The word "hair" often refers to two distinct structures:
- the part beneath the skin, called the hair follicle or when pulled from the skin, called the bulb. This organ is located in the dermis and maintains stem cells which not only re-grow the hair after it falls out, but also are recruited to regrow skin after a wound;
- the shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft may be divided roughly into three zones.
Hair fibers have a structure consisting of several layers. Starting from the outside:
- the cuticle which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles,
- the cortex, which contains the keratin bundles in cell structures that remain roughly rod-like; and in some cases,
- the medulla, a disorganized and open area at the fiber's center.
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