The extracellular matrix (ECM) and basement membrane (BM) have been considered to be little more than a static cellular scaffold. Although recent work has revealed instructive roles for the ECM, a basic understanding of the BM is lacking. Now, Stéphane Noselli and colleagues study the origin, assembly and function of the BM during Drosophila ovarian stalk morphogenesis. By labelling collagen type IV (ColIV, a key component of the BM) in different cell types, the authors show that the ECM is dynamic and mosaic. They reveal that ColIV originates from three distinct sources: the fat body, follicle and stalk cells, which contribute varying levels of ColIV to the BM at each stage. Using targeted loss-of-function approaches, the researchers reveal that ColIV from each source has unique functions to maintain structure and organisation during stalk morphogenesis; for example, stalk cell-derived ColIV is required for cell intercalation. Furthermore, genetic removal of ColIV from follicular cells cannot be rescued by the fat body, indicating that ColIV deposition, and BM assembly, is sequential and coordinated. Finally, removal of integrin produces similar phenotypes, suggesting that integrin-ColIV interactions instruct stalk morphogenesis. Together, these data reveal how dynamic assembly of a mosaic BM regulates complex tissue morphogenesis.
