Impaired invagination and neck formation in Myh9 cKO embryos. (A and B) Scanning electron microscopy images of Myh9 cKO embryos and control littermates at E12.5 (A) and E13.5 (B) with close-ups of mammary primordium 3. Scale bar, 1 mm. i, insert; scale bar, 100 µm. mb, mammary bud. (C) Planar and sagittal views of maximum intensity projections showing surface rendering (cyan) of mammary rudiments of Myh9 cKO embryos and control littermates at E11.5, E12.5, E13.0, and E13.5. Dashed line marks the epidermal contact area. Red line connects top and bottom domains of mammary primordium showing the depth of the rudiment. Scale bar, 20 µm. See also Video 4. (D–F) Quantification of epidermal contact area (D), bud depth (E), and bud volume (F). Statistical significances were calculated with Student’s t test to controls and Myh9 cKO embryos at each developmental stage. n_E11.5 = 6 (Ctrl) and 7 (Myh9 cKO), n_E12.5 = 6 (Ctrl) and 7 (Myh9 cKO), n_E13.0 = 10 (Ctrl) and 8 (Myh9 cKO), n_E13.5 = 8 (Ctrl) and 13 (Myh9 cKO). Data are shown as mean ± SD. *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001; ****, P ≤ 0.0001.