492
This chapter is centered around the mouse model, which is compared and con-
trasted with other mammals, Danio rerio and Xenopus laevis/tropicalis. There are
plenty of aspects important for a comparative summary of vertebrate OET, which
deserve attention. First, it must be appreciated that vertebrates are a highly diverse
group, which evolved over 500 million years. Vertebrates include aquatic, amphib-
ian, or terrestrial animals having external or internal fertilization. Accordingly,
embryonic development exists in three forms differentiated by parental involve-
ment—embryos develop externally or internally with or without connection to the
maternal cardiovascular system. Adaptations to different habitats and reproduc-
tive strategies can explain extreme diversity of vertebrate egg morphology
(Fig. 10.2), different amounts of deposited nutrients and information, as well as
pace of initial development (Fig. 10.3). For example, external development of
zebrafish and Xenopus embryos is very fast relative to the embryonic develop-
ment of placentals. Zebrafish and Xenopus embryos undergo intense organogen-
esis 1 day after fertilization. The 24-h-old mouse zygote has two cells and just
initiated its own mRNA synthesis; it will take it two more days to initiate separa-
Fig. 10.3 Variable timescales of vertebrate OETs. Shown is early development of the three com-
mon vertebrate models for studying early development. Maternal mRNA decay and zygotic
genome activation (ZGA) are drawn according to published data. In a strictly temporal timescale,
the earliest zygotic genome activation is observed in the zebrafish followed by the Xenopus model.
Zygotic transcription in mouse embryos is observed during the 1-cell stage; however, this tran-
scription does not seem to produce functional mRNAs replacing maternal mRNAs. ZGA in the
mouse embryo takes place at the 2-cell stage; in other mammals (e.g., cows, humans), ZGA takes
place later
P. Svoboda et al.