It is in the part of the follicle known as the pilar canal that the hair fibre (or shaft) becomes detached from its epithelial sheath and is free. It emerges at the scalp through the infundibular opening after having passed through the superficial layer of the epidermis. This shaft is surrounded by hyalinised (hardened) debris excreted from the inner root sheath and sebum secreted by the sebaceous gland.
The three types of hair
- the lanugo (primitive hair or first hair): fine, short and colourless, this type of hair is specific to embryonic life and covers the whole body.
- vellus hair is found on “hairless” areas. It is longer, slightly thicker and slightly pigmented. It represents less than 10% of all the hair of a young healthy adult.
- terminal or mature hair is characteristic of “hairy” areas: it thus develops on the scalp and as the eyelashes and the eyebrows. It is thick, long and pigmented. Only the palms of the hands and the soles of the feet have no hair at all.
The cross-section of a hair
Hair generally has the appearance of a highly pigmented shaft with a diameter that varies from 70 μm to more than 100 μm depending on which phase of the cycle it is undergoing. Its cross-section is more or less circular depending on the type of hair: Asian people tend to have rigid hair with a round cross-section and a diameter that may reach 100 μm, Europeans have supple hair with an oval cross-section and a diameter of between 70 μm and 90 μm, while curly African hair has an almost kidney-shaped cross-section.