The second most common cause is craniopharyngioma, a benign tumor derived from the oropharyngeal ectoderm of Rathke's pouch. (merckvetmanual.com)
It is interesting that YFP - expressing pluripotent cells were rarely detected in the lens, yet often present in the cornea since the surface ectoderm forms both the lens and the cornea. (journals.plos.org)
In other words, the basement membrane provides a physical dividing line between cells descendant from embryonic ectoderm (epidermis) and embryonic mesoderm (dermis). (keratin.com)