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First published online February 8, 2005
doi: 10.1242/10.1242/jcs.01680

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Signaling in stem cell niches: lessons from the Drosophila germline

Yukiko M. Yamashita, Margaret T. Fuller and D. Leanne Jones^*,

Department of Development Biology, Stanford University School of Medicine, Stanford, CA 94305, USA

View larger version (40K): [in a new window]   Fig. 1. GSC niches in the Drosophila ovary and testis. (A) Schematic of the early steps in Drosophila spermatogenesis. The GSCs (light green) surround and are in contact with a cluster of post-mitotic somatic cells known as the apical hub (red). The hub cells are a primary component of the male GSC niche. Each GSC is surrounded by two somatic stem cells known as the cyst progenitor cells (light gray). The GSC undergoes asymmetric cell division, giving rise to one daughter cell that will retain stem cell identity and one daughter cell, a gonialblast (dark green), which will undergo four rounds of cell division with incomplete cytokinesis to give rise to 16 spermatogonia. The gonialblast is surrounded by cyst cells (dark gray), which ensure spermatogonial differentiation. (B) Schematic of a Drosophila germarium, which houses the GSCs. Anterior is to the left and posterior is to the right. The terminal filament, cap and inner sheath cells (red) express molecules important for the maintenance and self-renewal of female GSCs and make up the stem cell niche. GSCs (light green) undergo asymmetric cell division, giving rise to one daughter cell that will retain stem cell identity and one daughter cell, a cystoblast, which will initiate differentiation (dark green). As these divisions are taking place, the more mature cysts are displaced towards the posterior of the germarium. Cyst encapsulation by the somatic stem cell derivatives (gray) occurs in region 2a/b. Mature encapsulated cysts budding off from the germarium make up region 3.

View larger version (48K): [in a new window]   Fig. 2. Signaling from the stem cell niche regulates Drosophila GSC self-renewal and maintenance. GSCs (yellow) attach to niche cells (blue) through adherens junctions (AJ, grey). The mitotic spindle (green) is oriented orthogonally to the supporting niche cells, ensuring that one stem cell daughter remains within the niche, whereas the other daughter cell is displaced out of the niche and differentiates. (A) Secretion of the Upd ligand from hub cells (blue) activates JAK-STAT signaling in the male GSC. Mitotic spindles are anchored to the GSC cortex by Apc2 (pink), and GSC daughters initiate differentiation as gonialblasts (light green). (B) Secretion of Dpp from cap cells (blue) activates TGF-ß signaling in female GSCs, which represses transcription of the differentiation factor bag of marbles (bam). Mitotic spindles are anchored to the GSC cortex by the spectrosome (orange), and GSC daughters initiate differentiation as cystoblasts (light green).

View larger version (77K): [in a new window]   Fig. 3. Mitotic spindle orientation is regulated in Drosophila GSCs. (A) Male GSCs expressing a GFP-tagged -tubulin were visualized using scanning laser confocal microscopy. Drosophila E-cadherin (red) localizes to the surfaces of hub cells and to the interface between hub cells and GSCs. The mitotic spindle visible in one GSC (arrow) is oriented orthogonally to the apical hub. (B) Immunofluorescence image of a Drosophila germarium. Germ cells are labeled with an antibody to the germ-cell-specific protein Vasa (green). Antibodies to the membrane protein -spectrin (red) label the somatic cells within the germarium, as well as the vesiculated, cytoplasmic, ball-shaped structure known as the spectrosome in GSCs (arrow) and cystoblasts.

View larger version (40K): [in a new window]   Fig. 4. Asymmetric division of Drosophila neuroblasts is regulated by intrinsic cell fate determinants. A Drosophila neuroblast divides asymmetrically, generating another neuroblast and a smaller ganglion mother cell (GMC) that will produce mature neurons. Cell fate determinants such as Numb and Prospero (blue crescent) are segregated only into the GMC. Spindle orientation is determined by the apical complex (red crescent), containing proteins such as Bazooka (Baz)/Par-3 and Inscuteable (Insc). Spindle orientation in the neuroblast is established by the programmed rotation of the spindle during mitosis so that it becomes perpendicular to the neuroepithelium, from which the neuroblast is derived. Pins, partner of Inscuteable; aPKC, a typical protein kinase C; Pon, partner of Numb.