Changes in the distribution of integrins and their basement membrane ligands during development of human thyroid follicular epithelium
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Interactions between cells and basement membrane components are crucial for the regulation of epithelial cell differentiation and polarization. We have studied by immunohistochemical methods the distribution of integrin adhesion proteins and some of their basement membrane ligands in foetal (16--19 weeks) and adult thyroid follicular epithelia. A diffuse immunoreactivity for only α3, αv and β1 integrins was found in foetal follicular epithelium, whereas in adult follicular epithelium these integrins were expressed basally in a polarized manner. Additionally, β3 integrin was seen in a more basolaterally confined manner in adult follicular epithelium. Among basement membrane components, laminin α1, β1, γ1 and β2 chains were found in epithelial basement membranes of the foetal thyroid gland, suggestive of the presence of laminins-1 and -3. In contrast, the basement membranes of adult follicular epithelium presented a much weaker immunoreactivity for the laminin β2 chain. Furthermore, immunoreactivity for the laminin α2 chain was occasionally seen in adult thyroid glands, apparently confined to myofibroblasts. Immunoreactivity for type IV collagen α1 and α2 (IV) chains was found in follicular basement membranes of foetal as well as adult thyroid gland. The results suggest that during maturation of foetal thyroid follicular epithelium a distinct polarization of integrins takes place. In mature thyroid follicular epithelium, the presumable adhesion-mediating integrin complexes are α3β1, αvβ1 and/or αvβ3 mediating adhesion to laminin-1 (α1-β1- γ1) and type IV collagen trimer α12 (IV)
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