Fig. 9. Model of sequential steps during lumen formation. A PAP is established in the contact surface of neighbouring cells via insertion of gp135-positive membranes originating from a compact ARE-like structure (bigger red spots) (I). PAP maturation occurs through formation of tight junctions around the PAP and the enrichment of it with water and ion channels (II). It triggers lumen initiation (III), which is followed by lumen enlargement (IV). The enlargement phase is subdivided into a phase without cell volume increase but with continued luminal growth (Phase a) and a phase with both cell and luminal growth (Phase b). Only two-cell aggregates are shown for simplicity and the object sizes represent the physical trends. The relative changes in aggregate volume, individual cell volume and lumen volume are represented below the scheme as bars of variable height indicating constancy, increase or decrease over time.