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Fig. 1. Schematic of the anterior part of a female ovariole and male testis in
Drosophila melanogaster. The description of the developmental stages
of the germ cells is to the left of each structure and the somatic cells are
to the right. Anterior is up. In the female ovariole (A) and male testis (B),
germ-line stem cells (GSCs) are located at the anterior tip of the gonad. Upon
division, the posterior daughter cell differentiates to a cystoblast (Cb,
females) or a gonialblast (males). The differentiation may be gradual, as
depicted by increasingly stronger shades of green. The daughter cell divides
further and forms a cyst. GSCs and their immediate daughter cells harbor a
spherical fusome (here marked in black). This organelle grows and extends into
every cell of the cyst (black marking within the cyst). In females, the niche
includes terminal filament (TF) and cap cells, which are located most
anteriorly. Inner sheath (IS) cells may also be part of the niche and may
perform similar functions to those of early somatic cyst cells in males.
Somatic stem cells (SSCs) are located `midway' down the germarium and they
give rise to the follicle cells that envelope the cyst. In males, SSCs are
attached to the hub, and their descendents (cyst cells) encapsulate the
gonialblast. Color-coding is used to mark cells that have a similar function
in males and females. Shades of red have been used for TF and cap cells as
these two populations, although similar, are not identical in their gene
expression profiles (Forbes et al.,
1996a).
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