Effects of Heavy-ion Irradiation on the Differentiation of Epidermal Cells in the Silkworm, Bombyx mori

The effect of heavy-ion irradiation on the differentiation of epidermal cells was examined in the silkworm, Bombyx mori. Local irradiation of fifth instar day 1 larvae with carbon-ions (400Gy) resulted in no marked morphological changes in the instar. The detection of few TUNEL-positive cells indicated that apoptosis of the epidermal cells did not occur in response to irradiation. However, the resultant moths were observed to loose their scales in the area irradiated in a dose dependent manner. Similar results were also obtained when newly hatched larvae were irradiated. Light- and scanning electron microscopy revealed that the pupal epidermal cells secreted adult cuticle, but that differentiation of scale and socket cells was completely inhibited at the site of irradiation. On the other hand, when effects of ion-beam irradiation were examined in an epidermal mutant strain (Knobbed: K), it was found that the paired protuberances (knobs) at the sites of larval markings disappeared in subsequent larval, pupal and adult stages when newly hatched larvae were locally irradiated. These findings suggest that knobs formed by the excessive proliferation of epidermal cells did not develop because cell division in these areas was suppressed by heavy-ion irradiation. It therefore appears that epidermal cells are considerably resistant to heavy-ion beams and that epidermal differentiation, such as that associated with scale and knob differentiation, are suppressed by the inhibition of cell division after irradiation.