Abstract:

To investigate the morphology and cellular composition of erythroblastic island （EBI） in the bone marrow and spleen of mice under physiological and stress conditions， and to elucidate the microstructural basis of extramedullary hematopoiesis （EMH）. Healthy adult C57BL/6J mice were assigned to a saline-treated control group and a phenylhydrazine （PHZ）-induced acute hemolytic anemia group. EBI from the bone marrow and spleen were enriched via density gradient sedimentation. Their cellular composition was analyzed by flow cytometry， and their morphology and abundance were assessed using imaging flow cytometry. The results showed that EBI primarily consisted of erythroid precursor cells， macrophages， and granulocytic/monocytic cells， with occasional lymphocyte association. No significant differences were observed in bone marrow EBI morphology or in the proportions of erythroid cells at different stages between physiological and hemolytic conditions. Under physiological conditions， the spleen of adult mice contained only a small number of EBI； following acute hemolytic stress， spleen weight increased by more than twofold， the number of EBIs increased markedly， and densely distributed "rosette-like" structures were observed， yet the proportions of erythroid cells at different stages within the islands remained stable， with proerythroblasts， basophilic erythroblasts， polychromatic erythroblasts， and reticulocytes exhibiting distribution patterns consistent with those in the bone marrow. These findings demonstrated that the spleen compensates for increased erythrocyte production through quantitative expansion of EBI， while erythroid precursors within the islands maintain their intrinsic differentiation program， thereby providing experimental evidence for understanding the cellular mechanisms of EMH.

Key words: erythropoiesis, erythroblastic island, spleen, bone marrow, stress hematopoiesis