The present work studied the induction of
cleft palate formation in embryos developed from
pregnant BALB/c mice treated orally with retinoic acid
(RA). Previous studies on mature somatic cell types
showed that RA exerted inhibitory effects on inducible
nitric oxide synthase (iNOS) production. For the first
time, our study has shown that RA actually stimulates
significant expression of iNOS at specific zones of the
affected embryonic palatal tissues at three consecutive
stages, from gestation day 13 (GD13) to day 16 (GD16).
Enzymatically, iNOS facilitates intracellular nitric oxide
(NO) synthesis from L-arginine. When NO reacts with
reactive superoxides it may result in irreparable cell
injury. NO was also reported to induce apoptosis in some
mammalian cell systems. Based on our findings, we
propose that such an increase in NO production might be
associated with apoptosis in the embryonic palatal
tissues in the RA-treated mice. The detrimental effects of
NO resulted in a reduction in proliferating palatal cells
and therefore disturbed the normal plasticity of the
palatal shelves. With iNOS overexpression, our findings
also showed that there was significant concomitant
down-regulation in the expressions of Bone
Morphogenetic Proteins (BMPs) –2, 4, and 7 with
regional variations particularly in the palatal
mesenchymal cells for those embryos developing cleft
palate. Since specific spatial and temporal expressions of BMPs –2, 4, and 7 are critical during normal palatal
morphogenesis, any deficiency in the epithelialmesenchymal interaction may result in retarding growth
at the embryonic palatal shelves. Taken together, our
study has demonstrated cleft palate formation in the
BALB/c embryos involved overexpression of iNOS and
down-regulation of BMPs- 2, 4 and 7