Alterations in the levels of stem cells (CFU-s) and plaque-forming cells (PFC) in mice during chronic phenylhydrazine-induced hemolytic anemia

Strategies of hemopoietic stress adaptation within the medullary cavity

Gravimetric determination of total bone water space was used as an index of available bone marrow space in mice following various specific stressors, i.e., splenectomy, hypoxia, bone fracture, and estrone-induced osteosclerosis. Data was corrected for bone weight and was reported as specific bone marrow volume (total bone water space/mg dry bone X 100). A direct relationship was observed between specific bone marrow volume and medullary hemopoietic activity induced by stress. Absolute and/or relative marrow space increased following splenectomy, hypoxia, and fracture. Osteosclerotic animals shift most hemopoietic activity from marrow to spleen, and splenectomized osteosclerotic animals become anemic. Both intact and splenectomized hypoxic animals develop increased specific bone marrow volume and successfully compensate for hypoxia with enhanced erythropoiesis. Animals sustaining a fracture callus increase both specific bone marrow volume and hemopoietic activity at the callus without an increase in hemopoietic demand. Increased specific bone marrow volume extends the marrow bone interface, where primitive stem cells accumulate, while expanding marrow stromal space, where stem cells lodge, proliferate and differentiate. Therefore, it would appear that availability of competent marrow space may play an integral part in passively permitting hematopoiesis and in determining hemopoietic reserve capacity. Stem cell migration increases during intensified hemopoietic demand, which also may be related to available marrow space. Mice have a low medullary hemopoietic reserve capacity; subsequently, when available medullary hematopoietic stroma becomes occupied, stem cells are more likely to migrate from the marrow to extra-medullary sites where they mature before entering the circulating pool.

Splenic plaque-forming cells (PFC) and stem cells (CFU-s) during acute phenylhydrazine-induced enhanced erythropoiesis

The erythroid status and levels of splenic plaque-forming cells (PFC) to sheep red blood cells (SRBC) were monitored in mice subsequent to acute phenylhydrazine (PHZ)-induced hemolytic anemia. From ferrokinetic measurements, we noted a shift in erythropoiesis from bone marrow to spleen. The levels of splenic PFC were significantly depressed following PHZ-induced erythroid differentiation. Although this immune depression may reflect competition at the stem cell levels, whereby pluripotent stem cells (CFU-s) are preferentially differentiated into the erythroid line at the expense of lymphopoietic pathways, other possibilities cannot be excluded. In this regard, we have shown that loading of the mononuclear phagocyte system (MPS) by PHZ-damaged erythrocytes effected profound depressions in splenic PFC numbers. Lastly, in addition to the well-documented increases in CFU-s migration from marrow to spleen during enhanced erythropoiesis, we noted increased migration of B lymphocytes (as assessed by PFC) in marrow-shielded lethally-irradiated mice given PHZ. We also provide data which show that PHZ-damaged RBC evoke increased migration of CFU-s in normal mice, indicating a possible involvement of the MPS in stem cell migration.