Alina Guna1, Masami Hazu2, Giovani Pinton Tomaleri2 and Rebecca M. Voorhees2 1Whitehead Institute for Biomedical Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, USA 2Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California 91125, USA Correspondence: voorhees{at}caltech.edu