Organization of basement membrane superstructure. A simplified and hypothetical diagram showing how BM might be assembled at the surface of a cell or an acinus. (a) Top: acinus surrounded by laminin-111 (green). Bottom: the laminin-111 polymerizes and engages integrins (blue) on the basal face of the epithelial cells. However, the laminin-111 polymer is not stably anchored into a supramolecular structure. In this case, apical polarity is not established and tight-junction proteins (pink) do not get organized on the apical surface of the acini. (b) Laminin-111 polymer (green) is anchored by type IV collagen (red); co-localization of the two proteins is shown by yellow. The proteins are now physically connected by nidogen (black). Basal integrins (blue) are organized and are likely to be held in a spatial orientation that allows proper signaling for establishment of apical polarity. Tight-junction proteins become organized apically in the acinus, and apical polarity is established.