Characterization of Embryonic Blood
(A–C) The rostral blood island (RBI) and intermediate cell mass (ICM) at 18 hr, visualized with (A) a myeloid marker, (B) a neutrophil marker, and (C) an erythroid marker. (D–F) Individual blood cells at 26 hr, visualized by in situ hybridization: (D) macrophage cells, (E) neutrophil cells, and (F) erythrocyte cells. (G–L) Individual blood cells in the live 30 hr embryo. Each image is a single frame from a real-time recording of circulating blood. Elapsed time is indicated in the lower frame, long arrows indicate the flow of circulation, and insets show a 200x-magnified view of specific cells. (G and H) Neutrophils (n) and circulating erythrocytes (e) in the (G) viteline vein over the yolk sac and in the (H) lumen of a tail blood sinus. The arrows in (G₁) indicate individual nuclear lobes of a neutrophil, and the small arrow in (H) indicates where the neutrophil anchors itself to the endothelial lumen. (I) A macrophage (m) patrolling the viteline vein. The last panel is magnified 200x. (J) A circulating thrombocyte (t and arrow). (K and L) Unidentified cells. (K) A large rare cell type (u), arrows in the inset indicate its diffuse nuclear envelope. A macrophage (m) is also visible in (K). (L) A dividing cell (arrow).

Lineage Tracing and Fate Map Analysis of the Embryonic Blood
(A–G′) (A–C and F–G) Progeny of a single mid-blastula- labeled cell. (A) The clone at 50% epiboly (face view), now one deep cell (arrow) and two EVLs (e) at the margin of the blastoderm; note that the single deep cell is being viewed through one of the EVL cells. (B) The clone at the shield stage (animal pole view), now two deep cells (arrow) and two EVLs (e) located 180° of arc from the dorsal midline (d). (C) The clone at the 13-somite stage (side view), now 11 deep cells (arrow) in the posterior IMC. The EVL portion of the clone (e), now periderm, is extraembryonic. (F and G) The clone at 36 hr, now endothelial cells and circulating blood, which include (F) neutrophils and (G) erythrocytes. (D and D′) Hematopoietic fate maps. Graphs depict the location of the same clones at two different stages of development. (D) 6 hr gastrula fate map, clones versus dorsoventral location. The presentation is a side view; dorsal is oriented toward the right, with clones on the right projected to the left. Each symbol is the average location of the deep cell portion of a clone relative to the margin (tier 0), in units of cell diameter, and the dorsal midline (0), in degrees of arc. The oversized symbol represents the clone shown in (A)–(C), and the small letters refer to the respective panel. (D′) 16 hr 14-somite stage fate map, clones versus primitive blood island. The horizontal line through each symbol shows the anteroposterior spread of each clone. All clones except one also included endothelial cells. (E) 24 hr hematopoietic fate map, fate of clone versus anteroposterior location (RBI, somite level within the ICM, or PBI). Each symbol represents a single clone at that anteroposterior location; however, clones that spread over several somite levels are represented with multiple symbols. The fate map is based on 27 macrophage clones, 41 erythrocyte clones, 16 neutrophil clones, and 12 thrombocyte clones. These clones include all that were later verified by in situ hybridization.
(H and I) Verification of in vivo blood morphology by one- or two-color in situ hybridization at 36 hr. (H) A multilineage erythrocyte, neutrophil, and thrombocyte clone (red fluorescence) that was visualized for coexpression of hbbe2, an erythrocyte-specific marker (green fluorescence), and mpx, a neutrophil-specific marker (NBT/BCIP purple substrate). Left panel, the white-light image shows two lineage-labeled neutrophils (arrows). Inset, same cells showing mpx expression alone. Right panel, the UV image shows two lineage-labeled erythrocytes (arrowheads) and a clonally related thrombocyte (hbbe2-negative; right of upper erythrocyte). The neutrophil portion of the clone is out of focus (arrows). (I) A unilineage macrophage clone (red fluorescence) that was visualized for coexpression of lcp, a macrophage marker (NBT/BCIP purple substrate). The white-light image shows three lineage-labeled macrophages (arrows) and two macrophages not in the clone.

Hematopoietic Progenitors Originate from Both the Dorsal and Ventral Gastrula
(A–E) Examples of individual derivatives. (A) Macrophages, (B) erythrocytes and thrombocytes, (C) neutrophils, (D) endothelial cells, and (E) pronephric cells. da, dorsal aorta; e, erythrocytes; sv, segmental vein; t, thrombocytes. Arrows indicate cellular protrusions.
(F–L) 6 hr gastrula fate maps. Clones that included (F) macrophage cells, (G) just erythrocyte cells, (H) both erythrocyte and neutrophil cells, (I) both erythrocyte and thrombocyte cells, (J) endothelial cells, and (K) pronephric cells. (L) Summary hematopoietic fate map. Superimposed symbols show individual clones that gave rise to multiple fates. The half-green/half-blue symbol indicates the four clones that included all three ventral-derived blood fates (erythrocytes, neutrophils, and thrombocytes). Some of the blood clones depicted on the fate maps were also later verified by in situ hybridization.

Not All Blood Is Committed to a Unipotential Lineage at 26 hr
(A–E) Single circulating blood cells give rise to neutrophil and erythrocyte progeny. (A–C) A newly photoconverted blood cell (arrow), within a ventral gastrula-derived clone. (A) Red wavelength, (B) green wavelength (note the other blood cells), and (C) composite image. (D and E) The resulting red-labeled clone at 48 hr included (D) circulating erythrocytes and (E) two neutrophils attached to the lumen of a blood vessel.
(F–I) Single macrophage cells give rise to macrophage progeny. (F–H) High-magnification view of an individual macrophage (arrow) in the process of being photoconverted from (F) green to (G) red. Nearby, another green-labeled macrophage (upper left panel) is preparing to divide. (H) Low-magnification view of the entire Kaede clone showing the newly photoconverted macrophage (arrow) among other green-labeled macrophages and out-of-focus endoderm and endothelium. (I) The resulting red-labeled clone at 48 hr was solely two macrophages near a blood vessel. Another green-labeled macrophage is out of focus.

Blood Formation in the Cell Cycle-Arrested Mutant harpy
(A–F) (A and B) Macrophage cells, (C and D) erythrocyte cells, and (E and F) neutrophil cells in wild-type and harpy (hrp) siblings. Note the super-sized cells in the hrp mutant.
(G) The approximate number of blood cells found at 24 hr. Error bars show SEM. For each blood type, a total of ten or more wild-type and mutant embryos were counted.