Ultrastructural analysis of MK-801–treated megakaryocytes. Ultrastructural characteristics of CD34⁺-derived megakaryocytes cultured for 14 days with 25 ng/mL TPO only (A), or TPO and 50 μM MK-801 (B). (A) TPO-only treated cells contained the expected characteristics of mature megakaryocytes: an indented multilobed nucleus (N), α-granule formation (arrows), dilated demarcation membrane system (arrowheads), and proplatelet formation (*). (B) However, cells treated with TPO and MK-801 had a rounded nucleus (N), lacked α-granules, demarcated membrane, and proplatelets, and also exhibited large cyoplasmic cisternae (*). Note different scales.