Practice question

Mammalian embryonic cleavage differs from canonical radial cleavage by incorporation of rotational division at second cell cycle. First division meridional produces two blastomeres; second division blastomeres divide at right angles to each other: one meridionally, other equatorially. This orthogonal orientation yields four-cell embryo with varied cell-cell contacts and slight asynchrony facilitating compaction. Rotational cleavage is hallmark of eutherian regulative development, enabling flexibility in fate allocation and inside-outside positioning before blastocyst formation. First, third and fourth cleavages are not defined by this rotational switch; second cleavage uniquely establishes mammalian cleavage geometry influencing early embryonic polarity and cell fate plasticity.