Morphogenesis and quantification of the development of post-implantation mouse embryos

This paper describes potential improvements in the quantitative assessment of the differentiation of rodent embryos used for in vitro embryotoxicity studies. A chart of schematic illustrations of the developmental stages of seventeen morphological features observed macroscopically in mouse embryos aged 8-10 days (0-30 somites) has been drawn up. The chart is based on the morphological scoring system proposed by Brown and Fabro (1981) for rat embryos and complements the original descriptions. Some intermediate stages have been added to the scoring system. The original and the modified scoring systems were applied to 310 mouse embryos in 31 groups of ten embryos, each with 0 to 30 somites. The modified score is consistently about 25% higher than the original score. The correlation of both the original and the modified scores with the number of somites is best expressed by an asymmetric sigmoid. The chart and the modified scoring system could also be used, with minor adaptations, to assess rat embryos at corresponding developmental stages.