CRITICAL PERIODS OF SENSITIVITY TO THE DEVELOPMENTAL TOXICITY OF INHALED METHANOL IN THE CD-1 MOUSE

Exposure of pregnant CD-1 mice to methanol (MeOH) by inhalation on ges tation days (gd) 6-15 results in dose-related increases in fetal cleft palate, exencephaly, and skeletal defects. Here, critical periods for the developmental toxicity of MeOH were assessed in pregnant CD-1 mic e exposed to 10,000 ppm MeOH or filtered air for 7 hr/day on 2 consecu tive days during gd 6-13, or to single day (7 hr) exposures to 10,000 ppm MeOH during gd 5-9. Mice received water but not food during exposu re. Maternal blood MeOH was determined at times during, at the end of, and subsequent to a single 7 hr exposure on gd 7. On gd 17, remaining mice were weighed, killed, and gravid uteri removed. Live, dead, and resorbed fetuses were counted, and live fetuses were examined, weighed , and preserved in 70% ethanol. All fetuses were examined externally a nd for cleft: palate, eviscerated, and stained with Alizarin red for s keletal examination. Pregnant mice lost an average of 0.3-2.9 g during 7 hr exposure to either filtered air or MeOH, but a MeOH treatment ef fect was evident only with 2-day exposure on gd 7-8. Peak maternal blo od MeOH concentration (at the end of exposure) was similar to 4 mg/ml, and MeOH was cleaved from maternal blood within 24 hr. Some fully res orbed litters were observed with 2-day MeOH exposures on gd 6-7 or 7-8 , or 1-day exposure an gd 7. With 1-day MeOH exposure on gd 7, the num ber live was lower than with exposure on any other day. As previously reported, cleft palate, exencephaly, and skeletal defects were the fet al anomalies observed in this mouse strain. Cleft palate occurred with 2-day exposures on gd 6-7 through gd 11-12 (peak on gd 7-8), and with 1-day exposure on gd 5 through gd 9 (peak on gd 7). Exencephaly occur red with 2-day exposures on gd 6-7 through gd 8-9 (peak gd 6-7) or 1-d ay exposure on gd 5 through gd 8 (peak on gd 7). Skeletal elements mal formed included the exoccipital (peak gd 6-7, gd 5), atlas (peak gd 6- 7, gd 5,6), axis (peak gd 6-7, gd 7), cervical vertebra 7 with a rib ( peak gd 6-7, gd 7), and lumbar vertebra 1 with a rib (peak gd 7-8, gd 7). An increased incidence of fetuses with 25 presacral vertebrae (nor mal = 26) was observed with methanol exposure on gd 5, whereas an incr eased incidence of fetuses with 27 presacral vertebrae was observed wi th MeOH exposure on gd 7. These results indicate that gastrulation and early organogenesis represent a period of increased embryonal sensiti vity to methanol. (C) 1997 Wiley-Liss, Inc.