Progenitor cell integration into a barrier epithelium during adult organ turnover

Barrier epithelial organs face the constant challenge of sealing the interior body from the external environment while simultaneously replacing the cells that contact this environment. These replacement cells--the progeny of basal stem cells--are born without apical, barrier-forming structures such as a protective, lumen-facing membrane and occluding junctions. How stem cell progeny acquire these structures to become part of the barrier is unknown. Here we use Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM), Correlative Light-Electron Microscopy (CLEM), and volumetric imaging of live and fixed organs to investigate progenitor integration in the intestinal epithelium of adult Drosophila. We find that stem cell daughters gestate their future lumenal-apical membrane beneath a transient, basal niche formed by an umbrella-shaped occluding junction that shelters the growing cell and adheres it to mature neighbor cells. The umbrella junction both targets formation of a deep, microvilli-lined, apical invagination and closes it off from the contents of the gut lumen. When the growing cell is sufficiently mature, the umbrella junction retracts to expose this Pre-Assembled Apical Compartment (PAAC) to the gut lumen, thus incorporating the new cell into the intestinal barrier. When we block umbrella junctions, stem cell daughters grow and attempt to differentiate but fail to integrate; when we block cell growth, no umbrella junctions form, and daughters arrest in early differentiation. Thus, stem cell progeny build new barrier structures in the shelter of a transient niche, where they are protected from lumenal insults until they are prepared to withstand them. By coordinating this dynamic junctional niche with progenitor cell differentiation, a physiologically active epithelial organ incorporates new cells while upholding integrity of its barrier.