John Shih

Mediolateral Intercalation of Mesodermal Cells in the Xenopus Laevis Gastrula

The prospective mesoderm is important in nearly every aspect of early vertebrate embryogenesis because of its morphogenetic functions and because of the pattern-forming, inductive functions of the Spemann “organizer” contained in its dorsal sector (Spemann, 1938). The objective of this paper is to focus on the morphogenesis of the mesoderm in early embryogenesis, specifically on the powerful convergence and extension movements of the prospective dorsal mesoderm. The two major morphogenetic processes in the gastrula mesoderm are the migration of the leading edge mesoderm across the roof of the blastocoel (see Nakatsuji, 1984; Keller and Winklbauer, 1991) and the convergence (narrowing) and extension (lengthening) movements of the prospective dorsal, axial and paraxial mesoderm (see Keller, 1986). The function, the cellular basis, and the tissue interactions involved in convergence and extension of the dorsal mesoderm of Xenopus have been investigated extensively in this laboratory and reviewed with emphasis on history and function (Keller, 1986), on the underlying motility (Keller et al.,1991a), and on cell interactions regulating cell motility and behavior (Keller et al.,1991b). Our objective here will be to summarize and simplify the fundamental features of the convergence and extension movements, including their cellular basis, their function, and their regulation by tissue interactions in Xenopus laevis, the African clawed frog.