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like the rest of your skin, the outermost layer of the epidermis - the stratum corneum
While it is made up of the epidermis and dermis like the rest of your skin, the outermost layer of the epidermis, the stratum corneum, is much thinner on your lips, so thin that it allows more visibility into our blood vessels, giving our lips that reddish color.
The outermost layer of your skin, the stratum corneum, is a layer 25 cells thick of dead skin.
The skin on their fingertips will respond by thickening the stratum corneum to over 100 cells!