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Effects of Heavy Ion Irradiation on the Precursor
Hemocytes of the Silkworm, Bombyx mori
S. Kobayashi a) , K. Fukamoto a) , K. Kiguchi a) , T. Funayama b) Y. Yokota b) , T. Sakashita b) ,
Y. Kobayashi b) and K. Shirai a)
a) Division of Applied Biology, Faculty of Textile Science and Technology,
Shinshu University, b) Radiation-Applied Biology Division, QuBS, JAEA
When compared with bacteria, insects are evolutionarily
much closer to mammals. Thus the response systems against
irradiation of the insect cells should be similar to that of
mammalian cells. Nevertheless, like bacteria, insect cells are
generally known to be quite resistant to radiation compared
with mammalian cells. However, the detailed mechanisms
(or reasons) of this resistance of insect cells have not been
fully understood.
Heavy ion beams can deposit more energy in target organs
than X-rays or gamma rays, and the irradiation system of
heavy ion beams in QuBS make it possible to irradiate the
desired region of the small biological samples. Therefore,
this system is an extremely useful tool to inactivate specific
organs or tissues such as larval imaginal discs in insects.
We have been studying the effects of heavy ion irradiation
on the hemopoietic organs of Bombyx mori by using heavy
1)
ion beams . From the results obtained to date, we have
shown that the hemopoietic organs can be regenerated
following the radio-surgery, even after irradiation with
2)
100 Gy carbon ions .
For the regeneration of the irradiated hemopoietic organs,
the apoptotic cell death of damaged precursor hemocyte in
the hemopoietic organs is the first and an important step.
However, it has also been made apparent that the insect cells,
such as epidermal cells of B. mori or cultured cells (Sf9 cells),
are more resistant to the irradiation including the carbon ions.
No effect of carbon ion irradiation on the viability of these
cells was observed even at a dose of 400 Gy. Therefore, the
information obtained from studies on the induction
mechanisms of apoptosis in the precursor hemocytes in the
irradiated hemopoietic organs would contribute to the further
understanding of the radiation response in insect cells. In
this report, we have investigated the effects of heavy ion
irradiation for the precursor hemocyte.
The viability of the irradiated precursor hemocytes up to
4 days after the irradiation was studied using carbon ions.
Apoptotic cells were observed from 2 days to 4 days after
irradiation with 100 Gy. The number of detected apoptotic
cells had increased after irradiation, peaked at 2 days after
irradiation, and gradually decreased afterwards. Whereas at a
dose of 1 or 10 Gy, no cell death was induced by the heavy
ion irradiation.
DNA replication in the irradiated precursor cells was
examined using thymidine analog, BrdU. This analog is
incorporated into DNA chains of the cells that are just
duplicating the genome DNA. At a dose of 100 Gy, BrdU
labeled cells had decreased remarkably at 2 days after
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irradiation. No significant effect on incorporation of BrdU
was observed within the specimens irradiated with 1 or 10 Gy
(Fig. 1).
Finally, the developmental change of cell numbers in the
irradiated hemopoietic organs was evaluated and compared
with those of non-irradiated organs. When the hemopoietic
organs were irradiated with 100 Gy of carbon ions,
proliferation of the irradiated precursor hemocytes in the
organs had almost stopped and the number of cells was static
until 3 days after irradiation. Moreover radiation-induced
cellular hypertrophy was clearly observed in the cells at
3 days after irradiation. Whereas, at a dose of 1 or 10 Gy,
the number of cells in the organs had increased along with
larval development and no remarkable effect was observed.
Initially, it had seemed to us that precursor hemocytes
were not resistant to heavy ion beams. However, these
results indicate the responses to heavy ion irradiation of
precursor hemocyte are quite similar to those of other insect
cells, such as epidermal cells and Sf9 cells.
References
1) K. Kiguchi et al., Nucl. Instrum. Meth. Phys. Res. B210
(2003) 312.
2) E. Ling et al., J. Insect Biotechnol. Sericol. 72 (2003) 95.
Fig. 1 BrdU labeling of precursor hemocytes in the
irradiated hemopoietic organ.