Crustacean haemocytes play important roles in the host immune response incl
uding recognition, phagocytosis, melanization, cytotoxicity and cell-cell c
ommunication. Classification of the haemocyte types in decapod crustaceans
is based mainly on the presence of cytoplasmic granules into hyaline cells,
semigranular cells, and granular cells. Each cell type is active in defenc
e reactions, for example; in crayfish, the hyaline cells are chiefly involv
ed in phagocytosis, the semigranular cells are the cells active in encapsul
ation, while the granular cells participate in storage and release of the p
rophenoloxidase (proPO) system and cytotoxicity. The haematopoietic tissue
has been described in several crustacean decapod species and shown to be th
e haemocyte-producing organ. Tentative stem cells have been shown to be pre
sent in this tissue. Using in situ hybridization, we demonstrated that proP
O is not present in the haematopoietic tissue of crayfish which suggests th
at protein expression is different between circulating haemocytes and the c
ells in the haematopoietic tissue. (C) 2000 Elsevier Science B.V. All right
s reserved.