Hairy portals for toxic chemicals.

Hairy portals for toxic chemicals

Toxicologists tend to focus on the nose and mouth as passageways through which hazardous chemicals enter the body. Skin is another portal for these chemicals, but its role is far less understood. Conventional wisdom has held that the skin's outermost layer of dead cells served as a passive barrier, and that simple diffusion set the rate at which chemicals passed through this inert filter. But that didn't explain why skin permeability at different parts of the body can vary dramatically. A study performed at Oak Ridge (Tenn.) National Laboratory now reveals that structural factors in skin--like hairiness--can affect absorption.

Skin does not present a uniform barrier. Among the "holes" penetrating it are hair follicles, sebaceous glands and sweat glands. John Kao (now at Smith Kline and French Laboratories in King of Prussia, Pa.) and his co-workers at Oak Ridge decided to examine the role of such holes by measuring the penetration of two chemicals -- the male hormone testosterone and the pollutant benzo[a]pyrene (BP), a suspected human carcinogen -- in skin cultured from hairy and hairless strains of mice.

Their findings, reported in the June 15 TOXICOLOGY AND APPLIED. PHARMACOLOGY, show that 4.4 to 9.4 percent of the BP penetrated the skin from hairy mice--two to three times more than was absorbed through the hairless-mouse skin. Testosterone, however, penetrated both types of mouse skin equally -- about 12 to 16 times more readily than BP passed through hairy-mouse skin.

The reason for the chemical differential, explains Jerry Hall, one of the study's authors, is metabolism. The researchers had previously found that healthy skin, far from being inert, can conduct "extensive" metabolism -- or biological alteration -- of chemicals. And this can affect a chemical's skin-penetrating ability.

As fat-seeking chemicals, both BP and testosterone will preferentially deposit at the hair follicles and sebaceous glands, focal sites of metabolic activity. BP must be altered by an enzyme in order to penetrate. Testosterone, however, can pass through directly. It now appears, Hall says, that hair follicles and glands can play a major role in chemical absorption through the skin, especially with those fat-seeking chemicals like BP that require metabolic alteration for penetration.