Human CD34⁺bone marrow cells were committed to the megakaryocytic lineage in serum-free liquid cultures by the following cytokines: thrombopoietin, erythropoietin, and IL-6. Megakaryocyte maturation has been described as being regulated by the extracellular matrix. These cells express receptors for laminin, collagen, and vitronectin, but they selectively adhere to and spread on fibronectin, a major component of the bone marrow environment. Function-perturbing antibodies against β1 integrins totally abolished the adhesion of megakaryocytes on fibronectin, whereas antibodies to β3 did not, suggesting that β1 integrins were responsible for the adhesive phenotype of these polyploid cells. β1-positive clusters were visualized in close contact with the extremities of stress fibers at the cell surface. In the course of cell spreading, we observed morphological modifications such as the disorganization of the compact nuclei structure and the appearance of holes in the cytoplasm leading to the release of αIIbβ3-positive cellular fragments. This process appeared to be a specific feature of megakaryocytes and is correlated neither to apoptosis nor to integrin signaling.