509Bachvarova et al. 1981 ). rRNA in growing mouse oocytes is highly stable until
ovulation (Bachvarova 1981 ) where the estimates for rRNA content range from 0.2
to 0.5 ng (Bachvarova 1974 ; Sternlicht and Schultz 1981 )—this would translate into
50–100 million ribosomes, which would be consistent with data from electron
microscopic morphometry, which estimated that the zygote contains ~55 million
ribosomes (Piko and Clegg 1982 ). However, several lines of evidence suggest that
proteosynthetic machinery is not stockpiled for mammalian OET but rather under-
goes rapid turnover and is quickly replaced by the embryonic one. Maternal ribo-
somes are destabilized upon resumption of meiosis (Bachvarova et al. 1985 ) and are
eliminated during OET at a much faster rate than zygotic rRNA synthesis occurs
(Gilbert et al. 2009 ; Piko and Clegg 1982 ). In mice, approximately 20 % of total
RNA is lost during meiotic maturation (Bachvarova et al. 1985 ), and the total RNA
is reduced approximately twofold between the fully grown oocyte and the late 2-cell
stage (Piko and Clegg 1982 ). Both RNA polymerase I transcription and pre-rRNA
processing machineries that produce pre-rRNA transcripts and mature rRNAs are
extensively eliminated by MII; another wave of degradation occurs after fertiliza-
tion (Zatsepina et al. 2000 ; Fulka and Langerova 2014 ). New transcripts for proteins
supporting rRNA production are made by the mouse embryo during ZGA as shown
by single-nucleotide polymorphism (SNP) analysis (Fulka and Langerova 2014 ).
Although zygotic rRNA expression is initiated at the 2-cell stage, the main upregu-
lation of rRNA synthesis takes place at the 4-cell stage (Abe et al. 2015 ; Zatsepina
et al. 2003 ). Thus, when the mouse 2-cell embryo initiates synthesis of translational
apparatus, at least a half of the maternal ribosomal pool is already eliminated, and it
will take an additional 24 h to have highly active rRNA synthesis (Abe et al. 2015 ).
A similar pattern is observed during bovine OET where the major ZGA occurs at
the 8-cell stage (Graf et al. 2014 ). rRNA amount in 8-cell embryos (estimated by
capillary electrophoresis) is more than threefold lower than that in the fully grown
oocyte (Gilbert et al. 2009 ) (Fig. 10.5). The notion of active elimination of the
maternal translational apparatus is supported also by microarray data, which sug-
gest active degradation of mRNAs encoding ribosomal protein-encoding genes dur-
ing meiotic maturation and after fertilization (Su et al. 2007 ; Zeng et al. 2004 ).
Taken together, if the maternal support would be compared to fueling a car for
a journey, mammalian oocytes would be reminiscent of a car with a tank having
just enough fuel to get to the nearest gas station. In zebrafish or Xenopus cars, the
tank would be full, and by the time this car would reach the gas station, the fuel
gage would not even move. A “full tank” concept observed in oviparous species
sounds self-evident. However, why would viviparous species adopt the other strat-
egy? One could speculate that the chosen “fueling strategy” is a cost-efficient
adaption of early development to the environment. In other words, in oviparous
species, there is presumably a high pressure to fuel the tank as much as possible as
the embryo developing in an external environment is highly dependent on mater-
nally deposited resources. Thus, more fuel likely translates into better fitness. In
contrast, mammalian embryos do not need to carry that much maternal supply as
they develop in a stable, nutrition-rich environment and, in fact, continue to receive
maternal support throughout pregnancy.
10 Clearance of Parental Products