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Drosophila
ovaries are made up of 14-16 ovarioles with the germarium as the anteriormost
structure. Germline stem cells reside in the anteriormost region of the
germarium, immediately under the terminal filament (Fig. 1A). Each stem
cell divides to form a cystoblast and another stem cell; the cystoblast
undergoes four synchronized divisions with incomplete cytokinesis to form
a cyst of 16 interconnected cells. Cysts move down the germarium and cells
of somatic origin, the follicle cells, surround them pinching off an egg
chamber at the posterior end of the germarium. The five to nine germ cells
(stem cells and early cystoblasts) immediately under the terminal filament,
show high levels of Sxl protein primarily in the cytoplasm (Fig. 1B). As
these cells mature, there is a marked drop in the level of cytoplasmic Sxl.
After this downregulation, Sxl is detected in mid-region 1 of the germarium
as bright nuclear foci (Bopp et al., 1993). The intensity of Sxl in the
nucleus and cytoplasm increases as the germ cells progress down the germarium
(Figs 1B, 2A). The remarkable change in both localization and levels of
Sxl in early germ cells suggests that the process is of functional significance.
We demonstrate that some of these changes are controlled by the Hedgehog
(Hh) signaling pathway. (Vied,
2001) |
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