493tion of extraembryonic and embryonic lineages and over another day to implant
and gastrulate.
10.1.2 Accumulation of Maternal Factors
Maternal factors, which accumulate in vertebrate oocytes, can be divided into
(1) energetic/synthetic resources (e.g., yolk, mitochondria, ribosomes) and (2)
control factors (e.g., mRNAs, RNA-binding proteins, transcription factors). The
yolk content in some vertebrate oocytes buttresses the notion of a warehouse
stockpiled with material and fuel for the embryo—as exemplified by the ostrich
egg, the largest egg, which is ~15 × 13 cm in diameter and weights 1.4 kg (Hyde
2004 ). It is astonishing that this entire structure filled with gigantic amount of
yolk evolved to support embryonic development from a single diploid nucleus.
Xenopus eggs, the largest of the three vertebrate models discussed in detail here,
are ~1.5 mm in diameter and contain ~4 μg of total RNA (Hough and Davidson
1975 ) and ~50 μg of non-yolk proteins (Smits et al. 2014 ). Zebrafish eggs are
smaller (~0.7 mm) and contain ~500 ng of total RNA (de Jong et al. 2010 ) and
~3 μg of non-yolk proteins (Link et al. 2006 ). An important characteristic of
Xenopus and zebrafish oocytes is that they are polarized. This polarization is
accompanied by differential distribution of factors, which constitute the mater-
nal preprogramming of the elementary body plan and the future germline
(reviewed in Pelegri 2003 ).
Mammalian oocytes offer a different picture. Apart from the relatively low
amount of yolk and lack of evidence for polarization that would determine embry-
onic axis formation, there does not seem to be such extensive stockpiling as observed
in zebrafish and Xenopus, which have external fertilization and development.
Mammalian oocytes are small relative to the examples above (80–120 μm in diam-
eter); it was estimated that they contain 0.3–2 ng of total RNA (Gilbert et al. 2009 ;
Lequarre et al. 2004 ; Piko and Clegg 1982 ) and 25–450 ng of total protein (Gilbert
et al. 2009 ; Schultz and Wassarman 1977 ).
Taken together, zebrafish and Xenopus zygotes are truly stockpiled with maternal
factors and develop at a fast pace. In contrast, mammalian zygotes develop rela-
tively slowly, and maternal stockpiling and parental governance of embryonic
development are rather moderate.
10.1.3 Aims and Scope of the Chapter
This chapter will explore elimination of parental factors. Among reviewed maternal
factors will be RNAs, proteins, and epigenetic marks. These factors can be divided
into oocyte-determining factors, which need to be removed to erase oocyte’s
10 Clearance of Parental Products