Organ formation and the establishment of adult stem cells

Figure 1A An illustration of an adult germarium Germ line stem cells (GSCs, light green) reside at the anterior of the germarium, in close contact with somatic niche cells (Terminal Filament, TF, and Cap cells, orange and maroon, respectively). GSCs are also in contact with the somatic Escort Cells (ESCs, pink). Upon stem cell division, one daughter cell remains a GSC, the other moves down the germarium, escorted by ESC daughter cells, and in contact with Inner Germarium Sheath cells (IGS, purple). Differentiating germ cells divide to form a germ line cyst (dark green), and midway down the germarium meet a third stem cell population, the Follicle stem cells (FSC, light blue). Follicle cells (FC, dark blue), the daughter cells of FSCs, and germ line cysts form the egg chamber.

Figure 1B A wild-type germarium Germ cells are labeled with Anti-Vasa (green). The germ line stem cell is the anterior-most cell in the germarium. Somatic cell membranes are outlined with 1B1 monoclonal antibody, directed against an adducin-like protein. Terminal filament, part of the somatic niche for GSCs, is marked by a bracket. 1B1 also labels the fusome, an organelle within germ cells, which is spherical in the GSC (arrowhead) and branched in germ line cysts (arrow).

Many organs employ stem cells for their normal function or for repair following damage. Understanding how stem cell numbers are determined, how they incorporate into their niches and how they are mobilized and differentiate is important for understanding normal body function, and imperative for applications of stem cell therapies.

We are using ovary formation in the fruit fly Drosophila melanogaster as a model system for organogenesis and stem cell establishment.

The ovary of Drosophila is composed of 18 egg-producing units. Each unit contains a structure, the germarium, which harbors two types of somatic stem cells as well as germ line stem cells (Figure 1A). Sophisticated fly genetics, combined with the accessibility of the niche and the stem cells to imaging (Figure 1B), turned the fly ovary into a leading system in the study of adult stem cells.

While the Drosophila ovary has been a successful model for the study of adult stem cell maintenance and differentiation, very little is known about how this complex organ forms.

Our research focuses on two major questions:

1. How different cell types within a growing organ coordinate their growth such that there is neither excess nor deficiency in any cell type.
    
2. How differentiation of a stem cell niche directs primordial cells to become adult germ line stem cells (GSCs).