Cardiac troponin I is a heart-specific marker in the Xenopus embryo: expression during abnormal heart morphogenesis

Cardiac troponin I (troponin Ic) expression is restricted to the heart at all stages of Xenopus development. Whole-mount in situ hybridization and Northern blot analysis indicates that troponin Ic is first expressed in tailbud embryos (stage 28) about the time of the first cytological heart differentiation and about 24 hr before beating tissue is observed. We have used this marker to examine abnormal heart morphogenesis in embryos treated with retinoic acid and lithium. When retinoic acid is administered to embryos prior to heart specification, heart tissue is reduced and often completely ablated. When embryos are treated after heart specification, but before the heart primordium migrates to the ventral midline, the migration is unaffected but smaller, abnormal hearts result. Lithium treatment of cleavage stage embryos causes an increase in heart tissue. In severely dorsalized embryos, heart tissue can be found around the entire embryo with the exception of a small gap at the most dorsal point. This gap indicates that migration of the heart to the ventral midline does not occur in these embryos. Later in development, a centrally located, beating heart is observed in dorsalized embryos. The timing of its appearance suggests that it is formed by movements normally associated with heart morphogenesis rather than migration.