Expression of CD34 mRNA during early cardiovascular development in the mouse embryo. Cranial/anterior is to the right in (A) and (B) (n, cranial neural fold). (A) 1-2–somite headfold stage: transcripts are detectable in two diffuse lines (arrowed), corresponding to the two presumptive dorsal aortae, continuous cranially with the presumptive paired endocardial heart tubes (arrowhead). (B) 3-somite stage: the paired dorsal aortae (da) are now clearly defined in the anterior half of the embryo, and are continuous with diffuse lines of stained cells, presumably pre-endothelial cells (arrowed), which join at the junction of the primitive streak (ps) and allantois; there is diffuse staining in the allantois and yolk sac (y). (C) 7-somite stage: caudally, the two dorsal aortae (da) are continuous with the omphalomesenteric artery (om) which is also stained; intersomitic arteries (is) have begun to branch from each dorsal aorta; endocardial (en) staining is clear in the ventricle and truncus arteriosus, which is continuous with the first aortic arch (arrow); there are also scattered stained cells within the cranial mesenchyme (c). (D) 9-somite stage: CD34 transcripts are now more abundant in the cranial mesenchyme (c) and in the endothelial lining (en) of the ventricle and truncus arteriosus of the heart; there are clear patches of stained cells in the yolk sac (y). (E) 14-somite stage: CD34 transcripts are abundant in the umbilical artery (ua), allantoic mesenchyme (al), umbilical vein (arrows) and proximal segment of the vitelline arteries (va); in the head, cranial mesenchymal cells expressing CD34 are beginning to condense around the developing eye close to the internal carotid artery (arrow); the endothelium of the first aortic arch artery (1) is stained, but transcript levels are low in the second aortic arch (2) and undetectable in the third aortic arch (3) or in the aortic sac that joins the second and third aortic arches; transcript levels are lower in the anterior cardinal vein (ac) and common cardinal vein (cc) than in the dorsal aorta (da) and umbilical vein (uv), and undetectable in the vitelline vein (vv); they are low in the ventricle (v), atrium (not visible) and sinus venosus (s), but high in the truncus arteriosus (ta). (F ) Day 9, 18-somite stage: transcripts are now present in the aortic sac (a) and all aortic arches, but in general are more abundant in capillary networks around the developing sense organs than in the larger vessels. (G) Day 10 embryo head: the hyaloid artery (arrow) is stained, but in general transcript levels are decreased in the head, being confined to capillary networks around the eye and brain and within the facial mesenchyme. e, eye; n, nasal pit; o, otic pit. Scale bars = 100 μm