Epidermis

The epidermis is a stratified squamous epithelium with no blood supply of its own, nourished by diffusion from capillaries in the papillary dermis. Keratinocytes make up the large majority of its cells, with basal melanocytes, Langerhans cells as resident antigen-presenting cells, and Merkel cells serving touch. Moving outward, the layers are the stratum basale, spinosum, granulosum, an additional stratum lucidum only in thick palmoplantar skin, and the stratum corneum. The basal layer rests on the basement membrane zone that anchors epidermis to dermis.

Over most of the body the epidermis is only around a tenth of a millimetre thick, thickening to well over a millimetre on palms and soles. A cell born at the basal layer takes roughly a month or more to reach the surface and shed, and transit slows with age. For topical delivery the useful division is between the stratum corneum, dead and constituting the barrier, and the viable epidermis beneath it, where living targets sit.

Naming the layer a mechanism requires is the quickest test of a claim. Tyrosinase inhibition needs to reach melanocytes at the basal layer, several cell layers down but still within the epidermis. Barrier lipid effects act within the granular layer and stratum corneum. A collagen claim needs the dermis, a compartment further still and across a basement membrane, which is a considerably harder ask.

Two reporting habits blur this. Studies quantifying an active recovered from tape strips are measuring stratum corneum retention and get cited as absorption. And epidermal penetration in a summary is often taken to mean dermal delivery, when they are separated by the layer that matters most for anti-ageing claims. Reconstructed epidermis models add a caveat: many lack melanocytes or immune cells entirely.