Ring cells display high actomyosin contractility. (A–C) Optical sections (planar and sagittal views; arrowheads indicate the section plane) of confocal images from wild-type embryos labeled with epithelial marker EpCAM (white), F-actin (phalloidin, represented in LUT, middle panel), and pMLC, represented in LUT, lower panel) at hillock (E12.5; A), bud (E13.0; B), and bulb (E13.5; C) stages. Scale bar, 50 µm. Dashed line marks the epithelial-mesenchymal border. Arrowheads highlight high-intensity levels, i and ii (insert). Scale bar, 50 µm. (D) Optical sections (planar and sagittal views; arrowheads indicate the section plane) of confocal images from a wild-type embryo stained with EpCAM (white) and NMIIA (green) at the bulb (E13.5) stage. Dashed line marks the epithelial-mesenchymal border. Inserts (i and ii) are close-ups of sagittal views marked in orange and blue boxes. Scale bar, 50 µm. (E) Quantification of NMIIA intensities in cells of the epidermis (EC), mammary rudiment (MR), and neck region from five different embryos per stage (n_{E12.5 EC} = 1,292; n_{E12.5 MR} = 485; n_{E13.5 EC} = 1,889; n_{E13.5 MR} = 533; and n_{E13.5 Neck} = 119). Data shown represent the median (line) with 25th and 75th percentiles (hinges) plus 1.5× interquartile ranges (whiskers). Statistical significance was assessed with the Mann-Whitney U test with Bonferroni correction. ****, P ≤ 0.0001.