Partial exogastrulation due to apical–basal polarity of F‐actin distribution disruption in sea urchin embryo by omeprazole

Gastrulation is a universal process in the morphogenesis of many animal embryos. Although morphological and molecular events in gastrulation have been well studied, the mechanical driving forces and underlying regulatory mechanisms are not fully understood. Here, we investigated the gastrulation of embryos of a sea urchin, Hemicentrotus pulcherrimus, which involves the invagination of a single‐layered vegetal plate into the blastocoel. We observed that omeprazole, a proton pump inhibitor capable of perturbing the left–right asymmetry of sea urchin embryo, induced “partial exogastrulation” where the secondary invagination proceeds outward. During early gastrulation, intracellular apical–basal polarity of F‐actin distribution in vegetal half was higher than those in animal half, while omeprazole treatment disturbed the apical–basal polarity of F‐actin distribution in vegetal half. Furthermore, gastrulation stopped and even partial exogastrulation did not occur when F‐actin polymerization or degradation in whole embryo was partially inhibited via RhoA or YAP1 knockout. A mathematical model of the early gastrulation reproduced the shapes of both normal and exogastrulating embryos using cell‐dependent cytoskeletal features based on F‐actin. Additionally, such cell position‐dependent intracellular F‐actin distributions might be regulated by intracellular pH distributions. Therefore, apical–basal polarity of F‐actin distribution disrupted by omeprazole may induce the partial exogastrulation via anomalous secondary invagination.