Effects of boric acid on axial skeletal development in rats

Prenatal exposure to elevated levels of boric acid (BA) causes reduced inci dences of supernumerary ribs and shortening/absence of the 13th rib in mult iple laboratory species. To explore this further, Sprague-Dawley rats recei ved 500 mg/kg b.i.d. on gestation days (gd) 5-9, 6-9, 6-10, or on single da ys between gd 6 and 11 (plug day = gd 0); gd-21 fetuses were stained for sk eletal examination. Following multiday exposures, malformations of the axia l skeleton involved the head, sternum, ribs, and vertebrae. Shortening/abse nce of the 13th rib was seen particularly in the gd 5-9 and 6-10 exposure g roups. Although most groups exposed on single days were generally unaffecte d, about 90% of the gd-9 exposed fetuses had only six cervical vertebrae; t he deficient region was usually C3-C5. In contrast, gd-10 treatment caused agenesis of a thoracic/lumbar vertebra in over 60% of the fetuses; the defi cient region was usually T11. For 13-ribbed fetuses, the length of rib 13 w as shortened compared to controls. Postnatal assessment suggested increased mortality for gd-10 exposed pups. Embryos in culture showed reduced develo pment when exposed to BA for 48 h. These findings demonstrate the critical periods for axial development in the rat and provide an experimental model for the study of homeotic shifts.