Extramedullary hematopoiesis in the spleen.

Spleens were harvested from wild-type and PU.1^+/−Spi-B^−/− mice, fixed in formalin, sectioned, and stained with hematoxylin and eosin. The presence of megakaryocytes under the capsule of the spleen is indicated by the black arrows. Occasional megakaryocytes are found in wild-type spleens, but the numbers are increased greatly in PU.1^+/−Spi-B^−/− spleens. (Original magnifications: panels A and B, × 20; panels C and D, × 40.)