Corneocyte

A keratinocyte spends roughly a month travelling from the base of the epidermis to the surface. On the way it loses its nucleus and its organelles, fills with keratin, and swaps its membrane for a tough cross-linked protein shell with a layer of covalently bound lipids on the outside. What arrives at the top is a corneocyte: not a living cell any more, but a durable, water-holding brick.

Fifteen to twenty layers of them sit in mortar made of intercellular lipids. A corneocyte's own water is held by natural moisturizing factor, a mix of amino acids and their derivatives produced when filaggrin is broken down inside the cell. A corneocyte short on that mixture is a stiff brick, and stiff bricks make a surface that looks rough, flakes and feels tight.

They are riveted to one another by corneodesmosomes, protein junctions that enzymes cut on a schedule so cells can be shed a few at a time rather than in sheets. Those enzymes need water and a mildly acidic surface to work, which is the mechanical link between a dry, alkaline surface and visible flaking.

Almost everything a moisturiser does, it does to these cells: humectants top up their water, occlusives slow its loss, exfoliating acids help the rivets let go. None of it reaches living tissue — which is both the limitation of topical skincare and its safety margin.