The early post-implantation mouse embryo consists of three tissues: the epiblast at the distal end, which forms the foetus; the extra-embryonic ectoderm at the proximal end; and the visceral endoderm, which surrounds them both. Initially, the anterior visceral endoderm (AVE) is positioned at the distal tip of the embryo, but it soon moves to the presumptive anterior where it induces anterior character in the epiblast. The mechanism of AVE motion has eluded researchers until now, but Srinivas and coauthors(p. 1157) have imaged developing embryos in real time to show that the cells of the AVE move actively, projecting filopodial processes in their direction of motion. AVE cells move in a single layer and maintain contact with the epiblast throughout migration, which the authors suggest might mean that the epiblast, or the extracellular matrix surrounding it, provides directional cues to the migrating AVE.

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