The mammary gland structurally and functionally remodels during pregnancy, during lactation and after weaning. There are three types of fibrillar collagens, types I, III, and V, in mammary stromal tissue. While the importance of the fibrillar structure of collagens for mammary morphogenesis has been suggested, the expression patterns of each type of fibrillar collagen in conjunction with mammary remodeling remain unclear. In this study, we investigated their expression patterns during pregnancy, parturition, lactation and involution. Type I collagen showed a well-developed fibril structure during pregnancy, but the fibrillar structure of type I collagen then became sparse at parturition and during lactation, which was concurrent with the downregulation of its mRNA and protein levels. The well-developed fibrillar structure of type I collagen reappeared after weaning. On the other hand, type V collagen showed a well-developed fibrillar structure and upregulation in the lactation period but not in the periods of pregnancy and involution. Type III collagen transiently developed a dense fibrillar network at the time of parturition and exhibited drastic increases in mRNA expression. These results indicate that each type of fibrillar collagen is distinctly involved in structural and functional remodeling in mammary glands during pregnancy, parturition, lactation, and involution after weaning. Furthermore, in vitro studies of mammary epithelial cells showed regulatory effects of type I collagen on cell adhesion, cell proliferation, ductal branching, and β-casein secretion. Each type of fibrillar collagen may have different roles in defining the cellular microenvironment in conjunction with structural and functional mammary gland remodeling.
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