TIME-DEPENDENT DIFFERENCES IN THE DEVELOPMENT OF SOMITES OF 4 DIFFERENT MOUSE STRAINS

In studies on reproductive toxicity and especially teratogenicity, ani mals are often treated at defined stages of pregnancy. As a result the exposure to a certain teratogen can lead to striking differences in t he formation of abnormalities in different strains of the same species . As a contribution to the discussion about the reasons for these diff erences, we examined the somite development of four different strains of mice during organogenesis. The embryos of pregnant females of the i nbred strains DBA/2J, BALB/cJ, and C57BL/6J and of the outbred strain Han:NMRI were studied on days 9, 10, 11, or 12 of gestation. As a crit erion for development the somite pairs were evaluated on the respectiv e days. There were remarkable differences in the somite number, even w ithin one litter. The largest variation (minimum vs. maximum) was 14 p airs of somites. The regression curves did not exhibit major differenc es in the speed of somite development from day 9 to day 12 between the four strains. We have to conclude from our results that the individua l embryonic stages within one litter may vary by nearly 1 day, and tha t there may be a delay of half a day in the embryonic development betw een different strains of mice. (C) 1993 Wiley-Liss, Inc.