The active migration of germ cells in the embryos of mice and men is a myth

Abstract

Much is written about the independent migration of cells during normal embryonic development. However, for any putative migration, few authors give a frame of reference for the movement or a velocity; both are necessary to prove active cell migration. As it is now known that (i) sclerotomal cells do not migrate to form the axial skeleton; (ii) cranial neural crest cells do not migrate to form facial mesectoderm; and (iii) mesodermal cells do not migrate from the primitive streak, it is essential to review the evidence for the active migration of germ cells. The use of a frame of reference in a re-examination of data in the seminal paper postulating amoeboid germ-cell migration in humans indicates that there is no active migration at all and that the displacement of germ cells can be explained by the global growth movements of the embryo. Such displacements are unrelated to the artefactual movements of explanted germ cells or the appearances of dead cells. The study of human embryos forces a re-examination of evidence for the active migration of germ cells in other species, such as the mouse, where the impact of growth movements appears to have been too casually dismissed.