Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 128

imaging have significantly improved our understanding of the scope and
prevalence of RNA localization. Recent studies in Drosophila embryos
found that over 70% of a set of 2500 mRNAs showed evidence of intra-
cellular localization during embryogenesis ( L´cuyer et al., 2007 ). Other
studies revealed mRNA localization in many types of asymmetric cell divi-
sions ( G¨nczy, 2008 ), in polarized epithelial cells ( L´cuyer et al., 2007;
Simmonds et al., 2001 ), and to a variety of subcellular compartments,
including the spindle, centrosomes, and various cellular organelles
( Martin and Ephrussi, 2009 ). It is thus becoming apparent that RNA local-
ization is not an isolated curiosity but a fundamental mechanism for control-
ling cell structure and function.
The egg is a cell and therefore must share general cellular mechanisms for
establishing polarity at some level. Since the last comprehensive reviews of
RNA localization in the vertebrate egg ( King et al., 2005; Kloc and Etkin,
2005 ), much has been learned about how polarity is established during ver-
tebrate oogenesis, how presumptive determinants are distributed in the
polarized egg, and how these determinants act in the egg to ultimately deter-
mine cell fate in the embryo. This chapter will summarize recent advances in
these areas, primarily focusing on work in Xenopus. Relevant work from
other amphibians, fish, and mammals will be addressed, and the outlook
for future genetic and postgenomic studies will be discussed.
2. Xenopus OOCYTES AND OOGENESIS
The overall cytoarchitecture of the amphibian oocyte forms much of
the basis for later regional cell differentiation; it is therefore important to
understand this organization and how it arises during oogenesis. In Xenopus ,
oocytes are produced continuously during reproductive life and oogenesis
occurs asynchronously, with all stages of oogenesis being present in
the ovary. Each ovary has about 24 lobes, each containing hundreds of
follicles ( Rasar and Hammes, 2006 ). Ovarian tissue is contained within an
epithelium continuous with the outer layer of the visceral peritoneum,
which covers theca cells (fibroblasts and blood vessels) and an inner epithelial
layer ( Rasar and Hammes, 2006 ). Oocytes themselves are surrounded
directly by a single layer of follicle cells. During early oogenesis, the oocyte
acquires the vitelline membrane (vitelline envelope), a noncellular muco-
polysaccharide layer secreted by the follicle cells ( Hausen and Riebesell,
1991 ). During follicular growth, follicle cells extend processes through