Why Does Hemocyte Density Rise during the Wandering Stage of the Silkworm, Bombyx mori?

抄録

Hemocyte density in the hemolymph changes drastically during larval development of the silkworm, Bombyx mori. Hemocyte density increases abruptly at the wandering stage and subsequently decreases during the spinning and prepupal stages. This pattern is also observed in larvae whose hemopoietic organs (HPOs) had been selectively subjected to heavy-ion irradiation, and in larvae whose entire HPOs had been extirpated surgically at the pre-molting stage during the 4th instar. These results and the relatively small number of hemocytes present in HPOs during the final 5th instar indicate that the HPOs do not play an important role in increasing the number of hemocytes in circulation, at least during the wandering stage. Application of juvenile hormone analog (JHA) called methoprene, during the early 5th instar, inhibited the sharp increase of the hemocyte density, while injecting 5th instar day 2 larvae with a low dose of 20-OH ecdysone brought about a slight but significant increase in hemocyte density. Tracking bromodeoxyuridine (BrdU) incorporated hemocytes reveals that the percentage of labeled hemocytes was the highest (26%) during the mid-instar (day 3) in the control larvae, but highest during the wandering stage in both HPOs-extirpated larvae (40%) and heavy-ion-irradiated larvae (37%). These results suggest that juvenile hormone and ecdysone are involved in the change of hemocyte density, and that mitosis of circulating hemocytes plays an essential role for elevation of hemocyte density during the wandering stage.