The epidermis is located at the most superficial layer of the skin, which directly reflects the skin’s appearance and health status, and gives the skin a layered feeling. It participates in the skin’s moisturizing and skin color production, and is a key medium in dermatocosmetics. So, what is the basic level of skin structure knowledge?

The epidermis belongs to stratified squamous epithelium, mainly composed of keratinocytes (keratinocytes), interspersed with different types of reticular cells, such as melanocytes (melanocytes), Langerhans cells (Langerhans cells), and Merkel cells (Merkel cells).

Keratinocyte

Keratinocytes are the main constituent components of the epidermis, accounting for more than 95% of the epidermal cells. Keratinocytes are connected with each other through various substances. During the different stages of division and maturation, the shape, size, and arrangement of keratinocytes change, and finally, keratinocytes containing keratin fall off to form the thin stratum corneum.

The orderly arrangement of keratins is a key factor in the skin’s resistance to external physical, chemical, and microbial damage. The epidermal keratin is the main structural protein of the epidermal cells, fibrous, with a diameter of 10nm, and belongs to the intermediate filament family.

Keratin proteins are divided into two types based on the homology of the nucleotide coding sequence of the keratin gene, type I with a smaller relative molecular mass and acidic (K10~K20), and type II with a larger relative molecular mass and neutral to alkaline (K1~K9). The keratin gene of type I and type II is located at the sex chromosome region of 17q12~q21 and 12q11~q13, respectively. The complete keratin fiber is composed of different dimers of type I and type II keratin in a 1:1 ratio, so keratin in the epidermis is expressed in pairs.

The differences in keratin protein expression mainly reflect the different stages of cell division in the epidermis. The keratin protein expressed in the dermal layer is K5/K14, and the co-expression of K1/K10 appears in the spinous layer with the differentiation of dermal cells. During the further division of somatic cells, the expression of K1/K10 gradually increases, while the expression of K5/K14 gradually decreases.

Therefore, based on the characteristics of the various developmental stages of keratinocytes, the epidermis is divided into five layers from the outer layer to the inner layer: thin stratum corneum, translucent layer, granular layer, spinous layer, and dermal layer.