49110.1.1 Oocyte-to-Embryo Transition in Brief
While a thorough review of OET in vertebrates is beyond the scope of this
chapter, we need to introduce essential landmarks appearing during the jour-
ney of a fully grown oocyte toward a “self-sufficient” embryo (Fig. 10.1).
OET begins at the end of the oocyte growth phase when transcription ceases
and the oocyte that was arrested in the prophase of the first meiotic division
resumes meiosis. These two events (transcriptional quiescence and resump-
tion of meiosis) may occur at different timepoints, and each of them is associ-
ated with signaling events/cytoplasmic changes with effects on stability of
maternal factors. Next, the oocyte progresses through meiosis and typically
becomes arrested at the metaphase of the second meiotic division (metaphase
II, MII). Fertilization triggers release from the second meiotic arrest and com-
pletion of meiosis. After fertilization, maternal and paternal genomes begin to
occupy the same nucleus (in mammals, the first true zygotic nucleus appears
at the 2-cell stage) and, at some cleavage stage, will become transcriptionally
active. Finally, zygotic expression will take over maternal gene products, and
embryo’s development will become fully controlled by genes expressed from
its own genome.
Fig. 10.2 Vertebrate oocyte diversity. Shown are schematic drawings of Xenopus, zebrafish, and
mouse oocytes. Oocytes are drawn to the same scale indicated by the scale bar 100 μm
10 Clearance of Parental Products