INDUCTION OF NOTOCHORD CELL INTERCALATION BEHAVIOR AND DIFFERENTIATION BY PROGRESSIVE SIGNALS IN THE GASTRULA OF XENOPUS-LAEVIS

We show that notochord-inducing signals are present during Xenopus lae vis gastrulation and that they are important for both inducing and org anizing cell behavior and differentiation in the notochord, Previous w ork showed that convergent extension of prospective notochordal and se mitic mesoderm occurs by mediolateral cell intercalation to produce a longer, narrower tissue, Mediolateral cell intercalation is driven by bipolar, mediolaterally directed protrusive activity that elongates ce lls and then pulls them between one another along the mediolateral axi s, This cell behavior, and subsequent notochordal cell differentiation , begins anteriorly and spreads posteriorly along the notochordal-somi tic boundary, and from this lateral boundary progresses medially towar ds the center of the notochord field, To examine whether these progres sions of cell behaviors and differentiation are induced and organized during gastrulation, we grafted labeled cells from the prospective not ochordal, semitic and epidermal regions of the gastrula into the notoc hordal region and monitored their behavior by low light, fluorescence videomicroscopy, Prospective notochordal, epidermal and semitic cells expressed mediolateral cell intercalation behavior in an anterior-to-p osterior and lateral-to-medial order established by the host notochord , Behavioral changes were induced first and most dramatically among ce lls grafted next to the notochordal-somitic boundary, particularly tho se in direct contact with the boundary, suggesting that the boundary m ay provide signals that both induce and organize notochordal cell beha viors, By physically impeding normal convergent extension movements, n otochordal cell behaviors and differentiation were restricted to the a nteriormost notochordal region and to the lateral notochordal-somitic boundary, These results show that mediolateral cell intercalation beha vior and notochordal differentiation can be induced in the gastrula st age, among cells not normally expressing these characteristics, and th at these characteristics are induced progressively, most likely by sig nals emanating from the notochordal-somitic boundary, In addition, the y show that morphogenetic movements during gastrulation are necessary for complete notochord formation and that the prospective notochord re gion is not determined by the onset of gastrulation.