Histology of the rat embryo cultivated in vitro (18–22 somites)

When rat embryos are grown in culture, the morphological scoring system of Brown & Fabro ( Teratology 1981, 24, 65–78) is usually employed at the end of culture to assess normal development and the effects of drugs and other chemicals. The morphological scoring system has proved very satisfactory, but its criteria only involves the external appearance of the embryo. It seems desirable to study also the internal anatomy of the embryo by histological methods. Thus, the object of this study was to gain data on the morphology of the main organ systems, with special attention given to the cardiovascular system during the developmental period most often employed in teratogenicity and embryotoxicity studies (day 9–10; 18–22-somites stage). The microscopic findings in this study agreed well with the morphological scoring system, with only one exception; whereas only two branchial arches were visible externally at the 18–22-somites stage, microscopy revealed that the third arch was also present.