155and Ziomek 1981a, b). Once the embryo has compacted and polarized, subsequent
cell divisions are influenced by the orientation of the cleavage plane so that the
established polarity is inherited in the daughter cells (Fig. 4.12). If a blastomere
divides at an angle parallel to its axis of polarity, both daughter cells will be polar
and remain on the outside of the embryo. However, if a blastomere undergoes mito-
sis perpendicular to its axis of polarity, one daughter cell will be polarized and
contribute to the outside of the embryo, whereas the other daughter cell will be api-
cal and become a part of the inside of the embryo (Johnson and Ziomek 1981a, b;
Sutherland et al. 1990 ). These symmetric versus asymmetric cleavage divisions
eventually result in the generation of two distinct cell populations; the cells on the
inside will become a part of the inner cell mass (ICM), and the cells on the outside
will contribute to the trophectoderm (TE) layer of the blastocyst.
As expected, much of what we know about embryo compaction and blastocyst
formation has been derived from studies in mice, and therefore relatively little is
Fig. 4.12 Cell division in a polarized epithelium results in different patterns depending on spindle
orientation. Spindle orientation perpendicular to the direction of cell apical–basal polarization
(green) results in a cleavage plane that maintains polarization in both daughter cells. Spindle ori-
entation parallel to the direction of cell polarization (red) results in one daughter cell that is polar-
ized and another that is not and which exits the epithelium. Spindle orientation is indicated by a
double arrow
4 Vertebrate Embryonic Cleavage Pattern Determination