Increased extramedullary splenic megakaryocytopoieis in PECAM-1 null spleens. (A,C) Low-power (5×) hematoxylin and eosin analyses of WT and CD31KO spleen sections. Circled are megakaryocytes identified by their morphologies. (B) Higher-power (20×) magnification of panel A. (D) Higher power (20×) magnifications of panel C. (E) The percentage of splenic megakaryocytic CD41a⁺ cells identified by FACS analyses was found to be increased in CD31KO spleens compared with that found in Het and WT littermate spleens (n = 2). (F) Splenectomies on WT and CD31KO mice were performed, and peripheral platelet counts were determined over a 70-day period (WT, n = 4; CD31KO, n = 3). (G) Representative bleeding time assay of age-matched splenectomized CD31KO (top panel) and WT mice (bottom panel) illustrating the increased volume of blood lost over time in the CD31KO animals. Numbers 1 through 5 denote consecutive 30-second intervals following tail clip starting at time 0 (arrowheads). Vertical lines in the bar graphs represent standard deviations.