• α7 integrin;
  • heart tube morphogenesis;
  • mesoderm re-specification;
  • neural plate bending;
  • somite epithelialization

Integrin α7â1 is a specific cellular receptor for laminin. In the present work, we studied the distribution pattern of the α7 subunit by immunofluorescence and immunoprecipitation and the role of the integrin by blocking antibodies in early chick embryos. α7 immunoreactivity was first detectable in the neural plate during neural furrow formation (stage HH5, early neurula, Hamburger & Hamilton 1951) and its expression was upregulated in the neural folds during primary neurulation. The α7 expression domain spanned the entire neural tube by stage HH8 (4 somites), and was then downregulated and confined to the neuroepithelial cells in the germinal region near the lumen and the ventrolateral margins of the neural tube in embryos by the onset of stage HH17 (29 somites). Expression of α7 in the neural tube was transient suggesting that α7 functions during neural tube closure and axon guidance and may not be required for neuronal differentiation or for the maintenance of the differentiated cell types. α7 immunoreactivity was strong in the newly formed epithelial somites, although this expression was restricted only to the myotome in the mature somites. The most intense α7 immunoreactivity was detectable in the paired heart primordia and the endoderm apposing the heart primordia in embryos at stage HH8. In the developing heart, α7 immunoreactivity was: (i) intense in the myocardium; (ii) milder in the endocardial cushions of the ventricle; (iii) intense in the sinus venosus; (iv) distinct in the associated blood vessels; and (v) undetectable in the dorsal mesocardium of embryos at stage HH17. Inhibition of function of α7 by blocking antibodies showed that α7 integrin–laminin signaling may play a critical role in tissue organization of the neural plate and neural tube closure, in tissue morphogenesis of the heart tube but not in the directional migration of pre-cardiac cells, and in somite epithelialization but not in segment formation in presomitic mesoderm. In embryos treated with α7 antibody, the formation of median somites in place of a notochord was intriguing and suggested that α7 integrin–laminin signaling may have played a role in segment re-specification in the mesoderm.