This month's winning image was submitted by Wei Yu (University of California (yuwei@itsa.ucsf.edu)). It shows actin stress fibres in a hollow cyst of Madin–Darby canine kidney (MDCK) cells.

Green fluorescence represents actin fibres in a hollow cyst of MDCK cells cultured in a collagen gel for 7–10 days. Actin was stained with Alexa Fluor 488-conjugated phalloidin. The image was obtained using a laser-scanning confocal microscope (LSM 510, Zeiss). Bar, 10 μm.

The winner would like to acknowledge the help of Keith Mostov (mostov@itsa.ucsf.edu) and Mirjam Zegers (zegers@itsa.ucsf.edu) (University of California).

We are pleased to acknowledge the help of our two external advisors, Ariel Ruiz i Altaba (Developmental Genetics Program, New York Medical Center) and Lelio Orci (Department of Morphology, University Medical Center, Geneva).

To see more Cell of the Month winners, please visit our Cell of the Month archive .