At 96 h or 72 h ALH, ball mutant cell lineages contained about half the number of GMCs as compared to wild type control cell lineages (Fig. 3G).
Thus, ES cells are able to undertake morphogenetic events in vitro that closely mimic the EPI transition from a ball of cell to a polarized epithelium both temporally and, most likely, mechanistically.
Furthermore, ball mutant cells did not undergo cell death during the time period as shown by staining for activated Caspase 3 (supplementary material Fig. S3).
"Lethal ice" is the term used to describe regions within the ice ball where cell death occurs.
In order to generate genetic mosaics, we used the FLP/FRT system (Xu and Rubin, 1993) to functionally identify ball mutant cell clones that derive from GSCs and CBs by the absence of an ovo D1 expressing transgene.
All tissues of the body originate from the pluripotent epiblast (EPI), a ball of cells positioned in the inner cell mass (ICM) of the blastocyst, whose identity is established during the first 4 days of development.
But if that ball of cells should form, a separate group of cells known as the inner cell mass assembles against one inside wall of the blastocyst, like bats huddling on the ceiling of a cave, and this is where the embryonic stem cells will appear.
An embryo would have to grow for about five days and, more important, from a ball of cells into a vesicle with two cell types.
We observed this phenomenon after generating ball mutant cells by MARCM in wing imaginal discs, showing that the mutant cells are capable to proliferate and to form cell clones.
