The early YS is the source of the first platelet forming cells in the early conceptus. (A) (i) Representative flow cytometry plots of the E9.5 YS, placenta, EP, and peripheral blood stained with markers for DPFCs: CD41 and CD42D. (ii) Quantification of absolute numbers of CD41^highCD42D⁺ DPFCs in the E9.5 conceptus, demonstrating a significant enrichment of cells in the YS and placenta (Plac). *P < .05, **P < .01 (n = 3 independent experiments, using 13 individual embryos). Bars represent standard deviation. (B) Representative 3-dimensional projection of a confocal z-stack showing the distribution of CD41 (red) and CD42D (green) coexpressing cells in the embryo proper (bar, 200 μm) and placenta (bar, 50 μm) at E9.5 (n = 9). VE-CADHERIN (VECAD) labels the vasculature (blue), and 4,6 diamidino-2-phenylindole (gray) was used to stain nuclei. (C) Representative plots from culture of 2 embryo equivalents of E8.5 (5-10 somite pairs) YS, placenta, and EP cell suspensions after 72 hours in proplatelet-forming conditions. Values represent the mean ± standard error of the mean (SEM) of CD41^highCD42D⁺ cells produced per embryo equivalent of input cells (n = 3).