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Two epithelial cell types cover the alveolar surface of the lung. Type II alveolar epithelial cells produce surfactant and, during development or following wounding, give rise to type I cells that are involved in gas exchange and alveolar fluid homeostasis. In culture, freshly isolated alveolar type II cells assume a more squamous (type I-like) appearance within 4 days after plating. They assemble numerous focal adhesions that associate with the actin cytoskeleton at the cell margins. These alveolar epithelial cells lose expression of type II cell markers including SP-C and after 4 days in culture express the type I cell marker T1alpha. Those cells that express T1alpha also deposit fibers of laminin-311 in their matrix. The latter appears to be related to their development of a type I phenotype because freshly isolated, primary type I cells also assemble laminin-311-rich fibers in vitro. A beta1 integrin antibody antagonist inhibits the assembly of laminin-311 matrix fibers. Moreover, the formation of laminin fibers is dependent on the activity of the small GTPases and is perturbed by ML-7, a myosin light chain kinase inhibitor. In summary, our data indicate that assembly of laminin-311 fibers by lung epithelial cells is integrin and actin cytoskeleton dependent, and that these fibers are characteristic of type I alveolar cells.

We examined the development of mouse submandibular gland by light and electron microscopy and determined the distribution of the alpha 6 integrin subunit and laminin in this process by immunofluorescence and immunoelectron microscopy. At Days 13.5 and 14 of gestation alpha 6 was localized over the entire cell surface of undifferentiated epithelial cells of the terminal cluster. On Day 15 the expression of alpha 6 could no longer be detected over central cells in the proximal portion of branched terminal cluster, whereas peripheral cells were stained over the entire surface. On Day 17 of gestation to day of birth, alpha 6 expression was restricted to the basolateral surface of the differentiated acinotubular structure. Its expression on acinar cells was uniformly distributed throughout the basolateral membrane, but on duct cells stronger staining towards the basal surface was noted. Similar expression was observed in adult acinar and duct cells. Expression of the alpha 6-subunit at the cell-cell contact in an early stage and its expression at the basolateral surface in an advanced stage indicate that integrin containing alpha 6 plays a significant role in cell-cell and cell-substrate interaction. Stage and cell type-specific change of integrin expression may be significant for submandibular development.

The temporal and spatial distribution of the basement membrane component laminin was examined in vivo in developing rat hearts at 11.5 and 15 days of embryonic development (ED), and in neonates and adults, by pre-embedding ultrastructural immunocytochemistry. In addition, the patterns observed at 11.5 days ED were compared to the distribution of laminin in embryos maintained in whole-embryo culture. At 11.5 days ED laminin was localized in punctate patches on the surface of the plasma membrane, with large gaps between areas of staining. The development of myocytes and localization of laminin in the whole embryo-cultured embryos was similar to that found in the in vivo embryos. At 15 days ED, laminin localization was limited to distinct patches of developing extracellular matrix material associated with the sarcolemma. Gaps between areas of localization were shorter than in the 11.5-day hearts. In neonates, distribution of laminin localization was more extensive with fewer gaps and was associated with the developing basement membrane. In adult hearts, laminin was localized along the entire length of the basement membrane and was heaviest in areas of morphological specialization, such as Z-bands, where collagen bundles contacted the sarcolemma.