Hypoglycemia: how little is too much for the embryo?

The effects of hypoglycemia on mammalian embryos undergoing neurulation (third to fourth week of human development) were investigated. Mouse embryos were maintained for 28 hours in whole embryo culture in serum collected from rats that had received 50 units of 100 United States Pharmacopeia insulin units per milliliter. Glucose concentrations used were 40, 60, 80, and 147 mg/dl (normal blood glucose in the pregnant mouse is 125 mg/dl). After the culture period embryos were evaluated for malformations and growth and compared with those maintained under euglycemic conditions. The results demonstrate that glucose concentrations approximately 50% of normal maternal levels were teratogenic but not growth inhibitory, whereas concentrations in the range of 30% to 40% of maternal levels were lethal to the embryo. Furthermore, a 14-hour exposure to reduced blood sugar in either the first or second half of the culture period produced malformations.