A complex relationship between the architecture of the basement membrane, its mechanical properties, and its ability to shape the Drosophila egg.

Basement membranes (BMs) are planar extracellular matrices that line the basal surfaces of epithelia and are essential components of most organs. During development, BMs can also play instructive roles in shaping the tissues to which they belong, but how they do so is incompletely understood. The Drosophila egg chamber has become a premier system to study this aspect of BM biology due to the ostensible simplicity of the BM's role in morphogenesis. The prevailing model posits that the egg chamber's outer layer of epithelial cells creates a symmetric stiffness gradient in the surrounding BM that preferentially channels egg chamber growth along one axis to create the elongated shape of the egg. There is evidence that the stiffening of the BM depends, in part, on a polarized array of fibrils that form within the BM network, and yet the exact role the BM fibrils play in egg chamber elongation has remained unclear. Here, we use genetic conditions that abrogate fibril formation to different extents to probe the relationship between the BM's fibril content, its mechanical properties, and the shape of the egg. The results of these experiments are consistent with a model in which BM fibrils influence egg shape by directly augmenting the mechanical properties of the BM. However, we then examine a final genetic condition that does not fit this simple narrative. We propose that the role of the BM in conferring final egg shape is more complicated than previously thought and that some approaches used to study this role should be re-evaluated for their efficacy.