Recent evidence has demonstrated that 13-cis-retinoic acid (13-cis-RA, or 
isotretinoin) is responsible for various craniofacial malformations in the 
rodent and human embryo. Our studies have been directed toward understanding 
this effect using mouse whole embryo and primary cell cultures. In whole embryo 
culture, 13-cis-RA caused significant overall embryonic growth retardation, 
especially in the primary and secondary palatal processes. In embryos explanted 
on day 10 of gestation and exposed for 24 or 48 hr, the mesenchyme beneath the 
epithelium of the nasal and maxillary processes contained pyknotic nuclei as 
well as a dramatically reduced number of nuclei incorporating 3H-thymidine. The 
secondary palatal processes and the roof of the oral-nasal cavity had fewer 
mesenchymal cells than control embryos. The incorporation of 3H-thymidine into 
TCA-insoluble macromolecules was 30% less in the retinoid-treated heads. In 
primary cell cultures from day-12 mouse secondary palatal mesenchyme, subsequent 
cell growth was decreased at concentrations of 13-cis-RA greater than 1 X 10(-5) 
M. After a 40-hr treatment period, labeling indices in retinoid-treated cells 
were significantly lower than control values (25% compared with 40%). Retinoic 
acid also caused a significant, concentration-dependent decrease in 3H-thymidine 
incorporation. The inhibitory effect of 13-cis-RA on proliferation of oral-nasal 
mesenchymal cells appears to be related to the production of craniofacial 
malformations.