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What’s Important?

  1. Keratinocytes have multiple receptors that give them several important functions.

  2. Every skincare product placed on the skin affects the keratinocytes in some way.

  3. Stem cells in cosmeceutical products have no activity.

What’s New?

  1. Circadian rhythms increase TEWL at night.

  2. Keratinocytes focus on cell protection in the daylight and cell repair at night.

  3. Cosmeceuticals can manipulate epidermal skin cells.

  4. Visible light injures mitochondria and lysosomes, and ages skin.

What’s Coming?

  1. New research on CBD in skincare products.

  2. New treatments to remove lipofuscin and rejuvenate lysosomes.

  3. New modalities to protect mitochondria.

  4. More studies on the effects of light on keratinocytes.

The skin is composed of three primary layers: epidermis, dermis, and subcutaneous tissue (Fig. 1-1). The epidermis is the outermost superficial layer of the skin. It is very important from a cosmetic standpoint because it is this layer that gives the skin its texture and moisture, contributes to color, and affects light reflection. If the surface of the epidermis is dry, the skin feels rough and poorly reflects light. When patients complain that their skin is “dull” or “not radiant,” the problem lies in the epidermis. This is the layer targeted by salespeople when they urge you to “just try” their product. The product is almost always an exfoliator that removes the uppermost layer of the epidermis providing instant smoothness and light reflection. The epidermis is the layer to target when patients want instant results or radiant skin overnight. However, the changes to the epidermis are temporary as the keratinization cycle continues to produce new cells and push away old cells to the skin’s surface. The best topical formulations and skincare procedures target both the epidermis and the dermis. The epidermis is much more complex than described in this chapter, which is meant to focus on what parts of the epidermis are important to enhance the skin’s beauty, appearance, and health with minimal focus on skin disease.


Keratinocytes, also known as corneocytes, are the cells that comprise most of the epidermis. The skin cells in the epidermis are called keratinocytes because they contain keratin, a protein found in the epidermis, nails, and hair. Keratin is also found in the beaks and feathers of birds, and shells. The word “keratin” comes from the Proto-Indo-European root ker, which means horn. Keratin composes 30–80% of the total protein of the human epidermis.

The epidermis has an abundance of the keratin filaments that make up an intermediate filament cytoskeleton. There are two classes of keratins: Type I is slightly acidic (K9–40) and Type II, more basic (K1–8 and K71–86). Both types of keratin filaments must be present for a keratin filament to develop.1 In other words, an acidic type and a ...

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