Corticosterone promotes myofibril maturation in foetal cardiomyocytes in a GR-dependent manner. ( a ) Representative images of control (untreated) primary foetal C57BL/6 cardiomyocytes (left panels), or following 100 nM corticosterone (cort) for 24 h (centre panels). To block GR-mediated effects, cells were pre-treated with 1 μ M RU38486 (RU486) for 30 min prior to addition of 100 nM corticosterone for 24 h (RU486+cort; right panels). Cells were stained with α -actinin (red, Z-discs) and DAPI (blue, nuclei). ( b ) Quantification of the effect of 100 nM cort on myofibrillar structure (top graph, see Materials and Methods for details of the scoring system), sarcomere length (middle graph), and Z-structure/Z-disc width (bottom graph). Data were analysed by one-way ANOVA with Bonferroni's post-hoc test; ** P