Final answer:
The free gingiva is lined with keratinized stratified squamous epithelium, which contains keratin to withstand chewing stress and protect against microbial invasion.
Step-by-step explanation:
The free gingiva is lined by a specific type of epithelial tissue. The epithelium in the mouth, pharynx, esophagus, and rectal canal is generally non-keratinized, stratified squamous epithelium. However, inside the mouth, transition areas exist. For the free gingiva, which is the part of the gum that is not directly attached to the tooth and can be moved, this epithelium is a specialized type that is keratinized to withstand the mechanical stress of mastication (chewing).
Keratinized epithelial cells contain keratin, which is a tough, protective protein. In the free gingiva, keratinized epithelial cells form a resilient barrier against the physical forces exerted during eating and help protect against potential invasion by microorganisms.
To summarize, while many areas of the mouth are lined with non-keratinized epithelium, the free gingiva features keratinized stratified squamous epithelium, equipped to deal with the demands of the oral environment.