Col4a2-eGFPmouse model reveals the molecular and functional dynamics of basement membrane remodelling in hair follicle morphogenesis

The precisely controlled remodelling of the basement membrane (BM) is considered vital for morphogenesis. However, the molecular and tissue-level dynamics of the BM during morphogenesis and their functional significance remain largely unknown, especially in mammals, due to limited visualization tools. We developed knock-in mouse lines in which the endogenous collagen IV gene (Col4a2) was fused with a fluorescent tag. Through live imaging of developing hair follicles, we revealed a spatial gradient in the turnover rate of COL4A2 that is closely coupled with the BM expansion rate. The proliferation of epithelial progenitors coincided with the increased expansion of their underlying BM. Epithelial progenitors displaced with directionally expanding BM, but did not actively migrate on stable BM. The addition of a matrix metalloproteinase inhibitor delayed the turnover of COL4A2, restrained the expansion of the BM, and induced a directional shift in the division angle of epithelial progenitors, altering the hair follicle morphology. Our findings revealed spatially distinct BM dynamics within the continuous epithelial BM and affirmed their significance in orchestrating the proliferation, movement and fate of progenitor cells, as well as the macro-level shape of organs during development.