Planar cell movements and oriented cell division during early primitive streak formation in the mammalian embryo

Formation of the mammalian primitive streak appears to rely on cell proliferation to a minor extent only, but compensating cell movements have not yet been directly observed. This study analyses individual cell migration and proliferation simultaneously, using multiphoton and differential interference contrast time‐lapse microscopy of late pregastrulation rabbit blastocysts. Epiblast cells in the posterior gastrula extension area accumulated medially and displayed complex planar movements including U‐turns and a novel type of processional cell movement. In the same area metaphase plates tended to be aligned parallel to the anterior–posterior axis, and statistical analysis showed that rotations of metaphase plates causing preferred orientation were near‐complete 8 min before anaphase onset; in some cases, rotations were strikingly rapid, achieving up to 45° per min. The mammalian primitive streak appears to be formed initially with its typically minimal anteroposterior elongation by a combination of oriented cell divisions with dedicated planar cell movements. Developmental Dynamics 240:1905–1916, 2011. © 2011 Wiley‐Liss, Inc.