A 12-year-old female patient presents for an

orthodontic check-up after braces placement. In one spot, part of her braces

are slightly pushing and interfering. There is a white spot that cannot be

wiped off. As a result of mechanical irritation, the mucous membrane of the

vestibular surface of the lip has become irritated and keratinised. Such spots

are known as leukoplakia and, in addition to mechanical irritation, a number of

other factors may be involved in their formation. However, the exact cause has

not yet been described. The transformation of which epithelium is involved

here, which normally forms the oral mucosa?