Functionally abnormal stromal cells and megakaryocyte size, ploidy, and ultrastructure in Sl/Sld mice

The first goal of the present studies was to determine if Sl/Sld megakaryocytes have features in common with the macrocytic megakaryocytes that genetically normal mice produce in response to acute platelet depletion. The second was to test the hypothesis that megakaryocyte abnormalities in Sl/Sld mice are due to genetically determined hemopoietic stromal cell abnormalities. Sizes and ploidies of mature Sl/Sld megakaryocytes were measured. Macrocytosis and a shift to higher ploidy values were found compared with normal. Within ploidy groups 16N-64N, Sl/Sld megakaryocytes were larger than normal megakaryocytes of the same ploidy. Transmission electron microscopy revealed that Sl/Sld megakaryocyte nuclei contain more and larger nucleoli, and the chromatin was more dispersed than in normal megakaryocyte nuclei of comparable maturity. Asynchronous megakaryocyte cytoplasmic maturation was found. Sl/Sld macrophages were also ultrastructurally abnormal. Megakaryocytic macrocytosis was reproduced in long-term bone marrow cultures in which the adherent layer was formed by Sl/Sld cells. It was the same if cultures were recharged with Sl/Sld or +/+ hemopoietic cells. Previously reported ambiguities in mixed cell cultures were avoided by recharging the adherent layers with only a million cells. These results were correlated with previously published observations. Sl/Sld megakaryocytes have features in common with megakaryocytes from acutely thrombocytopenic animals. One feature, macrocytosis, appears to be due to abnormal Sl/Sld stromal cells that are reproduced as adherent layer cells in long-term cultures. The responsible stromal cells in Sl/Sld mice may be counterparts of megakaryocytopoietic regulatory cells in the marrow stroma of normal animals.