Abstract
A system for the accurate and rapid measurement of the ploidy of cultured megakaryocytes derived from megakaryocytic colony-forming cells (CFU-M) has been developed. Thirty thousand murine marrow cells per milliliter were cultured for varying time periods in agar in the presence of horse serum and pokeweed mitogen-stimulated spleen cell- conditioned medium (PWM-SCM). To ensure the inclusion of all the megakaryocytic cells in the analysis, entire agar discs were transferred onto glass slides and dried. Cells of the megakaryocytic lineage were identified by staining for acetylcholinesterase (AchE) for two hours. Subsequently, the nuclei of the cells were stained using 1.7 X 10(-5) mol/L chromomycin A3, a specific DNA-binding fluorochrome. Megakaryocytic colonies (greater than or equal to 2 AchE+ cells) were located under transmission light. The fluorescence emission of each cell of the colony was then measured by a photometer interfaced with a computer. The mean fluorescence emission of about 20 random granulocytes per slide was used as a 2N standard. There was no significant cell loss, quenching of fluorescence by AchE staining, or overlapping of colonies or cells. Approximately 100 megakaryocytes per hour could be analyzed. Modal ploidy of cultured megakaryocytes increased from 2N to 32N between days 3 and 6 in culture. Varying concentrations of PWM-SCM from 5% to 20% did not affect the ploidy distribution when examined at day 5. The heterogeneity of the ploidy of cells within colonies increased continuously with increasing cell numbers per colony. Clonal analyses of mean ploidy and ploidy heterogeneity did not show distinct types or classes of colonies; rather, the data show that megakaryocytic colonies are structured as a continuum. An inverse correlation was found between the number of cells constituting the colonies and their mean DNA content. To determine if short-term in vivo exposure of CFU-M to a thrombocytopenic environment could affect the ploidy of their progeny, mice were given rabbit antimouse-platelet serum while control animals were given normal rabbit serum. Twenty-four hours after injection, marrow derived from these animals was cultured. At day 5, the ploidy distributions and ploidy heterogeneity were identical in both treated and control groups. Thus, factor(s) that promote CFU-M proliferation do not affect megakaryocytic endoreduplication, while stimuli that acutely influence megakaryocytic ploidy in vivo do not determine the ultimate ploidy potential of megakaryocytes derived from a CFU-M.
