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FIG. 4. RA treatment blocks expression of myocardial but not endocardial marker transcripts. Expression of the myocardial-specific sequence, XTnIc, or the endothelial marker sequence, X-msr, was detected by whole-mount in situ hybridization. (A) Transverse section through the heart region of a stage 32 embryo, stained for XTnIc expression. (B) Transverse section through the heart region of an RA-treated embryo stained for XTnIc transcripts. Although tissue layers similar to those in the control embryo are present, there is no detectable expression of XTnIc. (C) Stage 32 control embryo stained for the endothelial marker X-msr. The position of the endocardium (ec) is indicated. (D) RA-treated embryo assayed at stage 32 for the endothelial marker, X-msr. The position of the endocardium (ec) is indicated. (E) Transverse section through the heart region of a stage 32 control embryo stained for X-msr. (F) Transverse section through the heart region of a stage 32 RA-treated embryo, stained for X-msr. Note the presence of the endocardial tissue in both control and RA-treated embryos. (G) Transverse section through the heart region of a stage 35 control embryo stained for X-msr sequences. Note the separation of the endocardial tissue layer from the highly folded myocardial layer. (H) Transverse section through the heart region of a stage 35, RA-treated embryo, stained for X-msr sequences. The stained endocardial cells are located adjacent to the thick layer of unfolded tissue which occupies the position of the heart tube. |
