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. Re levant receptors, signal transduction pathways and patterns of gene express ion have been characterized over the past decade. Amphibian embryos have pr ovided an excellent model for analysis of embryonic induction because they are easily surgically manipulated and cultured in vitro, and with the addit ion of treatment with various inducing factors we have been able to control organogenesis and body patterning during early development in vitro. Activ in A, a TGF-P family protein, has a potent mesoderm-inducing activity on th e isolated ectoderm called the animal cap. Activin induces animal caps to d ifferentiate into various mesodermal and endodermal tissues, including beat ing hearts, in a dose-dependent fashion. Activin, in combination with retin oic acid, also induces the formation of the pronephros, a primitive embryon ic 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 organi zer. The potential use of in vitro induction systems to further our underst anding of vertebrate organogenesis and body patterning will be discussed.