Models of the septation process and alternative septations of fission yeast. (A) WT septation (top). Simultaneous coordinated synthesis (arrow) of PS (perpendicular to cell wall) and SS (parallel to PS) form a three-layered septum. Septum maturation proceeds by PS anchorage into the cell wall (yellow arrow) and a second round of SS synthesis. (middle) Septation in Bgs1 absence. The SS is synthesized parallel to the cell wall. The septum grows by successive parallel SS depositions (orange arrow). The MTD changes to a septum medial position, forming a dotted line of MD in the SS layers (Cortés et al., 2007). (bottom) Septation in Bgs4 absence. CAR and septum are oblique positioned in the cell middle. The septum grows as a weak twisted and misdirected PS (wavy arrows) that is delayed and uncoupled from CAR and PM ingression (red arrows). After septum completion, the defective middle region is repaired with new PS (orange arrows) and the PS is retracted from the cell wall. (B) WT cell separation (top). Controlled cell wall DR and PS degradation (arrow) and the osmotic pressure that curves the SS to the stable conformation ensure a safe separation. In Bgs1 absence, there is no cell separation (middle). Cell separation and lysis in Bgs4 absence (bottom). Uncontrolled cell wall DR degradation (arrow) leaves the PM exposed to the medium. Then the turgor pressure generates the PM rupture and cytoplasm release. F, fuscannel; FS, fission scar; Pr, turgor pressure. Bars, 1 µm.