Acute response of fetal rat telencephalon to ultrasound exposure in utero

Pregnant rats were exposed to ultrasound or microwaves from transducers located over one uterine horn. Ultrasound intensity (SPTA) was 0.78 W/cm2 for 30 min at a frequency of 2.5 MHz. Microwave exposure was used to reproduce approximately the rate of rise of uterine temperature of the rats exposed to ultrasound. The average peak temperature was 40.1 degrees C for ultrasound exposure and 42.2 degrees C for microwave exposure. On Gestational Day 16, 24 h after exposure, fetuses were removed and prepared for morphological examination of the developing cerebral cortical mantle. Morphometric measurements were made of nuclear area of subventricular zone cells, number of mitoses per mm in the ventricular zone, number of pyknotic cells in the mantle per mm, and number of ventricular macrophages per mm. Both exposures increased nuclear size and numbers of pyknotic cells and macrophages, and decreased the number of mitotic figures. The data from the four measurements were evidence of damage from ultrasound similar to the effects produced by microwave heating. The thermal effects of ultrasound, even at relatively low levels of rise in temperature, may have been the cause of the damage to the fetal cortex in these experiments or may have interacted with other effects of ultrasound energies to produce damage to developing neurons.