Romiplostim produced a dose-dependent increase in bone marrow fibrosis in rats. Photomicrograph of hematoxylin and eosin–stained bone marrow sections (original magnification ×20) of rat femurs from placebo-treated animals (Placebo Week 5), from animals treated for 4 weeks with romiplostim (Romiplostim 100 μg/kg Week 5), and from animals treated for 4 weeks with romiplostim followed by 4 weeks without treatment (Romiplostim 100 μg/kg Week 9). Areas of increased collagen fibrosis (arrows) and increased megakaryocytes (arrowheads) are indicated.