Effect of prolonged hypobaric hypoxia during postnatal development on myelination of the corpus callosum in rats.

Infant rats, together with the female, were exposed to a simulated altitude of 7,000 m every day from birth to the age of 17 days, excepting the 6th, 7th, 13th and 14th day. Animals were studied on the 18th day, 20 hours after the last exposure to hypoxia. Rats aged 12, 15 and 18 days acted as the controls. The number of myelinated axones of the corpus callosum rose from the 12th to the 18th day; the number in animals exposed to hypoxia did not differ significantly from the number in the 15-day-old controls. Axonal section area likewise increased from the 12th to the 18th day and again the value in animals exposed to hypoxia was no different from section area in 15-day-old animals. The number of myelin lamellae rose from the 12th to the 18th day, but in animals exposed to hypoxia it did not differ significantly from the value found in 12-day-old animals. The thickness of the myelin layer increased from the 12th to the 18th day; in the experimental animals it was the same as in the 12-day-old controls. The myelin lamellae likewise became thicker, between the 12th and the 15th day; in animals exposed to hypoxia this parameter was the same as the value in 12-day-old rats. In the electrophysiological part of the study, the development of the interhemispheric response was analysed. In animals exposed to hypoxia, the individual waves of the IHR had the same shape and latent periods as in 15-day-old animals. The results demonstrate that hypoxia has a profound effect on morphological and functional maturation of the corpus callosum in infant rats.