Expression of jak2a in circulating erythroblasts. The expression of jak2a in circulating cells of the primitive wave of hematopoiesis was examined in thin sections of animals after the onset of circulation (26 hpf) after whole mount in situ hybridization. Preparations are oriented with anterior to the left. (a) Transverse section of trunk and tail at the level of the dorsal aorta. Arrowheads demarcate the extent of the vasculature, indicating the anterior dorsal aorta and the posterior vascular sinus.jak2a-positive cells are confined within the vasculature (arrows). (b) Higher magnification of transverse section of the caudal vein showing large, rounded jak2a-positive cells (arrows) within the vasculature. pm, paraxial mesoderm. Scale bars: for (a), 100 μm; for (b), 50 μm.