Fig. 6. Putative model for gelsolin activation at the time of ectoplasmic specialization disassembly in Sertoli cells. The model involves hydrolysis of PIP₂ (phosphatidylinositol 4,5-bisphosphate) resulting both in the release of gelsolin and a surge in local levels of Ca²⁺. Although PIP₂ is shown in association with the plasma membrane, the model is not meant to exclude the possibility that PIP₂ is also present in the membrane of the ER. IP₃, inositol (1,4,5)-trisphosphate; IP₃R, inositol (1,4,5)-trisphosphate receptor; PIP₂, phosphatidylinositol 4,5-bisphosphate; PLC, phosphoinositide-specific phospholipase C.