Reexamination of the role of hematopoietic organs on the hematopoiesis in the silkworm, Bombyx mori

Larval hematopoietic organs (HPO) are thought as the only source of circulating hemocytes in most insects. In this paper, we re-checked the importance of hematopoietic organs to hematopoiesis in the silkworm through surgical operation to remove the organs from silkworm larvae at 12 h after 5(th) ecdysis. We observed that there was no significant decrease of hemocyte density but higher ratio of cell division in the HPO-removed wandering larvae. We checked and compared the total hemocytes in circulation and in 4 hematopoietic organs of each larva and found that even we suppose all hemocytes could be released from 4 organs at one time, it could not meet the circulating hemocytes increase in vivo due to huge difference. In order to monitor hemocytes movement in the hematopoietic organs to get information on hemocytes releasing in vivo, we labeled the dividing hemocytes with 5-bromo-2'-deoxyuridine ( BrdU) at 12 h after 5th ecdysis and observed BrdU-positive cells in the organs for several days. Our results show that the BrdU-labeled hemocytes were not released as quickly as we thought because there were still many BrdU-positive cells in the wandering organs and some cells even had almost no changed BrdU labeling. Therefore, the silkworm larvae have a novel hematopoiesis because circulating hemocyte division might contribute huge part to the hematopoiesis.