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5.3 Animal-Vegetal Oocyte Polarity
5.3.1 Key Events in Oogenesis
After their birth from a germ line stem cell, oocytes become polarized through the
differentiation process of oogenesis. The process of oogenesis begins when a germ
line stem cell divides to form an oogonial cell. Oogonia are larger than germ cell
stem cells and undergo additional mitotic divisions (Fig. 5.2a). The transition of
germ line stem cells into oogonia is uncharacterized in vertebrates, and its regula-
tion is unknown. During their mitotic divisions, oogonia undergo incomplete cyto-
kinesis, whereby individual oogonial cells remain interconnected by cytoplasmic
bridges to their sister cells, thus forming a cyst (Fig. 5.2a). The early development
of oocytes in a cyst is a conserved theme in animals (Pepling et al. 1999 ). Since cells
in a cyst can share their cytoplasm or specific factors within it, it is thought that the
cyst allows for their synchronized development. Oogonia divide several times, com-
prising the cyst, where it has been defined and is engulfed by somatic follicle cells
(Leu and Draper 2010 ).
After their final division, oogonia initiate meiosis I. Ample evidence in the mouse
establishes the retinoic acid (RA) signaling pathway as the major regulator of mei-
otic initiation (Anderson et al. 2008 ; Bowles and Koopman 2007 ; Koubova et al.
2014 ). While RA players are expressed in follicle cells of the zebrafish ovary
(Rodriguez-Mari et al. 2013 ), such data in zebrafish and frogs are scarce. Upon
initiation of meiosis, oocytes undergo a specialized and unusually long prophase.
Major developmental events occur during the elongated prophase I (Fig. 5.2). The
most prominent ones are the genetic process of recombination and the vast cellular
growth of the oocyte. The first meiotic division is completed in late stages of oogen-
esis, during oocyte maturation (Fig. 5.2c). The second meiotic division is only com-
pleted after fertilization.
The cyst in which oogonia reside eventually breaks down, and individual oocytes
begin folliculogenesis. Folliculogenesis involves the surrounding of individual
oocytes by somatic follicle cells. Follicle cells may play an active role in the com-
pletion of cytokinesis and breakdown of the cytoplasmic bridges in the cyst, as
found recently in germ line stem cells in the Drosophila testis (Lenhart and DiNardo
2015 ). Ultimately, several layers of follicle cells surround each oocyte, with granu-
losa cells in the inner layers and theca cells in the outer ones. Folliculogenesis pro-
gresses in parallel to oocyte growth.
5.3.2 Symmetry Breaking and Early Polarization
During Oocyte Differentiation
Until recently, the first indication of polarity in the oocyte was the detection of the
Balbiani body (Bb) at a mid-diplotene stage of prophase I or, an earlier more dis-
persed asymmetric localization of Bucky ball protein in zebrafish (Heim et al. 2014 ;
M. Escobar-Aguirre et al.