Expression of GATA-1 and globin during zebrafish embryogenesis. In situ hybridization was performed with fixed zebrafish embryos at the following stages: (A and B) Two somites. (C and D) Five somites. (E and F) Eighteen somites. (G and H) Twenty-three hours (before prim 5 stage). (I) Twenty-four hours (prim 5). (J) Twenty-five hours. (K) Twenty-six hours. (L-P) Twenty-six to 29 h. (B, D, F, and H) Cross-sections through the posterior region or tail bud (IM) of embryos corresponding to (A, C, E, and G). For each section, e denotes the embryo and y indicates the yolk. The arrows in B show GATA-1-expressing progenitors that reside in contact with the yolk syncytial layer. The closed arrow in G indicates the posterior boundary of GATA-1 expression in the IM. The open arrow in G points to undifferentiated, GATA-2-expressing progenitors in the posterior IM (see Fig. 3E). The arrows in J and K show hematopoietic progenitors migrating anteriorly and onto the yolk sac. Erythroid cells later enter the heart (P, arrow). [M, 0, and P are normal sibs from a cross of two spadetail (31) heterozygotes.] (Q and R) o-Dianisidine staining of 24-h wild-type zebrafish embryos. The brown color indicates erythroid cells containing hemoglobin in the IM region.

Expression of GATA-2 mRNA in zebrafish. (A) Seventyfive percent epiboly. Open arrows represent the border of the embryo, and closed black arrows indicate expression in ventral ectoderm. (B and C) Ninety percent epiboly. GATA-2 is expressed within the advancing yolk syncytial layer, with higher levels at the embryonic boundary (white arrows). (D) Five somites. A "baseball" pattern of expression encompasses the anterior (solid arrow) and posterior (open arrow) regions of the embryo. (E) Twenty somites. Note the expression throughout the IM, including the posterior undifferentiated progenitors (open arrow). The solid arrow indicates the posterior boundary of GATA-1 expression in the IM.