Observation of Hematocytes in Peripheral Blood and Hematopoiesis in Monopterus albus

To elucidate hematopoietic ontogeny in Monopterus albus, systematic cytological analysis was performed using Wright–Giemsa-stained peripheral blood smears and hematopoietic organ imprints. Peripheral blood smears revealed five major cell types: erythrocytes (60.05 ± 8.08%), lymphocytes (13.50 ± 4.27%), thrombocytes (8.95 ± 3.47%), neutrophils (6.40 ± 2.17%), and monocytes (5.32 ± 1.63%), with occasional basophils and no eosinophils. Microscopic analysis revealed that M. albus hematopoiesis progresses through three distinct developmental phases—primitive, juvenile (with granulocytic series exhibiting promyelocyte/myelocyte/metamyelocyte substages), and mature stages—across erythroid, monocytic, lymphoid, and granulocytic lineages. The head kidney served as the primary hematopoietic organ, containing pan-stage cells and supporting all lineages, whereas the spleen lacked myeloblasts but participated in non-granulocytic hematopoiesis, and the liver was restricted to juvenile-stage erythrocyte, lymphocyte, and neutrophil development. Notably, erythrocytes in the peripheral blood with elongated nucleus were observed in the peripheral blood.