legend Madin Darby canine kidney cells cultured in a collagen gel for 7–10 days form hollow cysts, consisting of a monolayer of polarized cells. Actin was stained with Alexa-Fluo-488–phalliodin. The 3D image was obtained with an LSM 510 laser-scanning confocal microscope (Zeiss, Tornwood, NY. Scale bar 10 μm. The contributor acknowledges the help of K. Mostov and M. Zegers.