Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 157

following compaction and apicobasal polarity initiation at the 8-16-cell
stage form TE. Nonpolarized cells on the interior, derived from tangential
cleavage of TE blastomeres, contribute to epiblast and PE in successive
waves of asymmetric divisions ( Morris et al., 2010 ). Epiblast and PE progen-
itors then undergo cell sorting to establish the proper arrangement of embry-
onic and extraembryonic cell lineages prior to implantation.
A series of recent experiments have suggested that 4-cell blastomeres
have slightly different developmental potencies ( Piotrowska-Nitsche
et al., 2005; Tabansky et al., 2012 ). In particular, the “vegetal” blastomere
of the 4-cell embryo cannot form viable embryos when made into chimeras
( Piotrowska-Nitsche et al., 2005 ), likely because it is deficient in generating
pluripotent ICM cells ( Morris et al., 2012 ). The basis for this preference for
TE fate is unclear, and epigenetic regulation of cell polarity gene expression
has been implicated ( Parfitt and Zernicka-Goetz, 2010 ).
There is no definitive evidence for localized determinants in the mam-
malian egg or oocyte at present. Although young mouse oocytes possess a
mitochondrial cloud, this structure disperses symmetrically in growing
oocytes ( Pepling et al., 2007 ), suggesting that it does not asymmetrically
contribute material to the cortex. Components of cell polarity signaling
are enriched at the animal pole, such as Pard6 ( Vinot et al., 2004 ), but these
likely have functions related to the meiotic spindle. A role for these proteins
in polarizing the blastomeres of the embryo has not been demonstrated.
Similarly, a recent microarray and qRT-PCR analysis of mouse oocytes
and early blastomeres found unique RNAs associated with the meiotic spin-
dles and polar bodies ( VerMilyea et al., 2011 ). Importantly, however, this
analysis failed to identify differential transcript expression in 2- and 3-cell
embryo blastomeres ( VerMilyea et al., 2011 ), suggesting that RNA locali-
zation is unlikely to direct cell fate specification at this stage.
RNA localization does, however, play a role in early TE specification in
the mammalian blastocyst. Caudal type homeobox 2 ( Cdx2 ) mRNA accumu-
lates at the prospective apical side of 8- to 16-cell blastomeres, and this local-
ization likely helps establish high Cdx2 levels and TE fate in the outer layer
and reduce the contribution of Cdx2 to pluripotent inner cells ( Jedrusik
et al., 2008; Skamagki et al., 2013 ). Interestingly, apical Cdx2 localization
is dependent on a 97-nt cis -element at the 3 0 -end of the open reading frame
( Skamagki et al., 2013 ). This is the earliest known RNA localization in
mammalian embryos and likely uses a general apical RNA localization
mechanism found in many epithelial cell types ( Medioni et al., 2012 ). Even
less is known about polarization of human eggs and oocytes. Human