The development of any organism relies on the formation of boundaries between homogeneous populations of cells. These boundaries generate compartments of cells that do not intermingle. Work recently published in Cell by Marco Milán, Stephen Cohen and colleagues (Cell 106, 785–794, 2001) identifies two transmembrane proteins with leucine-rich repeats, known as Capricious (Caps) and Tartan, that contribute to this process.

Marco Milán and Stephen Cohen

Formation of the dorsal/ventral compartment boundary in Drosophila melanogaster has been extensively studied to understand how cells are separated into different populations and lineages. The transcription factor Apterous (Ap) is required for the formation of the dorsal/ventral boundary and is expressed in the dorsal wing compartment. Ap controls the expression of genes regulating the development of the boundary and is essential for the development of the entire dorsal wing compartment. It has long been postulated that cells in different compartments of the Drosophila wing have altered affinities for one another, but this has not yet been confirmed.

Cohen and colleagues showed that both caps and tartan are expressed only in the dorsal compartment of the wing, and that this expression relies on apterous. Interestingly, when clones of cells expressing caps or tartan are generated in the ventral compartment of the wing, these clones try to migrate towards the dorsal compartment. The clones actually push into a stripe of Wingless at the dorsal/ventral boundary and move it dorsally. This stripe is required for supporting the growth and patterning of the wing and is a consequence of dorsal/ventral boundary formation. If the clones expressing caps or tartan are made early enough in development, the dorsal/ventral boundary is severely distorted.

As Caps and Tartan seem to have no effect on homophilic adhesion, Milán et al. propose that both molecules interact with an unknown protein expressed in the dorsal compartment. One insight into how these proteins might lead to cell sorting comes from analysing ventral clones expressing caps under the confocal microscope. Under the microscope these clones have membrane-bound cellular processes that extend towards the dorsal comparment (see figure, Caps protein (green), wingless (blue), apterous (red)). These processes are quite possibly filopodia and could act to increase the affinity of cells within the dorsal compartment for one another.

The identification of these two proteins, which seem to mediate the affinity between cells of separate compartments, opens the way to further insight into how cells of different lineage separate from each other.