The roles of phenotype and position in guiding the fate of 16-cell mouse blastomeres.

Background “Inside-Outside” hypothesis Cells located on the inside of the 16-cell morula differentiate into the inner cell mass (ICM) of the blastocyst while cells on the outside of the morula differentiate into the trophectoderm (TE) If a cell were moved to a different location in the morula, it would differentiate into a different kind of cell Hypothesis and corollary assumed that, for a time, all 16 cells in morula were identical. Later proved to be false; as soon 16-cell morula exists, the cells within the morula can be divided into 2 groups Apolar cells – smaller, label on surface uniformly, sparse but uniform distribution of microvilli, more adhesive, located on the inside Polar cells – larger, polar distribution of microvilli, less adhesive, tend to envelope aggregated apolar cells, located on the outside Recently outer cells were injected with horseradish peroxidase (HRP) and found that in 44% of embryos analyzed they contributed to both ICM and TE, and 53% contributed only to the TE