19 TGF-β Family Signaling in Xenopus and Zebrafish Embryos

Abstract

HISTORICAL OVERVIEW: AMPHIBIANS, EMBRYONIC INDUCTIONS, AND THE TGF-β FAMILY
The marking, explantation, and recombination of different regions of early amphibian embryos provided much of the foundation for vertebrate experimental embryology in the 20th century (Gerhart 1980; Hamburger 1988). The marking of early embryos with lineage traces allowed the definition of the developmental fate of early embryonic cells, and their surgical isolation allowed the study of their state of specification. Recombination of different tissues identified intracellular signaling events, referred to as embryonic inductions that specify developmental fate during early embryogenesis. The most famous of this class of experiments, the Spemann-Mangold organizer grafts, established that a small region from the prospective dorsoanterior region of the gastrulating embryo was sufficient to organize an ectopic dorsoanterior body axis, when transplanted to the prospective ventral side of the embryo (Spemann and Mangold 1924; Spemann 1938). A later, but also influential, set of Xenopus grafting experiments by Peter Nieuwkoop (1969) and K. Ogi (1967) demonstrated that prospective endodermal tissue could respecify prospective ectoderm as mesoderm, a process referred to as mesoderm induction. The Spemann-Mangold and Nieuwkoop-Ogi experiments provided a general framework for the study of intercellular signaling in early vertebrate embryos. Recent molecular embryology has been focused on identifying the molecular pathways that underlie these inductive events.

Despite attempts over many decades to isolate the agents responsible for inductive events in early embryos, the molecular basis for these signals only began to be addressed following identification and cloning of polypeptide growth factors in the late 1970s...