Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 123

Unfertilized egg

T = 0m

Yolk

Cortex Organelles

Fertilization and cortical rotation

60° − 90°
organelle
transport

T = 30 m

Microtubles

30° cortical rotation

Figure 3.3 In the unfertilized egg, there is no evident dorsoventral polarity, and at the
vegetal pole, there are small organelles between the cortex and the inner yolk. A rotation of
the cytoplasm relative to the cortex is initiated 30 min after fertilization (note how the cortex,
represented by the upward black arrow, has shifted 30° relative to the inner yolk, represented
by the downward black arrow) There is a 60-90° translocation of the organelles along the
parallel microtubule array in the vegetal hemisphere.
Source : From Moon and Kimelman (1998) .

Genomic Restoration in the Zygote

Both the sperm and egg pronuclei are encased in a nuclear membrane. The male pro-
nucleus is associated with the sperm centrosome, whose centrioles organize an aster
(a starlike microtubular structure), which slowly drives the male pronucleus from its
cortical position toward the egg pronucleus until they meet and fuse around the mid-
dle of the egg ( Figure 3.4 ). However, the female pronucleus is not a passive partner;
it also goes the “extra mile” to meet the male partner by moving along microtubules
of the sperm aster ( Reinsch and Gönczy, 1998 ). In species in which the egg cen-
trosome disintegrates ( Schatten, 1994 ) and is absorbed, the male centrosome serves
as the only organizing center of the egg for building the cytoskeleton, which drives
the sperm pronucleus in the direction of the egg pronucleus toward their fusion. On
their way to the middle of the egg, both pronuclei replicate their decondensed DNA,
which again is condensed into chromosomes when pronuclei fuse with each other.
Now that each pronucleus has restored the diploid set of chromosomes, the zygotic
nucleus has doubled the number of both paternal and maternal chromosomes and is
ready to start the first cleavage division, producing two diploid cells.
Cleavage divisions may be equal when it produces two cells that are similar
both genetically and phenotypically, but often this division is unequal and leads to