Every developing heart is subtly different. Hearts and their blood vessels don’t always grow at the same rate, to exactly the same size or precise shape, and this can complicate things if we want to identify an abnormality. To be sure if any feature of a heart is abnormal, first we need to understand the range of differences that we might see in normal hearts as they grow.
Surprisingly, this is a much-understudied area – something that it has only recently been possible to determine using modern imaging techniques. A new article published in Journal of Anatomy  uses high resolution 3D imaging to study more than 200 genetically normal mouse embryos from the DMDD programme, identifying the typical range and occurrence of different variations in the heart’s development. The image below shows the hearts of two genetically normal mouse embryos that were determined to be at exactly the same stage of development. However, one feature of their hearts that is very different is the extent to which the ventricular septum has grown to separate what was initially a single cavity into the right and left ventricles. The heart on the left has only a very small gap left in the developing septum, while the heart on the right has a much larger gap. Without this sort of study we wouldn’t be able to tell whether the heart on the right is normal or whether it has a ventricular septal defect – the most common congenital heart defect in newborns.
The new data will be a valuable reference when identifying phenotypes in the heart and vessels of mouse embryos around the 15th day of gestation.
 Morphology, topology and dimensions of the heart and arteries of genetically normal and mutant mouse embryos at stages S21-S23, S. H. Geyer et al., J. Anat (2017), doi: 10.1111/joa.12663