Hydroxyl ions in calcium hydroxyapatite crystals can be exchanged with fluoride ions to form fluorapatite crystals. In which tissue layer of a tooth does this exchange of ions occur?

The exchange of hydroxyl ions with fluoride ions to form fluorapatite primarily occurs in the enamel layer of a tooth. Enamel is the outermost layer, composed mostly of hydroxyapatite crystals, which provide strength and durability to the tooth structure.

When fluoride is introduced, such as through fluoride treatments or exposure from toothpaste, it can replace the hydroxyl ions in the hydroxyapatite. This substitution forms fluorapatite, which is not only less soluble but also more resistant to acid, thereby providing enhanced protection against dental caries.

The process reflects the unique ability of enamel to undergo remineralization and ion exchange due to its crystalline structure. Other layers of the tooth, such as cementum, dentine, and pulp, do not have the same capacity for fluoride ion exchange in the context of forming fluorapatite. Cementum and dentine are also mineralized tissues, but they have a different composition and structure compared to enamel. The pulp, being the innermost part containing nerves and blood vessels, does not participate in the mineralization processes that occur in the enamel. Thus, the enamel layer is the correct answer as it is the site where this significant ion exchange takes place.