Increased apoptosis in Stat5a^−/−5b^−/−neonatal spleens.

(A) Hematoxylin and eosin-stained sections of neonatal spleen from either wild-type or Stat5a^−/−5b^−/− mice. Original magnification at the time of photography was × 250. (B) Flow cytometry density plots of wild-type and Stat5a^−/−5b^−/−neonatal spleen cells simultaneously labeled for Ter119, CD71, and annexin V binding. Regions II to IV are indicated and correspond to the analysis of annexin V binding on these cells shown in panel C. Ter119 expression was variably reduced in Stat5a^−/−5b^−/− spleens. In the representative plots shown, the geometric mean fluorescence for Ter119 in the Ter119-positive populations is 73 and 68 for wild-type spleens 1 and 2, and 14 and 50 for Stat5a^−/−5b^−/−spleens 1 and 2, respectively. (C) Annexin V binding of Ter119^high erythroblasts in regions II to IV for the spleen cells analyzed in panel B. Inset shows an example of annexin V binding histogram for region II. Cells within the marker M1′ were considered annexin V–postiive.