S ummary . Megakaryocytes have been studied in rats using objective methods, with assessment of numbers of cells, cell diameter and DNA content. Stimulation and suppression of platelet production have been shown to be accompanied by changes in cell diameter, and in numbers of megakaryocytes in the bone marrow and spleen. A method is described permitting measurement of DNA content of the cells under these conditions, using Feulgen staining and an integrating micro‐densitometer. It has been demonstrated that alteration in size is secondary to alteration in ploidy value. The reduction in ploidy following transfusion of platelets was apparent morphologically as a decrease in the number of lobes in the nucleus, but following stimulation of platelet production, an increase in ploidy occurred which was not associated with a discernible increase in nuclear lobulation. Microdensitometry provides the only certain method of objective assessment of megakaryocyte ploidy. It is concluded that the system regulating megakaryocyte proliferation governs DNA replication in both megakaryoblast and stem cells.
Penington et al. (Wed,) studied this question.