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We have isolated cDNAs encoding the bHLH protein Hand2 in the amphibian Xenopus laevis and analysed Hand2 expression in early development from the onset of gastrulation to feeding tadpole stages. XHand2 is expressed in the branchial archmesenchyme and also in small bilateral populations of cells in the anterior, ventrolateral region of early tailbud embryos. At later stages, these punctate Hand2-expressing cells are located at the sites of the forming common cardinal veins, suggesting that they may constitute progenitors of vascular smooth muscle cells. Other Hand2-expressing cells are also associated with further components of the forming anteriorvasculature but are not detected in mature blood vessels. Interestingly, no myocardial expression of XHand2 can be detected in the developing tadpoleheart, in marked contrast to results obtained with chick and mouse embryos.
Fig. 3. Whole-mount in situ hybridisation analysis of XHand2 expression.
(A,C,F,H) Leftlateral views of Xenopus embryos at stage 22, 24, 28,
and 30, respectively. (B,E,G,I) Higher magnification ventral views of the
same embryos depicted in A,C,F, and H. Anterior is to the top (B,E) or the
left (G,I). (D) Detail of a stage 24 embryo in which punctate Hand2-
expressing cells are undergoing synchronous cell division. (J,K,L,M) Leftlateral views of the head of embryos at stage 32, 34, 38, and 41,
respectively. Black arrows indicate XHand2 expression. BA, branchial arches; CCV, common cardinal vein; H, heart; A, anterior; P, posterior.
Fig. 4. Location of punctate XHand2-expressing cells. Transverse sections through the anteriortrunk of stage 24 (A) and stage 30 (B) embryos. (C and magnified view D) Transverse section at the level of the pronephric sinus of a stage 32 embryo. (E) Transverse section though the heart-forming region of a stage 30 embryo. Sections C to E have been counterstained to reveal cell nuclei, whereas small arrows indicate XHand2-positive cells. (Pd, pronephric duct).
Fig. 5. Transverse sections through the branchial arch and heartforming region of Xenopus embryos after whole-mount in situ hybridisation for XHand2. Four representative sections marking progressively
posterior slices through the heart are shown for stage 30 (A), stage 32 (E), and stage 38 (I) embryos. End, Endoderm; Mes, mesoderm;
Ect, ectoderm; Ec, endocardium; Mc, myocardium; BA, branchial arch mesenchyme; S.Mes, somatic mesoderm; Sv, sinus venosus; AS, aortic sac; OT, outflow tract; V.Ec, ventricular endocardium; V.Mc, ventricular myocardium, AA, aortic arch artery; Liv, liver; A.Mc, atrial myocardium.
