Theoretical exploration of blastocyst morphogenesis.

A theoretical exploration of cell distribution on the mouse blastocyst is conducted. A model ball of cells represents the morula which develops into a 32-cell model blastocyst that is enclosed in a spherical surface with a hemispherical cavity at one end. In the combinatorial analysis it is assumed that each cell of the 2-cell embryo forms 16 cells in the blastocyst and that these 16 cells touch each other. The results of the analysis identify a tendency for one set of 16 cells to contribute twice as many cells to the basal solid end of the blastocyst than the other set, a developmental bias that is also found by some observers of natural blastocysts. In the geometric analysis, half the volume of the inner group of cells of the morula and blastocyst and half the volume of the surrounding shell cells, the trophectoderm, is assumed to be formed from the progeny of each 2-cell stage cell. Making various assumptions about morphogenesis, it is found that there is a tendency for a curved frontier between the volumes from each 2-cell stage cell, the clonal volumes, to lie at an angle of 43.4 degrees to the equator of the blastocyst and for the bulk of the frontier circumference to lie on either side of the equator. These tendencies are also found by some observers of real blastocysts.