**3. Structure of hair**

Hair is defined as an accessory structure of the integument along with the sebaceous glands, sweat glands, and nails [4]. The shaft of the hair (hard filamentous part that extends above the skin surface) consists of three layers, starting from the outside: the cuticle (having several layers of flat, thin cells, overlapping one another), the cortex (containing the keratin bundles in rodlike cell structures), and the medulla (a disorganized and open area at the fiber's center) [9, 10].

In the dermis, we find the bulb of the hair, which contains the dermal papilla. It has an important role in hair formation, growth, and hair cycle [11]. Besides maintaining stem cells that regrow the hair after it falls out, it also nourishes the hair follicle (providing nutrients and oxygen to epidermal cells in the lower layer) due to the blood vessels present at the bottom of the dermal papilla [1].
