, signal transduction pathways and patterns of gene expression have been characterized over the past decade. Amphibian embryos have provided an excellent model for analysis of embryonic induction because they are easily surgically manipulated and cultured in vitro, and with the addition of treatment with various inducing factors we have been able to control organogenesis and body patterning during early development in vitro. Activin A, a TGF-beta family protein, has a potent mesoderm-inducing activity on the isolated ectoderm called the animal cap. Activin induces animal caps to differentiate into various mesodermal and endodermal tissues, including beating hearts, in a dose-dependent fashion. Activin, in combination with retinoic acid, also induces the formation of the pronephros, a primitive embryonic kidney. The in vitro induced kidney was confirmed to function in vivo in a transplantation experiment. Furthermore, the activin-induced animal caps organize heads or trunk-and-tails in exactly the same manner as the organizer. The potential use of in vitro induction systems to further our understanding of vertebrate organogenesis and body patterning will be discussed.