Corneodesmosome

Corneocytes are not simply stacked like bricks. Each is pinned to its neighbours by modified desmosomes — corneodesmosomes — assembled from adhesion proteins including corneodesmosin, desmoglein 1 and desmocollin 1. Those rivets are what make the stratum corneum a coherent sheet rather than a pile of loose scale.

Shedding is therefore demolition rather than erosion. Enzymes resident in the corneum, chiefly the kallikrein-related peptidases, cut those proteins so that cells release singly and invisibly. The enzymes need water in order to work and a particular surface acidity in order to be active at all, and both of those are properties of the outermost layer — which is exactly where cleansers and climate act.

The rivets are also not spread evenly. Their number falls away through the depth of the corneum, so the outermost cells are the least attached of all, which is what makes shedding orderly rather than random.

That single dependency accounts for a great deal of ordinary roughness. On dry skin the connections are not cleaved on time, so cells depart in sheets rather than as individuals, and a sheet large enough to see is a flake. A surface pushed toward alkaline by a harsh wash reaches the same place by a different route.

It is also why humectants and hydroxy acids end up in the same paragraph. Hydrating the surface lets its own enzymes do the cutting; an acid loosens the connections chemically. Two routes to the same rivet.