Somite as a Morphological Reference for Staging and Axial Levels of Developing Structures in Mouse Embryos

Background: The embryonic staging system based on the number of days post coitum is not always adequate to describe the rapid changes that occur during early organogenesis, and appropriate morphological features that can be easily referred to are important in descriptive embryology. Aims: The present study aims to ascertain whether somites can be used to determine both the developmental stage of mouse embryos and the axial level of embryonic structures from 9.5 days post coitum onwards. Results: The somite number could be easily counted after cresyl violet staining from 9.5 to 12.5 days post coitum, and the first few somites started to de-epithelialize by 10.0 days post coitum. However, from 12.5 days post coitum onwards, somites became difficult to count because of the thick surface epithelium. Vital dye labelling followed by whole embryo culture demonstrated that somites, limb buds and various regions of the neural tube did not undergo any major positional shift in relation to each other. Conclusions: From 9.5 to 12.5 days post coitum, the ease of somite count and the minimal positional shift in somites, limb buds and the neural tube implicate that the somite can be a convenient morphological reference for both the embryonic stage and the axial level of developing structures including limb buds and different regions of the neural tube during this period.