Hormonal Regulation of Embryogenesis: The Formation of Mesoderm inXenopus laeuis

EMBRYONIC induction, the process wherein one cell instructs a responding cell to adopt a particular fate, bears striking parallels to a number of endocrine regulatory systems (1). In addition to utilizing similar intracellular signal transduction mechanisms, many of the extracellular signals used are the same. Regulation of metamorphosis in frogs by thyroid hormone is a classical example which has seen renewed interest recently, and activin, follistatin, fibroblast growth factor (FGF), and epidermal growth factor all appear to have roles in the early embryo. Embryonic induction bears other parallels to endocrine systems as well; for example, the responding cell is sensitive to very low levels of inducing signals. In addition, the type of response to a given signal is determined by the responding cell, by the type of receptors present, regulation of the intracellular signaling components, or activation of different downstream targets. Finally, the distinctions between endocrine, paracrine, and autocrine phenomena are critical to understanding inductive signaling; the question of how an inductive signal is propagated through the embryo remains an important and unsolved problem in modern embryology.