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Fig. 7. Analysis of the heart morphology and xId2 expression patterns of stage 33 Xenopus embryos, following neural crest ablation and in situ hybridization (ISH) for xId2. (A, B) Transverse sections of a stage 33 control embryonic heart immunostained with the myocardial marker MF20. White arrows indicate (MF20 negative) undifferentiated splanchnic mesoderm cells continuous with the newly formed cardiac myocardial cells expressing myosin heavy chain (MF20 positive). (C, D) Sections through a stage 33 control embryo following ISH for xId2. In the control embryos, there is a visible myocardium (m), endocardium (e), and pharynx (P). [See Mohun et al. (2000) for reference to Xenopus heart morphology.] There is xId2 expression in the splachnic mesoderm (red arrows) as well as the outflow track myocardium (green arrow). (E, F) Sections through a stage 33 embryo following stage 15 neural crest ablation and ISH for xId2. There is a lack of xId2 expression in the splachnic mesoderm (red arrowheads) and the outflow tract myocardium (green arrowhead). There are also morphological differences in heart development following neural crest ablation. Note the enlarged myocardium (m), the thickened pericardial layer (Pc), and the pericardial edema (PcE). Morphology is variable among neural crest ablated embryos; (E) sometimes there is visible formation of a myocardium (m) and endocardium (e), (F) whereas in other embryos there is only an undistinguishable mass of heart cells (h). |
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Fig. 8. Analysis of morphology and xId2 expression patterns of stage 37 Xenopus embryos following neural crest ablation and in situ hybridization (ISH) for xId2. (A, B) Transverse sections of a stage 37 control embryonic heart immunostained with the myocardial marker MF20. White arrows indicate (MF20 negative) undifferentiated splanchnic mesoderm cells continuous with the newly formed cardiac myocardial cells expressing myosin heavy chain (MF20 positive). (C, D) Sections through a stage 37 control embryo following ISH for xId2. (C) There is Id2 expression in the branchial arches (BA), and in a more caudal section (D) there is expression in the splachnic mesoderm (red arrows), myocardium of the outflow tract (green arrow), and the branchial arches (white arrow). Note that the expression in the splachnic mesoderm is continuous with the distal outflow tract myocardium. There is a visible pharynx (P), pericardium (Pc), myocardium (m), endocardium (e), aortic sac (as, black arrow), and truncus arteriosus (ta). [See Mohun et al. (2000) for reference to Xenopus heart morphology.] (E, F) Sections through a stage 37 embryo following stage 15 neural crest ablation and ISH for xId2. There is a lack of xId2 expression in both the splachnic mesoderm (red arrowheads) and myocardium of the outflow tract (green arrowheads). Morphologically, there is pericardial edema (PcE), an enlarged myocardium (m), and an irregular pericardial layer (pc). (E) The heart that formed in this ablated embryo is an unlooped linear heart tube. |