Activin A induces craniofacial cartilage from undifferentiated Xenopus ectoderm in vitro

Activin A has potent mesoderm-inducing activity in amphibian embryos and induces various mesodermal tissues in vitro from the isolated presumptive ectoderm. By using a sandwich culture method established to examine activin A activity, we previously demonstrated that activin-treated ectoderm can function as both a head and trunk-tail organizer, depending on the concentration of activin A. By using activin A and undifferentiated presumptive ectoderm, it is theoretically possible to reproduce embryonic induction. Here, we test this hypothesis by studying the induction of cartilage tissue by using the sandwich-culture method. In the sandwiched explants, the mesenchymal cell condensation expressed type II collagen and cartilage homeoprotein-1 mRNA, and subsequently, cartilage was induced as they are in vivo. goosecoid (gsc) mRNA was prominently expressed in the cartilage in the explants. Xenopus distal-less 4 (X-dII4) mRNA was expressed throughout the explants. In Xenopus embryos, gsc expression is restricted to the cartilage of the lower jaw, and X-dII4 is widely expressed in the ventral head region, including craniofacial cartilage. These finding suggest that the craniofacial cartilage, especially lower jaw cartilage, was induced in the activin-treated sandwiched explants. In addition, a normal developmental pattern was recapitulated at the histological and genetic level. This work also suggests that the craniofacial cartilage-induction pathway is downstream of activin A. This study presents a model system suitable for the in vitro analysis of craniofacial cartilage induction in vertebrates.