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REVIEWS
CLEAVAGE PATTERN AND EMERGING ASYMMETRY OF THE MOUSE EMBRYO
DETERMINANTS
Spatially localized factors that direct the fate of a cell.
ANIMAL POLE
The part of the egg where the meiotic divisions have occurred.
VEGETAL POLE
The opposite pole of the egg to the animal pole.
BLASTOMERES
Cells of the early-cleavage-stage embryo.
INNER CELL MASS
Inside cells of the blastocyst that retain pluripotency and give rise to all cell types of the future body.
Magdalena Zernicka-Goetz
Abstract | Early mammalian development is regulative it is flexible and responsive to experimental intervention. This flexibility could be explained if embryogenesis were originally completely unbiased and disordered; order and determination of cells only arising later. Alternatively, regulative behaviour could be consistent with the embryo having some order or bias from the very beginning, with inflexibility and cell determination increasing steadily over time. Recent evidence supports the second view and indicates that the sequence and the orientations of cell divisions help to build the first asymmetries.
In many organisms, the fate of cells is determined from the time they are born because they inherit cytoplasmic factors that are spatially localized in the oocyte or in the fertilized egg. Such factors function as DETERMINANTS when they guarantee specific cell fate.Differences in the inheritance of determinants lead to cells having distinct properties, and experiments can be devised to show that mislocalization or elimination of such determinative factors changes the fate of the cell.For example, the destruction of spatially distributed RNA at the pole plasm of insects or the germ plasm of amphibian eggs generates sterile animals12.
However, the development of the mammalian embryo is different: it is regulative. The evidence for this comes from experiments in which the mouse embryo is reorganized by changing the arrangement or the number of cells, after which it can recover and complete development37. Even the removal of the ANIMAL or
VEGETAL POLES of the embryo does not prevent development, which argues against localized determinants89,
and this raises fascinating questions about how cell fate develops in the absence of such factors. Are the first cell-fate decisions taken entirely at random with nothing to pilot development of the mammalian embryo?Or is there some non-rigid pattern that guides development yet allows flexibility? Here, I review recent
progress...