Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 124

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Figure 3.4 Movements of male and female pronuclei in the newly fertilized Xenopus egg.
The positions of the male and female pronuclei are depicted 30 min (A) and 40 min (B)
after fertilization. Sections of Xenopus eggs stained with Azure B are shown on top, and
corresponding schematic representations below. Thin black lines represent microtubules;
white and black disks represent male and female pronuclei. The male pronucleus is associated
with the centrosome, which nucleates microtubules to form the sperm aster. The growth of
the sperm aster drives the male pronucleus from the cell cortex toward the center of the egg.
Organelle-like motility propels the female pronucleus along microtubules from the cell cortex
toward the centrosome located in the center of the sperm aster. After pronuclear meeting, the
aster and the associated male and female pronuclei continue to migrate until the center of the
egg is reached.
Source : From Reinsch and Gönczy (1998) .

the formation of two cells, which are genetically identical but epigenetically (and
hence phenotypically) different. Thus, in many metazoans, differentiation of geneti-
cally identical cells may begin as early as the first cleavage division, suggesting that
the zygote is epigenetically programmed to produce two different cells of the same
genotype.
The DNA in the sperm nucleus is packed densely with protamines, which pre-
vent transcription, although it retains part of the histones ( Miller et al., 2010 ) that
it possessed before transforming into a mature sperm cell. After entering the egg
cell, it has to be epigenetically reprogrammed in order to replicate its DNA. This,
obviously, requires establishment of a new, “zygotic” epigenetic state in the epige-
netically condensed sperm genome. In mammals, the male pronucleus begins DNA
demethylation and the replacement of protamines by histones ( Oswald et al., 2000;