NMHC IIB is the predominant form of NMII in the embryonic heart and epicardium.

<p>A-C: Immunohistochemical comparison of the three different NMHC II isoforms (green) in the <i>Myh10</i>∆/+ E14.5 heart. NMHC IIB is the predominant isoform, and shows intense staining in the epicardium (B, arrows). D-F: E14.5 embryonic lung tissue was used as a control for the NMII isoform immunohistochemical analysis (arrows). Cell nuclei are stained with DAPI (blue). G-I: Western blot analysis of NMII isoforms in E11.5 <i>Myh10</i>∆ heterozygous control and homozygous mutant whole heart protein extracts. Protein extract from adult mouse lung was run as a positive (+ve) control. NMIIA and IIB are found in relative abundance at this developmental stage in heterozygous hearts (G and H). As expected, NMIIB is absent from mutant samples (H, lane 3), which also show a reduction in NMIIA (G, lane 3). NMIIC is not detectable in either control or mutant samples (I). Protein extracts pooled from at least n = 3 hearts for each genotype. J-O: Immunocytochemistry for the predominant IIA and IIB isoforms in <i>Myh10</i>∆ heterozygote (J, K, M and N) and mutant (L, O) epicardial cell cultures from E11.5 heart explants. Note localisation of NMIIA to the periphery of epicardial cells (J, K, arrows), whilst NMIIB is localised throughout the whole cell body (M, N, asterisks), and is frequently associated with actin stress fibres (M, N, arrow). Consistent with western blot analysis (H), NMHC IIB is not detectable in mutant samples (O). Images representative of cell cultures obtained from four hearts per genotype. P: NMIIB immunofluorescence (green) in E11.5 control sample. NMIIB is expressed in the epicardium (arrow). Nuclei are labeled with DAPI (blue). Q: NMIIB expression in <i>Myh10</i>∆ homozygous mutant heart at E11.5. R: NMIIB immunofluorescence (green) at E14.5 in control sample. NMIIB is highly expressed in the epicardium (arrow). Nuclei are labeled with DAPI (blue). S: NMIIB expression in <i>Myh10</i>∆ homozygous mutant heart at E14.5. NMIIB is not detectable in <i>Myh10</i>∆ homozygous mutant samples, consistent with western blot (H) and cell culture results (O). Scale bars: A-F, J, L, M and O = 100 μm, K, N, P-S = 50 μm. Images in A-C and D-F obtained at same magnification. Abbreviations: ∆: <i>Myh10</i>∆.