Hemocytes and hematopoiesis in the silkworm, Bombyx mori

The silkworm, Bombyx mori, is a typical Lepidopteran insect. In the silkworm hemolymph, there are 5 types of circulating hemocytes that are classified as prohemocytes, granulocytes, plasmatocytes, spherulocytes and oenocytoids. All of them are involved in humoral and cellular immunity either directly or indirectly. Insect hematopoietic organs can produce hemocytes that are continuously released into the circulation. Recent studies indicate that in the hematopoietic organs of silkworm larvae, there are mainly prohemocytes and oenocytoids. Based on in vitro observations, silkworm prohemocytes can differentiate into plasmatocytes and granulocytes, and granulocytes can differentiate into spherulocytes. The silkworm also has a novel type of hematopoiesis. When its hematopoietic organs are extirpated through a surgical operation, circulating hemocytes can still remain at a high level through the wandering stage due to an increase in the level of cell division. Previously, oenocytoids have been considered as the only source of prophenoloxidase (PPO) which is an important immunity protein in insects. However, recent studies in different insect species, as well as in the silkworm, show that additional hemocyte types contain PPO. Furthermore, PPO can be produced by epidermal cells in the hindgut of the silkworm. Consequently, the silkworm is a valuable model to study hemocyte development and cellular and humoral immune responses.