Localization of megakaryocytes (GP1bβ-positive cells) in mouse embryos. Individual megakaryocytes were first identified in the yolk sac of E9.5 conceptuses (arrows, top row, left panel). No GP1bβ-positive cells were detected in the E9.5 embryo proper (top row, middle panel). Maternally derived GP1bβ-positive platelets were detected in the surrounding E9.5 decidua (top, right panel). At E10.5 (middle row), clusters of GP1bβ-positive cells were evident in the yolk sac and intraembryonic megakaryocytes were seen in the circulation (arrowhead), associated with the wall of the aorta (arrows), and in the fetal liver (arrowhead). GP1bβ antibodies label both small (open arrowhead) and large cells in the E11.5 to E12.5 fetal liver, and predominantly large cells in the E13.5 to E14.5 fetal liver (lower row). Platelets are seen in E11.5 to 14.5 livers (arrows). Scale bars represent 50 μm.