Differential expression of tenascin-C in the developing human lung: an immunohistochemical study

Much of the specification for the basic embryonic body plan is the result of a hierarchy of developmental decisions at different developmental times. The extracellular matrix (ECM) appears to be a very dynamic structure during embryogenesis. One of the mesenchymal ECM proteins, tenascin, is reported to be transiently expressed during embryonic tissue development, and is absent or much reduced in most fully developed organs. The respiratory system is an outgrowth of the ventral wall of the foregut, and the epithelium of the larynx, trachea, bronchi and alveoli is of endodermal origin. The cartilaginous and muscular components are of mesodermal origin. The aim of this study was to investigate the role of tenascin-C (TNC) in the developing human lung, during the pseudoglandular, canalicular and saccular stage of lung maturation. Formalin-fixed, paraffin-embedded tissue from the lungs of 30 embryos (10 corresponding to the 10th to the 16th gestational week (pseudoglandular stage), 10 to the 17th to the 23rd gestational week (canalicular stage), and 10 to the 24th to the 27th gestational week (saccular stage), were investigated by conventional histology and immunohistology for the expression levels of TNC. The changes observed in the distribution patterns suggest that during embryogenesis, the rate of tenascin synthesis changes significantly. During the pseudoglandular stage, the density of cells expressing TNC was higher in the condensing mesenchyme surrounding the epithelial glands than in the epithelial cells, whereas the inverse result was observed during the canalicular stage. During the saccular stage the pattern of immunoreactivity with TNC was lower than those of the pseudoglandular and canalicular stage, either in epithelial or mesenchymal cells, but it was highly expressed in the basement membranes. This restricted spatiotemporal distribution suggests that tenascin has a key role (1) in mesenchymal tissue remodeling during the pseudoglandular stage, a period that describes the development of the complete bronchial tree and (2) on the epithelial cell shape and function during the canalicular stage, a period that describes the formation of pneumocytes type I and pneumocytes type II. The later, will produce the surfactant, a phospholipid-rich fluid capable of lowering surface tension at the air-alveolar interface. During the saccular stage, tenascin was present mainly in the basement membranes surrounding the acinar and vascular structures, indicating a supporting and mechanical role.