Practice question

Upon fertilization, sperm entry triggers calcium wave and formation of microtubule array from sperm aster at vegetal cortex. Dynein-dependent cortical rotation shifts vegetal cortex 30 degrees relative to inner cytoplasm, transporting dorsal determinants like Dishevelled, GBP and Wnt11 mRNA toward opposite side of sperm entry. This opposite side shows depigmentation as melanosomes move, forming gray crescent rich in beta-catenin stabilizing factors. Future dorsal organizer arises there. Sperm entry thus defines ventral pole, gray crescent dorsal pole marking organizer position for gastrulation patterning.