F. Spatial distribution of cells

Human oocytes are polarized from their earliest stages of formation and consist of an animal and vegetal pole (Antczak and Van Blerkom, 1997, 1999; Edwards and Beard, 1997). This animal and vegetal gradient is distributed differently to specific 4-cell blastomeres via the combination of meridional and equatorial cleavage divisions (Gulyas, 1975). The first cleavage occurs meridionally and results in two nearly identical daughter blastomeres each inheriting similar polarities of animal and vegetal cytoplasm. In the second cleavage, one cell divides meridionally while the other cell divides equatorially which results in four cells with different polarity (Diagram 3). The two daughter cells resulting from the meridional cleavage have inherited full polarity, while the two daughter cells from the equatorial cleavage differ in polarity with one cell containing mostly animal cytoplasm and the other cell containing mostly vegetal cytoplasm (Gulyas, 1975; Edwards and Hansis, 2005). These cleavages lead to a typical pyramidal or tetrahedral arrangement of three blastomeres with animal cytoplasm associating with the polar body; and one blastomere, inheriting only vegetal cytoplasm, located distant to the polar body (Figs 281 and 282; Edwards and Hansis, 2005). Other ways of meridional or equatorial cleavage divisions may lead to a different distribution of animal and vegetal poles in the daughter cells and may result in non-tetrahedral or ‘clover’ shaped 4-cell stage embryos (Figs 283–285). The clover shape can be maintained in the following division (Fig. 286).

Another feature sometimes seen is ovoid embryos, originating from ovoid oocytes (Figs 287–290). In these embryos, the spatial distribution of blastomeres is necessarily abnormal.