Hair Shaft and Nail Plate as Hardened Protein

A Thread and a Plate Made From the Same Raw Material

Roll a hair between your fingers or tap a fingernail against the edge of a table, and you are touching tissue in which no living cell is at work anymore. Both structures are made of cells that first filled themselves with fibrous proteins, gave up their nucleus and lost most of their water. What remains is tightly packed keratin.

Keratin is not one uniform substance, though. The outermost layers of the skin contain so-called soft keratins, which stay pliable and flake off over time. Hair and nails, by contrast, are built from hard keratins. The difference comes down mainly to one building block: the amino acid cysteine, which carries a sulfur atom and occurs with particular frequency in hard keratins and in the proteins that accompany them.

Labradoritium follows the spot where these sulfur atoms get linked to one another, how the cells at the base of the follicle and beneath the nail fold end up there, and where selenium turns up in these events, as well as where zinc does. The material is written for anyone who wants to follow such connections without prior training in biochemistry.

Where Hard Keratin Takes Its Shape

Hard keratin is produced in meaningful amounts at only two sites in the body: in the bulb of a hair follicle and in the nail matrix. Both sites lie sheltered below the surface, and both release their product in a single direction only.

The Zone Just Above the Papilla

At the lower end of the follicle, matrix cells wrap around a small knob of connective tissue called the papilla. The cells that leave this area heading upward sort themselves into three layers: on the outside the cuticle, overlapping like roof tiles; beneath it the cortex, which makes up most of the shaft; and in some hairs a loose medulla at the center.

A few rows of cells higher up, the cortex cells begin to manufacture keratin filaments in large numbers. Specialists call this stretch the keratogenous zone. Here the filaments still sit in a moist, moldable environment.

Beneath the Nail Fold

The nail matrix lies mostly hidden under the fold of skin at the base of the nail. Only its front edge shows on some fingers, as a pale half-moon. Its cells flatten as they move forward, interlock closely with their neighbors and become onychocytes, the horn cells of the nail plate. Unlike hair, no separate medulla or cortex can be made out; the plate consists of layers stacked on top of one another.

How Two Sulfur Atoms Form a Bridge

For as long as the cells are alive, the sulfur groups of cysteine are present mostly as free thiol groups. During keratinization, pairs of them are oxidized and joined into a disulfide bridge; two cysteine residues become cystine in the process. These bridges tie keratin filaments to the sulfur-rich accompanying proteins, forming a network that can barely be deformed once it has dried out.

Exactly how this oxidation is controlled has not been worked out in every detail. What is certain is that a tissue in which so many sulfur groups are meant to react at the right moment needs a balanced stock of redox enzymes and metal ions. This is the point where the two trace elements come in, each presented separately in the next section.

Arranged Element by Element

After a review by EFSA, the Commission of the European Union authorized two wordings from this subject area for selenium, and a single one for zinc. Each card quotes only what applies to its own element.

• Se · Selenium
• Zn · Zinc