Representative MKs. Representative MKs in consecutive semithin (A,C) and ultrathin (B,D) spleen sections from wild-type (A-B) and GATA-1^low (C-D) mice. The arrow in panel A indicates an isolated MK in the semithin section from a wild-type spleen further analyzed by TEM in panel B. The ultrastructural morphology (cytoplasm rich in α granules and platelet territories, PF) characterizes this cell as a stage III MK. The arrow and arrowhead in panel C indicate MKs within a cluster in a semithin spleen section from GATA-1^low mice further analyzed by TEM in panel D. Two populations of mutant MKs were identified by TEM, one with normal (arrow) and the other with high (arrowhead) electron density of the nucleus (nu) and cytoplasm. The M in panel C indicates a mitotic MK. The asterisk in panel D indicates structures with plateletlike morphology. Similar observations were obtained with spleens and marrows from 2 additional wild-type and GATA-1^low littermates. Original magnifications × 100 (A,C) and × 3000 (B,D).