Splenomegaly and extramedullary erythropoiesis in tg mice. (A) The spleens from a wt and a tg mouse are shown in the photograph. The wet weights of the spleens are depicted in the histogram. Wt versus tg: 87 ± 18.5 mg vs 407 ± 43 mg (n = 7 in each group; P < .001). (B) Exemplary sections obtained from wt (left images) and tg spleens (right images) were stained either with hematoxylin-eosin (HE; original magnification, × 63) or with the erythropoiesis-associated antibody ER-HR3 (red; original magnification, × 400). The proportion of the red pulp as well as the density of ER-HR3–positive cells is largely increased in tg mice, indicating, together with the 5-fold higher wet weight of the tg spleen, the massive extramedullary erythropoesis in tg mice.