The Questions of Developmental Biology

Photoreceptor induction in Drosophila
The Drosophila retina consists of about 800 units called ommatidia
(Figure 6.16). Each ommatidium is composed of 20 cells arranged in a
precise pattern. Eight of those cells are photoreceptors; the rest are lens cells.
The eye develops in the flat epithelial layer of the eye imaginal disc of the
larva. There are no cells directly above or below this layer, so the
interactions are confined to neighboring cells in the same plane. The
differentiation of these randomly arranged epithelial cells into the retinal
photoreceptors and their surrounding lens tissue occurs during the last (third)
larval stage. An indentation forms at the posterior margin of the imaginal
disc, and this morphogenetic furrow begins to travel forward toward the anterior of the
epithelium (Figure 6.17). The movement of the furrow depends on the interactions between two
paracrine factors, Hedgehog and Decapentaplegic. Hedgehog is expressed by the cells
immediately posterior to the furrow (i.e., those that have just differentiated), and it induces the
expression of the Decapentaplegic protein within the furrow (Heberlein et al. 1993; Ma et al.
1993). Thus, as retinal cells begin to differentiate behind the furrow, they secrete the Hedgehog
protein, which drives the furrow anteriorly (Brown et al. 1995).
As the morphogenetic furrow passes through a region of cells, those cells begin to
differentiate in a specific order. The first cell to differentiate is the central (R8) photoreceptor
(Chen and Chien 1999). (It is not yet known how the furrow instructs certain cells to become R8
photoreceptors, but it is possible that the Decapentaplegic and Hedgehog proteins in the furrow
region induce R8 determination.) The R8 cell is thought to induce the cell before it and the cell
after it (with respect to the furrow) to become the R2 and R5 photoreceptors, respectively.
The R2 and R5 photoreceptors are functionally equivalent, so the signal from R8 is
probably the same to both cells (Tomlinson and Ready 1987). Signals from these cells induce
four more adjacent cells to become the R3, R4, and then the R1 and R6 photoreceptors. Last, the
R7 photoreceptor appears. The other cells around these photoreceptors become the lens cells.
Lens determination is the "default" condition if the cells are not induced.