Ultrastructure of circulating platelets from MPZenNeo (A) or MPZenTPO (B) reconstituted mice. db, dense body; er, endoplasmic reticulum; gly, glycogen. The figure emphasizes the abnormally large size of some platelets seen in MPZenTPO reconstituted mice. (C) Enlargement of a giant platelet showing a richness in glycogen and in endoplasmic reticulum. Platelet shape and ultrastructure did not reveal any signs of activation other than the rare occurrence of dense bodies in MPZenTPO platelets.