Apical extracellular matrix development.

The apical extracellular matrix (aECM) plays crucial roles during morphogenesis and forms the interface through which tissues communicate with the environment. The aECM exhibits complex molecular composition and structure, including multi-layered organization, localized cell-type-specific specializations, three-dimensional elaborations and dynamic temporal remodeling. Yet, until recently, the aECM received relatively little attention compared to the much more well-studied basal ECM (i.e. the basement membrane and stromal matrix). Here, we discuss aECM composition and variation across species, and summarize emerging ideas from a growing community of aECM researchers related to potential mechanisms that control aECM development.