The aim of this study was to evaluate the ultrastructural features of human megakaryocytes cultured in vitro. For this purpose, pluripotent CD34⁺ (cluster of differentiation 34) hematopoietic progenitor cells, obtained from the peripheral blood of healthy adult donors, were differentiated along the megakaryocytic lineage in liquid cultures by the addition of the megakaryocyte‐specific growth factor thrombopoietin (TPO, 100 ng/ml). After only 6–8 days, virtually all of the CD34‐derived cells expressed the early megakaryocytic CD61 antigen, while, after 15–16 days, most cells also expressed the late megakaryocytic CD42a antigen. Ultrastructural analysis of cells obtained after 7 days of culture showed aspects typical of developing megakaryocytes (MK), such as formation of platelet territories and cytoplasmic fragmentation. At later (15–16 day) culture times, two distinct cell populations were observed: fully developed megakaryocytes releasing platelets into the culture medium and senescent megakaryocytes, characterized by morphological features of apoptosis. Analysis of DNA fragmentation in these cells revealed that apoptosis in megakaryocytes occurred in the absence of the internucleosomic cleavage, which is characteristic of most, but not all, types of apoptosis in cells of hematopoietic origin. On the other hand, flow cytometry of the DNA content of senescent megakaryocytes showed a subdiploid peak that was likely due to a loss of micronuclei during processing. Anat Rec 258:90–99, 2000. © 2000 Wiley‐Liss, Inc.