Fig. 1. GFP labelling of cardiac progenitors in HH3 primitive streak. (A) Schematic illustration of the electroporation and grafting procedure; embryos were placed in EC culture with their ventral side up. (B) HH3 donor embryo (left) after electroporation of primitive streak with pCS2⁺-GFP, and HH3 host embryo (right). Green fluorescence identifies labelled cells; small grafts were dissected and transplanted into a stage-matched host. The white dotted semi-circles show the anterior margin of the primitive streak; the white dotted oval indicates the graft. (C) Transverse cryosection (15 µm) through an HH5 embryo shows GFP-positive cells in the mesoderm of the lateral plate. (D) HH7 embryo with GFP-positive cells in the area of the bilateral heart fields, indicated by a white line (Stalsberg and DeHaan, 1969). (E) GFP-positive cells in the heart tube at HH10. (F) Transverse cryosection (30 µm) of a similar embryo, stained with anti-GFP (green) and MF20 (red) antibodies, showing that GFP-positive cells were present in endocardium and myocardium. White line in E indicates the approximate level of the section shown in F. ao, area opaca; ec, ectoderm; en, endoderm; end, endocardium; fg, foregut; h, heart; hf, heart field; my, myocardium; nt, neural tube; ps, primitive streak.