Practice question

During classic Spemann-type transplantation, contact between neuroectoderm-derived optic vesicle and overlying head surface ectoderm initiates lens development. The optic vesicle secretes BMP4 and FGFs that upregulate Pax6 and Sox2 in competent ectoderm, driving thickening into lens placode and subsequent invagination. Removal of the vesicle blocks lens formation in competent regions, while trunk ectoderm lacks competence and cannot respond. This inductive interaction remains the textbook example of tissue interaction establishing eye primordia, initiating crystallin expression and highlighting sequential signals from endoderm and neural plate.