What unique property do keratin-filled cysts, known as milia, possess?

Milia are unique in that they have no visible opening on the skin surface. This characteristic differentiates them from other types of cysts or lesions that may have a drainage point. Milia are small, white or yellowish cysts that often appear on the face, particularly around the eyes and cheeks, but they do not break open like some other skin lesions. The lack of a visible opening means that they do not easily drain or release their contents, which are primarily composed of keratin—a protective protein found in skin, hair, and nails. This closed nature contributes to their persistence and the way they can subtly protrude beneath the surface of the skin without creating an obvious wound or pore.