Without lots of apical membrane, long tubes cannot form (right).

Andrew/Elsevier

Delivery of apical membrane drives tube growth, according to Monn Monn Myat and Deborah Andrew (Johns Hopkins University, Baltimore, MD). Although the initial step of tube formation involves constriction of the apical membrane to allow cell involution, the subsequent expansion of apical membrane supplies the raw material for tube elongation.

The tubes in question are the salivary glands of flies, but Andrew thinks her results extend far beyond fly spit. “I think it's going to be a unifying phenomenon,” she says. “You can do a lot by controlling when and where membrane is delivered.”

Earlier workers had put forward the apical membrane hypothesis. But the Johns Hopkins team has now supplied the field with some handy molecular markers for the process. They found that the patterning gene hairy suppresses both excessive branching and expansion of tubes, at least in part by turning off huckebein (hkb).

Hkb acts to turn on two genes that increase apical membrane growth: klarsicht (klar) and crumbs (crb). Klar helps transport everything from nuclei to lipid droplets; in this case it is probably essential for delivering secretory vesicles and thus apical membranes. Crb, meanwhile, has been shown by others to increase apical membrane growth by a poorly defined mechanism that may involve building a cytoskeletal framework. ▪

Reference:

Myat, M.M., et al.
2002
.
Cell.
111
:
879
–891.