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Fig. 8. Proposed model for the function of HB-EGF in leading edge formation during
eyelid closure. (A) Before E15.0, the leading edge is still not formed, and
HB-EGF expression is not detectable. (B) At E15.0, the leading edge starts to
extend, and HB-EGF expression appears at the tip of the leading edge. (C) From
E15.0 to E16.0, HB-EGF is constitutively expressed at the tip of the extending
leading edge. On the surface of the cells located in this region, ectodomain
shedding of proHB-EGF may be mediated by ADAM17 (TACE), allowing sHB-EGF to
diffuse throughout the leading edge region. sHB-EGF binds to and activates
EGFR and the ERK pathway, resulting in F-actin polymerization and promotion of
epithelial sheet migration.