Extended Data Fig. 8 |. Mechanisms of epithelial cell incorporation.

Three mechanisms to incorporate stem cell progeny into a mature epithelium are shown. Apical membrane (cyan), occluding junction (red), differentiating cell (blue), terminally-differentiated cells (gray), and stem cells (white). a, Symmetric inheritance. Stem cells possess occluding junctions, which are inherited by their progeny. b, Radial intercalation. Stem cells lack occluding junctions. As stem cell progeny differentiate, they grow apically, wedging themselves between terminally-differentiated cells. When they reach the occluding junction of the epithelium, the differentiating cell forms occluding junctions with its neighbors. These junctions expand radially in a ring around the cell’s nascent apical membrane. c, Pre-assembled Apical Compartment (PAC) integration. Stem cells lack occluding junctions Differentiating cells create a transient, occluding junction niche that supports development of the new cell’s future, lumen-facing apical surface. This Pre-assembled Apical Compartment (PAC) is formed from deep, apical plasma membrane pit in the differentiating cell that is covered by overlying mature cells. As the new cell grows and differentiates, the transitional junction mediates a basal-to-apical neighbor exchange between the new cell and mature cells that exposes the PAC to the gut lumen and seamlessly integrates the new cell into the epithelial barrier. d, PACs are asymmetric structures with split apical/basolateral character. The pre-enterocyte’s apical membrane pit accounts for most of the PAC’s surface area.