The glycoprotein Ib/IX/V complex (GPIb-IX-V) mediates platelet attachment to von Willebrand factor in exposed subendothelium. Molecular defects in the genes for GPIbalpha, GPIbbeta, and GPIX give rise to the Bernard-Soulier syndrome, in which thrombocytopenia and giant platelets suggest that this receptor also is involved in platelet production. To study how giant platelets are produced in vivo, we used a model of GPIbalpha-deficient mice (GPIbalpha(null)) and mice rescued with the human GPIbalpha transgene (GPIbalpha(null;hTg)).Using electron microscopy and immunogold labeling, we examined megakaryocytopoiesis in the bone marrow of these mice and developed a method to quantify the membranes of megakaryocytes (MK) and proplatelets by computer analysis.Abnormal membrane development in the perinuclear zone was found in immature MK of GPIbalpha(null) mice. This led to a poorly developed demarcation membrane system and other ultrastructural changes. As a result, well-organized platelet territories were rarely seen within the cytoplasm of mature MK. Membrane quantification confirmed that MK of GPIbalpha(null) mice had a reduced internal membrane pool. Whereas these MK normally crossed the endothelial barrier, their migration was accompanied by the production of unusually large MK fragments or proplatelets in the vascular sinus with an approximately 50% decrease in internal membrane content compared to wild-type. In the rescued GPIbalpha(null;hTg) model, GPIbalpha was normally localized in MK, and there was a total correction of the ultrastructural defects.GPIbalpha is essential for membrane development and distribution in maturing MK. Its absence leads to abnormal partitioning of the membrane systems and abnormal proplatelet production.