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In-Depth Information
At the cell biological level, our understanding of RNA localization is pri-
marily descriptive. The organization of the three main cytoskeletal systems
during
Xenopus
oogenesis has been characterized in detail. However, the
role of critical cytoskeletal effector proteins in communicating this organi-
zation to organelles and cellular processes remains mostly unknown. Part of
this deficiency is technical, since oogenesis takes place over an extended
period of time and is not easily accessible to experimentation, in contrast
to mature oocytes and eggs. Also, the cytoskeleton is inherently dynamic,
but mostly, we have only a static understanding of oocyte organization
derived from immunostaining of ovary and oocytes. There are a number
of interesting studies examining cytoskeletal dynamics and interactions in liv-
ing, full-grown
Xenopus
oocytes and eggs, using fluorescent protein fusion
probes (
Bement et al., 2003; Benink and Bement, 2005
). However, these have
mostly focused on events during wounding or oocyte maturation and have
not been extended into early oogenesis or developmental outcomes.
Although the opacity of
Xenopus
oocytes is commonly cited as a disadvantage,
many critical events happen in the cortex, which is imaged easily.
In addition to deficits in understanding the molecular and cellular mech-
anisms of RNA during oogenesis, there are additional questions regarding
the overall oocyte development and the establishment of overall oocyte
polarity. First, the mechanisms regulating ongoing oogonial stem cell divi-
sion in adult fish and frog ovaries remain mostly unexplored. The recent
description of
bona fide
germline stem cells and associated niches in teleosts
(
Beer and Draper, 2013; Nakamura et al., 2010
) suggests that vertebrate
models of this system could have a significant impact on understanding
oogenesis. The existence of adult oogenesis is highly controversial in mice
and humans (
Woods et al., 2013; Zhang et al., 2012
). Insights from frog and
fish oogonial stem cells could help understand potential deficiencies in mam-
malian oogonial stem cell renewal. Second, the role of the putative verte-
brate spectrosome and mitochondrial cloud components in regulating cell
polarity through the oogonial-oocyte transition remains unexplored. The
studies on
macf1
in zebrafish suggest that intracellular transport to the periph-
ery may be involved (
Gupta et al., 2010
). Third, it is unclear how the cen-
trosome inactivated in early diplotene and how definitive AV polarity
reestablished in the oocyte. There are hints of a role for signaling between
the oocyte and the follicle cells in this process (
Bontems et al., 2009
). Addi-
tional questions remain regarding the formation and function of germline
granules and the functions of noncoding RNAs in the oocyte.
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