WHAT MECHANISMS DRIVE CELL-MIGRATION AND CELL-INTERACTIONS IN PLEURODELES

Embryogenesis implies a strict control of cell interactions and cell m igration. The spatial and temporal regulation of morphogenetic movemen ts occurring during gastrulation is directly dependent on the early ce ll 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 n ow clear that there are no sharp boundaries between germ layers at the onset of gastrulation but rather diffuse transition zones. The coordi nation of cell movements during gastrulation is closely related to the establishment of dorsoventral polarity, Ventralization by U.V. irradi ation 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 c ells need to adhere to a fibrillar extracellular matrix (ECM) to under go migration during gastrulation. Extracellular fibrils contain lamini n and fibronectin (FN). Interaction of cells with ECM involves recepto rs of the beta 1 integrin family. A Pleurodeles homolog of the alpha(v ) integrin subunit has been recently identified. Protein alpha(v) expr ession is restricted to the surface of mesodermal cells during gastrul ation. Integrin-mediated interactions of cells with FN are essential f or 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 i njections of several probes including antibodies to FN or integrin alp ha(5) beta(1), competitive peptides to the major cell binding site of FN or the antiadhesive protein tenascin all block mesodermal cell migr ation. This results in a complete arrest of gastrulation indicating th at mesodermal cell migration is a major driving force in urodele gastr ulation. 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 express ed and might be involved in mesodermal induction. During gastrulation, FGFR-3 and FGFR-4 have a restricted pattern of expression, whereas FG FR-1 mRNA is nearly uniformly distributed. Splicing variants FGFR-2III b 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.