Regulation of Cell Polarity and RNA Localization in Vertebrate Oocytes - International Review of Cell and Molecular Biology - page 137

incomplete, leaving the cells connected by intracellular bridges termed ring
canals ( Kloc et al., 2004a,b ). In Drosophila , where germline cyst development
has been extensively studied, the cystocyte with the most connections to
other cells becomes the oocyte and the others become nurse cells
( Pepling et al., 1999 ). In vertebrates, however, all cells in the germline cyst
become oocytes.
Oocytes in the germline cyst are asymmetrically organized before enter-
ing meiosis, having a distinct “pear-shaped” structure ( al-Mukhtar and
Webb, 1971; Coggins, 1973 ). Cells in the cyst divide in a synchronous
and ordered fashion, ultimately forming a typical “rosette” arrangement,
with the narrow, or tail (or cap, in the older literature), end of the cells
consistently oriented toward the center of the cyst. The nuclei of these cells
occupy the wider head end, with cellular organelles (endoplasmic reticulum,
Golgi, mitochondria) and the centrosomes clustering in the tail end.
The mitochondria, in particular, are clustered into an aggregate that likely
perdured from similar aggregates in primordial germ cells and are considered
to be the precursor of the Balbiani body, or mitochondrial cloud, of meiotic
oocytes ( al-Mukhtar and Webb, 1971; Coggins, 1973 ). The centrosome
is typically associated with the mitochondrial aggregate. Interestingly,
nuclear components are also polarized in these early oocytes. Once meiosis
is underway, the synaptonemal complexes of pachytene chromosomes
cluster toward the centrosomal/mitochondrial side of the nucleus, an inter-
esting but unexplained phenomenon ( al-Mukhtar and Webb, 1971;
Coggins, 1973 ).
Following entry into diplotene of meiosis I (early stage I), visible polarity
is lost transiently as follicle cells invade ( al-Mukhtar and Webb, 1971;
Coggins, 1973 ). The intracellular bridges are disassembled, separating the
oocytes from one another and allowing relatively asynchronous develop-
ment. Among other changes, the cells become rounder and the GV is more
centrally positioned. Also, the mitochondrial aggregate disperses into smaller
aggregates that surround the GV and become indistinguishable both visibly
and ultrastructurally ( Heasman et al., 1984 ). Furthermore, a subset of RNAs
that are localized to the definitive mitochondrial cloud in stage I oocytes are
localized to all of these “premitochondrial clouds” ( Kloc et al., 1996 ),
suggesting the preclouds are functionally similar. Additionally, analysis of
microtubule nucleation activity in living oocytes and immunostaining
against centrosomal antigens indicates that the centrosome becomes
inactivated at this time ( Gard et al., 1995 ). The molecular basis for centro-
some inactivation is unknown.