Embryos were fixed as described. After rinsing in PBS, heads were dissected in half, embedded in 4% agarose, and allowed to set overnight at 4°C. Thick sections (120 μm) were cut using a tissue slicer (Electron Microscopy Sciences, Hatfield, PA). Sections containing the lens were blocked in 5% normal goat serum, 0.5% Triton X-100 for permeabilization, and 0.03% sodium azide for 1 hour at RT and were incubated with primary antibodies overnight at 4°C. After rinsing, sections were incubated with fluorescence-labeled secondary antibodies for 1 hour at RT and counterstained with DRAQ-5 (Biostatus Limited, Shepshed, Leicestershire, UK), a vital, fluorescent DNA dye. Sections were mounted in medium (Vectashield; Vector Laboratories) mixed with PBS at a 1:1 ratio, mounted on glass coverslips, and visualized using a confocal microscope (LSM 510; Carl Zeiss, Inc., Thornwood, NY). Primary antibodies used for immunofluorescence in this study were rabbit anti–phospho-histone H3 (3475B; Cell Signaling, Danvers, MA) at 1:1000, rabbit anti–FoxE3 (a gift from Peter Carlsson) at 1:1000, mouse anti–Prox1 (MAB5652; Chemicon InternationalA) at 1:1000, rabbit anti–β-catenin (9587; Cell Signaling) at 1:1000, anti–E-cadherin (13–1900; Invitrogen, Carlsbad, CA) at 1:1000, and anti–ZO-1 (61–7300; Zymed Laboratories, San Francisco, CA) at 1:500. AlexaFluor-labeled phalloidin (Invitrogen) was used to stain for F-actin at 1:1000. AlexaFluor-conjugated anti–mouse and anti–rabbit secondary antibodies (Invitrogen) were used at 1:1000.