Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 126

function. This chapter summarizes recent molecular and genetic experiments regarding:
(1) mechanisms that regulate polarity during different stages of vertebrate oogenesis,
(2) pathways that localize presumptive protein and RNA determinants within the polar-
ized oocyte and egg, and (3) how these determinants act in the embryo to determine
the ultimate cell fates. Emphasis is placed on studies done in Xenopus, where extensive
work has been done in these areas, and comparisons are drawn with fish and mammals.
The prospects for future work using in vivo genome manipulation and other

postgenomic approaches are also discussed.

1. INTRODUCTION
The animal egg is totipotent, fromwhich all the tissues of the body are
derived. Although often portrayed as simple and unorganized, the egg cell is
highly differentiated in its overall architecture and physiology. Many of these
specializations are needed for meiosis, fertilization, and egg activation. Less
appreciated is that cell polarity and organization in the egg and embryo are
necessary for successful development to occur and for the egg's totipotency
to be realized during subsequent embryonic development.
Some of the earliest embryologists appreciated the idea that polarization
of the egg might influence the development of different cells in the embryo.
Naturalists such as von Baer and Remak noted the visible organization of
amphibian eggs and described the tendency of these regions to contribute
to different germ layers. Typically, the pigmented upper “animal” half
would form the nervous and sensory systems, whereas the pale, yolky lower
“vegetal” hemisphere would form the gut and gut-derived organs ( Wilson,
1928 ). It had also been noticed that the germinal vesicle (GV or oocyte
nucleus) is often eccentrically positioned toward one side, typically the side
of eventual polar body extrusion from the egg. This was observed com-
monly at the animal pole, thus the site of polar body formation became
the accepted definition for the animal pole.
By the early twentieth century, experiments conducted mainly in marine
invertebrates led to the general conclusion that differential inheritance of
maternal cytoplasmic components from these different regions of the egg
leads to diverging cell fates in blastomeres during cleavage ( Wilson, 1928 ).
Several seminal observations provided evidence for such localized cytoplasmic
determinants in vertebrates as well, particularly in amphibian eggs. First, the
rearrangement of vegetal cortical egg cytoplasmwas suggested to contribute to
the formation of the dorsoventral axis ( Ancel and Vintemberger, 1948 ).