Dermis

The dermis lies between the basement membrane zone at the base of the epidermis and the subcutaneous fat. Its upper papillary layer is thin and loosely organised, with fine collagen fibrils, interdigitating with the epidermal rete ridges and containing the capillary loops that supply the avascular epidermis above. Beneath it, the reticular dermis is thicker and built from coarse, densely interwoven collagen bundles carrying most of the skin's tensile strength. Fibroblasts, mast cells, nerve endings, hair follicles, and glands all sit in this compartment.

By mass the dermis is mostly collagen, with elastic fibres a small percentage of dry weight and glycosaminoglycans and proteoglycans forming the hydrated ground substance between fibres. Thickness varies severalfold by body site, from very thin on the eyelid to several millimetres on the back. Two vascular plexuses, superficial and deep, run through it, which is why injectable fillers target this layer and why substances reaching it enter the systemic circulation.

That vascular point is the tradeoff people skip. The dermis is simultaneously the target for every anti-ageing claim and the layer a topical product is least likely to reach, since the stratum corneum exists to prevent exactly that. And any molecule that genuinely does arrive there has a route into the bloodstream, so a product cannot coherently claim deep dermal action and no systemic exposure at the same time.

In practice, marketing language about penetrating deep into the skin nearly always describes the stratum corneum or, at best, the viable epidermis. Asking which layer a claimed mechanism actually requires usually settles whether the claim is plausible.