Hyperkeratosis refers to the thickening of the skin due to excess production of keratin. Keratin is a strong protein and a key structural material for the skin, hair, and nails. In the skin, it bonds cells together and so forms the structure of the skin. It’s made by cells called keratinocytes, and those cells make up a large part of skin, hair, and nails.

In plain English, we can say that follicle refers to a ‘tube’ made of cells, such as in the hair canal. So follicular hyperkeratosis refers to excess keratin, and the problems it causes, in the skin follicles.

Follicular hyperkeratosis is a feature in many skin problems, such as psoriasis, acne, hidradenitis suppurativa (acne inversa), and keratosis pilaris. Hyperkeratosis is one of the earliest, if not the first, step in the acne formation process.

Excess keratin contributes to comedo formation in 2 ways:

• Keratinocytes accumulate on the walls of the skin follicle. As more and more accumulate, the walls thicken, and the tube opening narrows and eventually closes completely.
• Keratin prevents dead skin cells from separating, and, when mixed with sticky sebum, forms clumps of dead cells. These clumps block the skin follicle and form a microcomedo.

There’s no single cause to hyperkeratosis. Researchers have identified several factors that either cause or contribute to the process (source):

• Exposure to squalene peroxide. Squalene is a fatty substance that’s part of sebum. Exposure to oxidative damage (e.g. UV radiation, cigarette smoke, air pollution) degrades squalene into squalene peroxide, which is a highly comedogenic substance. I covered this in more detail in the inflammation and oxidative stress page.
• Stimulation by hormones.
• Contact with P. Acnes bacteria.
• Linoleic acid deficiency in the skin follicles.
• Inflammation.