ISSN 1022-7954, Russian Journal of Genetics, 2008, Vol. 44, No. 9, pp. 1007–1019. © Pleiades Publishing, Inc., 2008.
Original Russian Text © O.B. Simonova, Ju.E. Vorontsova, 2008, published in Genetika, 2008, Vol. 44, No. 9, pp. 1157–1171.
Oocyte determination remained an enigma in biol-
ogy over more than one hundred years. The oocyte
originates from a 16-cell syncytium (germline cyst),
which develops from one germline cell via four rounds
of division with incomplete cytokinesis [1, 2]. The
other 15 develop into nurse cells (trophocytes), which
supply nutrients and cytoplasmic components to the
oocyte. This suggests that the oocyte is chosen as a
result of polarization of the cytoplasm and membrane
of the cyst . Recent data make it possible to assume
that the oocyte polarity is partly determined by asym-
metry of the cyst. Thus, determination and polarization
of the oocyte are interrelated processes, which are con-
sidered in this review.
EARLY ONTOGENY IN
ovary consists of 16–20 ovarioles,
each containing a chain of maturing egg chambers.
New egg chambers are formed in the so-called germar-
ium, which is the anterior part of the ovariole and is
divided into four regions (Fig. 1). Oogenesis starts in
region 1, where a germline stem cell (GSC) divides
asymmetrically to produce a cystoblast, which is
shifted backwards, and a new GSC, which remains
associated with the adjacent somatic cell in the anterior
region . The cystoblast undergoes exactly four
rounds of mitotic division with incomplete cytokinesis,
which yields a cyst of 16 germline cells connected with
each other via stable cytoplasmic bridges, known as
ring canals. The constant division pattern is due to the
fusome, a speciﬁc cytoplasmic structure that anchors
one of the poles of the mitotic spindle . The resulting
16-cell cyst includes two cells with four ring canals,
two cells with three ring canals, four cells with two ring
canals, and eight cells with one ring canal. The con-
stancy of the division pattern is important, because the
oocyte is always formed from one of the two cells (pro-
oocytes) possessing four ring canals.
Once formed, the 16-cell cyst passes to germarium
region 2a. All cells of the cyst look similarly at this
stage, but one of them is determined to be oocyte as
soon as the cyst reaches region 2b. This event is accom-
panied by several characteristic changes. First, oocyte-
speciﬁc proteins (BicD, Orb, Btz, and Cup) and
) are initially concentrated
in the two pro-oocytes and occur at both sides of the
large ring canal between them. These molecules accu-
mulate exclusively in the oocyte at the end of region 2a
. Mitochondria similarly accumulate in the oocyte.
Second, microtubules, which are diffusely distributed
through the cyst, group close to the fusome, and their
minus ends gradually accumulate in the prospective
Am not I
A ﬂy like thee?
Or art not thou
A man like me?
Source of Asymmetry in Ontogeny: Early Polarization
of the Germline Cyst and Oocyte in
O. B. Simonova and Ju. E. Vorontsova
Kol’tsov Institute of Developmental Biology, Russian Academy of Sciences, Moscow, 119991 Russia;
Received August 13, 2007
—One or more body axes are already formed in the egg in many vertebrates and invertebrates. In
, the anterior-posterior and dorsoventral axes are determined during oogenesis owing to the asym-
metric localization of the
mRNAs in the oocyte (prospective egg). The localization
of these transcripts depends on the polarized organization of the oocyte cytoskeleton and, consequently, the
oocyte polarity. Initial asymmetry, leading to the oocyte polarity, is established in early ontogeny, during oocyte
determination. The review considers the steps of early polarization and oocyte differentiation in
genetic control of these processes, and the ﬁndings that suggest an early oocyte polarity for vertebrates.