Fig. 4. Emergence of TER119-expressing erythroid cells and CD41^bright cells in the E8.0-8.25 embryo. (A) A portion of the emerging blood island in an E8.0 embryo (1-somite-pair stage) with the first TER119-expressing cells. (B) Extent of TER119 expression in cells (note the presence of TER119^bright maternal erythrocytes). (C) Emergence of a new population of CD41^bright cells among the CD41^dim cells. (D) E8.25 (4 somite pairs) embryo that has been rendered planar with the anterior (ant) and posterior (post) regions, allantoic stalk (all), and somites (s) identified. Massive expansion in the TER119-expressing cells and extensive vasculogenesis with a network of capillary-like structures is evident as is the emergence of CD41^bright cell clusters (asterisks). White box indicates region depicted in E-H. (E) High magnification composite image of a blood island containing TER119- (red), Flk-1- (green), and CD41- (blue) expressing cells. Isolated channels are shown in F-H. A red arrow in E-H identifies a cell expressing TER119 (red) but not CD41 (blue). A blue arrow (EH) indicates a cell expressing only CD41 (blue). A yellow arrow indicates a cell that is expressing TER119 (red) and CD41 (blue). Of interest the CD41^bright cell in panel G (asterisk) also expresses Flk-1 (E,H). Scale bar: 100 µm (A-D) 33 µm (E-H). (I) A representative dot plot of sorted E8.25 whole embryos with quadrants color-coded to match arrows in E-H.