Stratum Corneum

The stratum corneum is what remains after a keratinocyte completes terminal differentiation: an anucleate, organelle-free cell packed with keratin and enclosed in a covalently crosslinked cornified envelope to which ceramides are chemically bound. On most body sites the layer is ten to twenty micrometres thick and roughly fifteen to twenty cells deep, thickening to several hundred micrometres on palms and soles. Corneocytes are held by corneodesmosomes and separated by the lipid lamellae that form the actual diffusion barrier.

It is a turning-over structure, not a static shell. Corneodesmosomes are progressively degraded by kallikrein-family proteases and cells are shed individually, a process taking roughly two weeks to move a cell from the base of the layer to the surface within a full epidermal turnover on the order of a month or more. That timescale sets how quickly any change confined to this layer, deposited pigment included, can visibly resolve.

Because it is rate-limiting, the fate of a topical peptide is decided in a layer of dead cells before any living target is reached. Barrier state and formulation are therefore not secondary details: the same molecule on intact versus stripped skin behaves like two different drugs, and the practical ceiling on passive delivery of large hydrophilic molecules is set here, not by anything about the target receptor.

The claim to distrust is penetration of the stratum corneum offered as evidence of activity. Getting into this layer is the easy part, and it is where tape-stripping assays and most confocal images find their signal; material sitting in dead corneocytes is a reservoir on its way to being shed, not delivery to a fibroblast. A related misconception is that exfoliation removes the barrier, when the machinery beneath resupplies it.