INSULIN-LIKE GROWTH FACTOR-I INCREASES BRAIN GROWTH AND CENTRAL-NERVOUS-SYSTEM MYELINATION IN TRANSGENIC MICE

Insulin-like growth factor I (IGF-I) is a potent regulator of oligoden drocyte development and myelination in vitro, but its effect on myelin ation in vivo has never been tested directly. Therefore, we examined b rain growth and myelination in a transgenic mouse line that overexpres ses IGF-I. By postnatal day 55, when brain growth and myelination are essentially complete in normal mice, the brains of transgenic mice wer e 55% larger than those of controls owing to an increase in cell size and apparently in cell number. Most or all brain structures appeared t o be affected. At the same time, total myelin content of the transgeni c mice was 130% greater than that of controls. Oligodendrocyte number as a percentage of total cell number was not increased in the transgen ic mouse brains; the increase in myelin content was primarily the resu lt of an increase in myelin production per oligodendrocyte. These find ings indicate that IGF-I is a potent inducer of brain growth and myeli nation in vivo.