Models of the septation apparatus and the apical growth of fission yeast. (A) Septation and cell separation in WT cells. A balance between the osmotic pressure that curves the secondary septum to the stable spherical conformation and the degradation of the primary septum ensures a symmetrical and steady separation. (B) Alternative septation and side-explosive cell separation in the absence of Ags1 and the corresponding α-glucan. Asymmetrical septum edging degradation and mechanical tear of a weak primary septum that cannot hold the turgor pressure, leading to an instantaneous side-explosive separation to adopt the stable spherical conformation in both new ends. The cells stay attached by the septum edging for the next cell cycle. (C) Apical growth in WT and Bgs4- and Ags1-depleted cells. (1) The WT cell wall thickness is uniform, ensuring a balance between the turgor pressure and the strength of the growing cell wall. (2) Bgs4 absence produces a thin wall at the pole tip that cannot withstand the turgor pressure, leading to the wall rupture and cytoplasm release (unpublished data). (3) Ags1 absence causes wall cavities in the pole side region due to a defective final cell separation, leading to the cell lysis and cytoplasm release. CW, cell wall; Cyt, cytoplasm; F, fuscannel; Fs, fission scar; ICW, remedial internal cell wall layer; Mtd, materiel triangulaire dense; Ne, new end; Pr, turgor pressure; Pm, plasma membrane; Ps, primary septum; RSs, remedial secondary septum; Se, septum edging; Ss, secondary septum.