Development and Stem Cells Research Article

INTRODUCTION The development of naïve cells towards a terminally differentiated fate often proceeds through a number of distinct steps. Complex transcriptional and post-transcriptional hierarchies regulate transitions between these stages so that differentiation occurs in a linear and tissue-specific manner. Drosophila ovarian germline cyst development has served as a useful platform for studying how diverse mechanisms coordinate to establish specific cell fates, particularly with regards to stem cells and their differentiating progeny. Ovarian cyst development begins in the germarium with the asymmetric division of a germline stem cell (GSC) (for a review, see Wong et al., 2005). This division results in one of the daughters being displaced away from the cap cell niche. This cell, called the cystoblast, proceeds through four incomplete mitotic divisions to form an interconnected 16-cell cyst. Within this cyst, one cell becomes the oocyte, whereas the remaining cells become supportive nurse cells. Once encapsulated by follicle cells, the cyst buds off of the germarium to become an egg chamber. Several molecular and morphological markers highlight changes within differentiating germline cysts. One widely used marker has been the fusome, a germline-specific organelle that has many properties of the endoplasmic reticulum (Lighthouse et al., 2008; Snapp et al., 2004). The fusome plays roles in regulating the mitotic cell cycle within germline cysts and in oocyte specification (Lin and

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