The action of low-intensity pulsed ultrasound on amphibian embryonic tissues

Amphibian embryos at different stages of development, and ectomesodermal explants (small pieces of embryonic tissue extirpated from lateral parts of the embryos just after neurulation), have been irradiated with continuous and pulsed ultrasound at a frequency of 0.88 MHz, spatial and temporal average intensities being within the range 0.025-0.1 W cm2. Ultrasound pulse repetition frequencies were varied within 10-1000 Hz, duty factor being constant and equal to 0.5. Destruction of embryonic tissue as an immediate effect, and interruption or distortion of development as delayed (24 hr) effects, were registered. Significant damaging effects of non-cavitational and non-thermal origin and dependent on pulse repetition frequencies, were observed. Highly frequency-dependent resonance-type effects were obtained for irradiated explants. The maximum effect on Rana temporaria explants was obtained at pulse repetition frequencies of 10-20 Hz and on Xenopus laevis explants at 110-130 Hz.