490
The term OET is closely intertwined with an interchangeably used term
maternal- to- zygotic transition (MZT). MZT is defined as a period that begins
with the elimination of maternal transcripts, continues through the production
of zygotic transcripts, and ends with the first major requirement for zygotic
transcripts in embryonic development (Tadros and Lipshitz 2009 ). As our review
on clearance of parental products goes beyond transcriptome changes, we opt
for using OET, which is a broader term for transition of a fully grown oocyte to
an embryo. This will also simplify reviewing different vertebrate model systems
together given differences in stage nomenclature and different usage of terms
such as zygote or embryo in different communities studying vertebrate
models.
expressionmaternal zygotictime
oocytezygote embryo
1-cell morulamouse GV oocyte 1-cell 2-cell 8-cell morula blastocystfertilization genome activation2-cellMII oocyte 4-cellFig. 10.1 Vertebrate oocyte-to-embryo (OET) transition. Vertebrate OET spans a period from
the end of oocyte’s growth phase to the formation of an embryo where gene expression control
is relieved from maternal factors (represented by the red gradient). Maternal factors, as maternal
mRNAs (represented by red curves), are eliminated differentially, frequently in response to
developmental events such as transcription or genome activation. Zygotic gene expression is
represented by gray curves representing differential gene expression appearing from the onset of
the zygotic genome activation. The scheme corresponds well to the stages in the mouse model
depicted at the bottom of the scheme. Above the mouse model is a more generalized labeling of
the OET graph where the rectangles indicate loosely defined usage of the terms zygote and
embryo with regard to embryonic stages. The term zygote can be used for the fertilized egg or
for all cleavage stages embryos until the genome activation (or midblastula transition), which
takes place at different timepoints/early developmental stage in different species. As a conse-
quence, research on different model systems recognizes slightly different preferences for the
terminology
P. Svoboda et al.