9-cis-retinoic acid: a direct-acting dysmorphogen.

Experiments in vitro with cultured rat conceptuses demonstrated that 9-cis-retinoic acid (9-cis-RA) (300 ng/mL amniotic fluid) produced branchial arch and somite defects similar to those elicited by equal concentrations of all-trans-retinoic acid (all-trans-RA), but with an increase in cephalic defects that included missing optic vesicles. After conceptuses were intraamniotically microinjected with 600 ng 9-cis-RA/mL amniotic fluid on day 10 of gestation, an unusual heart defect was also observed. HPLC analyses indicated that 9-cis-RA readily underwent conversion to the less active metabolite, 13-cis-retinoic acid (13-cis-RA), in cultured conceptuses during the first 4 hr after treatment but only after 6 hr could elevated levels of the potent dysmorphogen all-trans-RA be detected. In separate experiments, conversion of 13-cis-RA or of all-trans-RA to 9-cis-RA could not be detected during a 6-hr embryo culture period. Endogenous levels of 9-cis-RA in whole rat embryos also were below limits of detection but small quantities of this isomer could be detected in neonatal rat eye and human embryonic brain. Our present study strongly suggests that 9-cis-RA is a direct-acting dysmorphogen with probable specific target sites of action.