Cell motility and matrix assembly have traditionally been studied in isolation because of a lack of suitable model systems in which both can be observed simultaneously. With embryonic tissues from the gastrulating frog Xenopus laevis we observe stages of fibronectin fibrillogenesis coincident with protrusive activity in the overlying cells. Using live confocal time-lapse images collected from Cy3-tagged fibronectin and plasma membrane tethered green fluorescent protein, we describe the movement and the elaboration of a complex fibrillar network undergoing topological rearrangements of fibrils on the surface of an embryonic tissue. Discrete processes of annealing, polymerization, stretching, breaking, and recoiling are recorded. Elaboration and maintenance of the complex topology of the extracellular matrix appears to require filamentous actin. These findings support a mechanical-model in which cell tractive forces elaborate the complex topological fibrillar network and are part of a homeostatic mechanism for the regulation of the extracellular matrix. Copyright (c) 2008 Wiley-Liss, Inc.