Prenatal exposure to low levels of carbon monoxide alters sciatic nerve myelination in rat offspring

Prenatal exposure to low concentrations of carbon monoxide (CO, 75 and 150 ppm from day 0 to day 20 of gestation), resulting in maternal blood HbCO concentrations equivalent to those maintained by human cigarette smokers, leads to subtle myelin alterations in the sciatic nerve of male rat off- spring. The rapid growth spurt in pup body weight was related to the period of maximal increase in myelin sheath thickness in both control and CO-exposed animals. A signiÞcant reduction in myelin sheath thickness of sciatic nerve Þbers, paralleled by changes in the frequency distribution, occurred in both 40- and 90-day-old rats exposed in utero to CO (75 and 150 ppm). Myelin deÞcit observed in 75 and 150 ppm CO-exposed animals showed up only after the major spurt in myelination but not early during development. The subtle myelin alterations observed in CO-exposed offspring were not ac- companied by changes in developmental pattern of axon diameters and did not result in a gross impair- ment of motor activity. These results suggest that the myelination process is selectively targeted by a prenatal exposure model simulating the CO exposure observed in human cigarette smokers.