Motile behavior and protrusive activity of migratory mesoderm cells from the Xenopus gastrula

During Xenopus gastrulation, the mesoderm migrates across a fibronectin (FN)-containing substrate, the inner surface of the blastocoel roof (BCR). A possible role for FN is to promote the extension of cytoplasmic processes which serve as locomotory organelles for mesoderm cells. To test this idea, the interaction of prospective head mesoderm (HM) cells with FN was examined in vitro. Nonattached HM cells extend filiform processes from an active region of the cell surface. This spontaneous activity is modulated by cell attachment to FN. Additional active regions appear, and cytoplasmic lamellae extend from these sites, leading to cell spreading and translocation. Thus, although FN seems not to induce processes de novo, it modulates a spontaneous protrusive activity to yield the extension of lamellae along the substrate surface. As putative locomotory organelles, HM cell protrusions were characterized functionally. They adhere rapidly and selectively to in situ substrates, preferentially to FN, and retract upon attachment. During translocation, the passive cell body is moved by the activity of the protrusions. Lamellae continuously extend, retract, or split into parts. This leads to an intermittent, nonpersistent mode of translocation. The polarity of HM cells, as expressed in the arrangement of protrusions, bears no constant relationship to the orientation of the cell body, and a cell can change its direction of movement without a corresponding rotation of the cell body. This may be relevant with respect to the mechanism by which mesoderm cells translate guidance cues of the BCR into a polarized, oriented cell structure during directional migration in situ.