Megakaryocytes in steel mutant mice.

The megakaryocytes in the bone marrow of steel mutant mice (S1/S1d) and their normal littermates (+/+) were studied by light and electron microscopy with special emphasis on their maturity and distribution in the hematopoietic cords. A higher percentage of megakaryocytes lying against the sinus wall, a higher percentage of the sinus perimeter covered by megakaryocytes and a higher percentage of large megakaryocytes were found in S1/S1d mice than in +/+ mice. In addition, more large megakaryocytes as well as senile megakaryocytes were observed in the spleen of S1/S1d mice than in that of +/+ mice. These observations suggest that more platelets are produced in S1/S1d mice than in +/+ mice on the basis of per unit area of the marrow tissue. Heretofore, the fate of the senile megakaryocytes in the marrow was not known. However, in S1/S1d mice senile megakaryocytes were often found entering the marrow sinuses from the hematopoietic cords. They were also seen in the lung and the spleen where degradation of senile megakaryocytes was observed. These observations suggest that senile megakaryocytes in S1/S1d mice leave the marrow and are removed by the reticuloendothelial system outside the marrow.