Recent clinical observations strongly suggest that isotretinoin [13-cis-retinoic 
acid (cis RA)] is a human teratogen causing primarily heart and craniofacial 
malformations including ear and palatal defects. The purpose of the present 
study was to determine if cis RA could induce similar craniofacial malformations 
in mouse embryo culture. Day 8 CD-1 mouse embryos were cultured for 48 hours in 
rat serum in the presence or absence of various concentrations of cis RA 
dissolved in DMSO. DMSO by itself had no effect on embryonic development; 
however, cis RA at 2 X 10(-5) M (6 micrograms/ml) was clearly toxic. At 2 X 
10(-6) M cis RA, growth retardation was minimal, and approximately one-third of 
the embryos exhibited very specific defects including a dramatic reduction in 
the size of the first and second visceral arches, which eventually give rise to 
the maxilla, mandible, and ear. Similar observations were also made with 
4-oxo-13-cis RA, which is a major metabolite of cis RA in the mouse and human. 
These malformations would be expected to result in defects similar to those 
observed in the human, and preliminary observations suggest these defects are 
due to cis RA-induced inhibition of cranial neural crest cell migration. Using 
day-10 mouse embryos cultured for 48 hours in Waymouth's medium containing 50% 
fetal calf serum, we observed that cis RA at 2 X 10(-5) M produced a high 
percentage of embryos with limb defects and median cleft lip. Our results 
demonstrate that labeled cis RA enters the tissues of the embryo both in vivo 
and in vitro. Cis RA inhibited proliferation of the frontonasal mesenchyme cells 
in primary culture with 31% inhibition occurring at 2 X 10(-5) M cis RA.