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embryos and conclude with a discussion of how the regulation of key maternal cell-
fate determinants at the level of translation functions in Xenopus embryogenesis. A
key theme is that the molecular asymmetries critical for forming the body axes are
established and further elaborated upon by the selective temporal and spatial regula-
tion of maternal mRNA translation.
Keywords Xenopus • Maternal mRNA • Regulated translation • Embryonic
asymmetry
2.1 Introduction
2.1.1 Oogenesis and Oocyte Maturation: Maternal mRNAs Set
the Stage
A significant amount of the early development of Xenopus is deterministic, meaning
that embryonic cells destined to specific cell fates accumulate distinct proteins
called maternal determinants (Heasman 2006a; White and Heasman 2008 ). Maternal
determinants, regulatory proteins that function in cell-cell signaling, translational
control, mRNA processing, chromatin remodeling, and other processes that can
alter cell fates are translated from stored maternal mRNAs that are accumulated in
eggs prior to fertilization during oogenesis (Fig. 2.1).
Oocytes form and grow over a period of months to develop into fully mature
stage 6 oocytes with a distinctive darkly pigmented animal hemisphere and a lightly
pigmented vegetal hemisphere (Fig. 2.1). These pigmentation differences mark the
animal-vegetal axis of the oocyte and are carried over into eggs during oocyte matu-
ration and then passed on to embryos after fertilization. The animal and vegetal
hemispheres are a striking visual manifestation of the elaborate molecular events
that create animal-vegetal distributions of macromolecules that function in cell-fate
decisions, particularly localized mRNAs that encode cell-fate determinants (Medioni
et al. 2012 ; King et al. 2005 ; Houston 2013 ) (Fig. 2.2a).
Fig. 2.1 Maternal stages of Xenopus development. Summary diagram of oogenesis, oocyte matu-
ration, fertilization, and cleavage stages of embryogenesis. The maternal period of embryonic
development begins at fertilization and continues until the maternal mRNAs and proteins are elim-
inated and replaced by zygotic products during the maternal to zygotic transition
M.D. Sheets et al.