What mechanisms drive cell migration and cell interactions in Pleurodeles?

Embryogenesis implies a strict control of cell interaction and cell migration. The spatial and temporal regulation of morphogenetic movements occurring during gastrulation is directly dependent on the early cell interactions that take place in the blastula. The newt Pleurodeles waltl is a favorable model for the study of these early morphogenetic events. The combination of orthotopic grafting and fluorescent lineage tracers has led to precise early gastrula mesoderm fate maps. It is now clear that there are no sharp boundaries between germ layers at the onset of gastrulation but rather diffuse transition zones. The coordination of cell movements during gastrulation is closely related to the establishment of dorsoventral polarity. Ventralization by U.V. irradiation or dorsalization by lithium treatment modifies the capacity for autonomous migration on the fibronectin coated substratum of marginal zone cells accordingly. It is now firmly established that mesodermal cells need to adhere to a fibrillar extracellular matrix (ECM) to undergo migration during gastrulation. Extracellular fibrils contain laminin and fibronectin (FN). Interaction of cells with ECM involves receptors of the beta 1 integrin family. A Pleurodeles homolog of the alpha v integrin subunit has been recently identified. Protein alpha v expression is restricted to the surface of mesodermal cells during gastrulation. Integrin-mediated interactions of cells with FN are essential for ECM assembly and mesodermal cell migration. Intracellular injection of antibodies to the cytoplasmic domain of beta 1 into early cleavage embryos causes inhibition of FN fibril formation. Intrablastocoelic injections of several probes including antibodies to FN or integrin alpha 5 beta 1, competitive peptides to the major cell binding site of FN or the antiadhesive protein tenascin all block mesodermal cell migration. This results in a complete arrest of gastrulation indicating that mesodermal cell migration is a major driving force in urodele gastrulation. It is now possible to approach the role of fibroblast growth factor (FGF) during cell interactions taking place in urodele embryos. Four different FGF receptors (FGFR) have been cloned in Pleurodeles. Each of them has a unique mRNA expression pattern. FGFR-1, FGFR-3, and the variant of FGFR-2 containing the IIIb exon are maternally expressed and might be involved in mesodermal induction. During gastrulation, FGFR-3 and FGFR-4 have a restricted pattern of expression, whereas FGFR-1 mRNA is nearly uniformly distributed. Splicing variants FGFR-2IIIb and FGFR-2IIIc have exclusive expression patterns during neurulation. IIIb is expressed in epidermis and IIIc in neural tissue, suggesting a function in the differentiation of ectodermal derivatives.