In Vivo and In VitroExpression of Tenascin by HumanThymic Microenvironmental Cells

摘要

Increasing evidence reveals that extracellular matrix components can be regarded as a groupof mediators in intrathymic T-cell migration and/or differentiation. Yet, little is kown aboutthe expression and putative function of one particular extracellular matrix protein, namely,tenascin in the thymus. Herein we investigated, by means of immunocytochemistry,tenascin expression in normal infant and fetal human thymuses, as well as in cultures ofthymic microenvironmental cells.In situ, tenascin distribution is restricted to the medulla and cortico-medullary regions ofnormal thymuses. This pattern thus differed from that of fibronectin, laminin and type IVcollagen, in which subseptal basement membranes were strongly labeled. Interestingly,tenascin did not co-localize with the cytokeratin-defined thymic epithelial cell network. Thiswas in keeping with thein vitrodata showing that tenascin-bearing cells were nonepithelial(and probably nonfibroblastic) microenvironmental elements.Studies with fetal thymuses revealed a developmentally regulated expression of tenascin,with a faint but consistent network labeling, in thymic rudiments as early as 12 weeks ofgestational age, that progressed to a strong TN expression at 18 weeks of fetal development,which was similar to the distribution pattern observed thereafter, including postnatally.Our results clearly indicated that tenascin is constitutively expressed in the humanthymus, since early stages of thymic ontogeny, and suggest that the cell type responsible forits secretion is a nonepithelial microenvironmental cell.