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cytoskeletal differences are eliminated during maturation. The functional
significance of this arrangement is also unclear. Because secretion is blocked
at oocyte maturation, likely at the trans -Golgi-to-plasma membrane step
( Colman et al., 1985; Leaf et al., 1990 ), it is impossible to test whether dif-
ferential membrane traffic toward animal or vegetal hemispheres occurs in
the egg. Additionally, after fertilization, the entire egg membrane behaves as
apical membrane and newmembrane as basolateral membrane ( M ¨ ller et al.,
1993; Roberts et al., 1992 ), suggesting that any animal-vegetal differences in
polarity proteins are not sufficient to impart functional membrane domains
at this stage. It may be the polarization of the egg is related to meiotic events
at the animal pole or events involving sperm receptors, which are also
animally localized.
2.2.3 Egg cell polarity in fish and mammals
The acquisition of polarity through egg cortex inheritance is likely to occur
in zebrafish as well. Blastomeres can polarize cell autonomously in transplant
assays ( Ho, 1992a ), and nonpolarized blastomeres are not polarized by expo-
sure to the external environment ( Sagerstr¨m et al., 2005 ). Additionally,
superficial epithelial fate in both frogs and fish requires the maternally
encoded transcription factor Interferon regulatory factor 6 (Irf6; Sabel
et al., 2009 ).
By contrast, mammalian (mouse) apical and basal polarity markers are not
differentially sorted between the egg cortex and new membrane ( Vinot
et al., 2005 ). Furthermore, mouse blastomeres depend critically on cell-cell
contact to initiate polarization during compaction ( Ziomek and Johnson,
1980 ). The differences in polarization mechanisms between mammals and
frogs are not known but could reflect differences in timing or rates of devel-
opment or
in the aquatic versus
intrauterine environments of
the
developing embryos.
2.3. Generation of definitive animal
vegetal polarity
In Xenopus , as in many other organisms, oocyte development and differen-
tiation take place within a germline cyst (or nest, in the older literature) of
16 interconnected cells ( Pepling et al., 1999 ). An overview of oogenesis in
both frogs and fish is depicted in Fig. 4.2 . Prior to entering meiosis, oocytes
undergo a clonal expansion from a single primary oogonium (cytoblast),
which is likely derived through asymmetric cell division of an oogonial stem
cell. The cystocyte daughters divide synchronously four times to produce a
germline cyst containing 16 cells (cystocytes). Cell division is typically
-
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