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 degrees 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. (C) 2011 Wiley-Liss, Inc.