Ultrastructural changes in 9-day old mouse embryos following maternal tobacco smoke inhalation

Mice were exposed to tobacco smoke inhalation for three ten-minute episodes on days 6, 7 and 8 of pregnancy. The effects of a higher tar cigarette (tar 12.9 mg, nicotine 1.19 mg, carbon monoxide 9.01 mg/cigarette) and a modified brand (4.8, 0.54 and 4.03 mg/cigarette respectively) were compared. Specific cells of the embryos were examined by scanning and transmission electron microscopy on day 9, 20 hours after the last smoking episode. Cells of the neural plate, surface ectoderm, pericardium and heart all showed marked surface changes with the higher tar cigarettes which suggested depressed metabolic activity. The changes were sometimes less marked with lower tar cigarettes but not in all cell types. Transmission electron microscopy of the neural tube also showed that significant changes were associated with the higher tar cigarettes, particularly of the mitochondria which became elongate and the cristae more common and less distinct. These findings suggest that maternal smoking may cause anoxia in the embryo, and that the cellular changes this produces persist even 20 hours after smoking has ceased. However, cell counts of sections of the closed neural tube showed no change in the total cell number or number of dead cells or alteration in the mitotic index with either type of cigarette. Cigarette modification does reduce to some extent the detrimental effects of maternal smoking observed in the embryo, but it is by no means all-embracing.