Extramedullary Hemopoiesis, Spleen, Rat

The normal spleen is usually a tongue-shaped organ with well-defined edges. When extramedullary hemopoiesis is present, the spleen is enlarged two or more times, but still in its normal anatomic position, and darker than normal. The splenic margins are rounded, and the surface is smooth and regular. Seen from the cut surface, the firm red pulp is prominent in comparison with the inconspicuous white pulp (Fig. 248).