Imaginal discs are composed of two cell layers: the columnar epithelium and the peripodial epithelium. There have been numerous studies characterizing the development of the columnar epithelium and this has contributed to our understanding of disc patterning, cell signaling mechanisms and cell cycle regulation. However, very little is known about these same processes in the peripodial epithelium.

We are interested in the extent to which the peripodial epithelium cell number or cell size regulates the size of the disc proper. We overexpressed cell-cycle regulating genes specifically in the peripodial epithelium and monitored size changes in the columnar cells that form the adult organ. We found that one of the cell-cycle genes expressed in the peripodial epithelium produces a larger wing. These initial observations indicate that cell signaling from the peripodial epithelium regulates organ size. Now we will analyze how changes in cell size and cell number in the two layers lead to a larger wing.

Stem cells and their regenerative capabilities are of great interest to developmental biologists, giving hope for the treatment of injuries and diseases. Imaginal disc cells have rigidly determined fates. Yet in each imaginal disc we identified a few specific cells that have the developmental plasticity to regenerate missing parts. In doing so they can change from one disc identity to another in a process known as transdetermination.

We have characterized the cell cycle of regenerating and transdetermining cells. We find that the cell cycle of regenerating cells does not differ from normal growing disc cells. However, if regenerating cells transdetermine, we observe a novel cell cycle with a higher proportion of cells in S-phase and G2 (green curve) compared to non-transdetermining cells (black curve).

The phenomenon of regeneration has been known for centuries and experiments examining the process of regeneration have been performed for over 200 years. However, a molecular and genetic understanding of the process is still in its infancy. Initial observations in many regeneration systems have shown that Wingless (Wg/Wnt1) is a significant signaling molecule that is necessary and significant for the initiation of regeneration. To find Wg targets in the process of regeneration, a model system using Drosophila imaginal discs is ideal. Progress in the regeneration field requires the identification of Wg target genes, their products and function in regeneration. Currently, the nature of regeneration genes is not clearly known. Are regeneration genes developmental genes previously active during embryogenesis and then reactivated during the process of regeneration (a recapitulation of earlier developmental events) or are they simply up or down-regulated during regeneration. Novel and possibly ideal for therapeutic reasons are those genes that are activated in the target organs only during the regeneration process, possibly these are the true regeneration genes.

As in other regeneration systems, imaginal disc cells respond to fragmentation first by activating wg and later by forming a blastema that proliferates to rebuild the missing part. In genome profiling experiments with blastema cells, we identified many candidate regeneration genes. Our recent work focuses on genes that are not expressed in growing leg discs, but are induced specifically in regenerating cells. Interestingly, many of these genes are also conserved in the process of mammalian tissue regeneration.

We have shown that mutations of these genes change when and where a blastema is formed. We hope to use this information to understand the mechanisms that permit regeneration.