There are three potential pathways of diffusion through the skin:
- Across the epidermal cells
Water can move freely in and out of stratum corneum cells under the influence
of humidity, temperature, and each cell's complement of water-attracting
substances. Thus, water-soluble substances can slowly make their way through
the cells into the dermis.
- Through the intercellular spaces between the stacked layers of corneocytes
Diffusion can occur between in the lipid-filled spaces between corneocytes.
The intracellular lipids provide a type of semi-permeable membrane.
That is, the membrane allows some substances to pass through while inhibiting
the passage of others.
- Through sebum and sweat
The deposition of both sebum and sweat on the skin is accomplished through
sebaceous ducts emptying into hair follicles and pores extending from eccrine
sweat glands in the dermis, respectively. Absorption of water-soluble substances,
for example, can occur through diffusion through the pores of sweat glands.
