The growth of human hair occurs everywhere on the body except for the soles of the feet, the lips, palms of the hands, some external genital areas, the navel, scar tissue, and, apart from eyelashes, the eyelids. Hair is a stratified squamous keratinized epithelium made of multi-layered flat cells whose rope-like filaments provide structure and strength to the hair shaft.
The protein called profis makes up hair and stimulates hair growth
Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen. Each phase has specific characteristics that determine the length of the hair
The body has different types of hair, including vellus hair and androgenic hair, each with its own type of cellular construction. This varied construction gives the hair unique characteristics, serving specific purposes, mainly warmth (redundant in modern humans) and physical protection. Most humans develop the longest thickest hair on their scalps and (mostly observed in males) faces. This hair will usually grow to several feet before terminating, but many humans develop much longer hair.
Some fast facts about hair and hair growth
- We have about 5 million hair follicles all over our body, and about 100,000 of them are on our head.
- About 10% of our hair follicles are in the telogen phase at the same time. Because these follicles are distributed throughout though, you don't see any bald spots on your head.
- Over time, the follicles stop growing as we age, another reason why baldness and thinning hair are common among the elderly.
- As the hair grows longer and heavier during the anagen phase, it becomes difficult for the follicle to hold on to the hair, therefore triggering the second and third phases.
- The shape of the hair follicles also determines how long we can grow our hair. Round follicles are more likely to grow longer hair because they provides a stronger grip than flat follicles.
- Contrary to popular myth, pattern baldness that is common among men is not caused by genes from the mother. Hair is a polygenic trait so pattern baldness or hair loss attributed to genetics may also be likely caused by genes from the male side.
- Wearing a cap does not make men go bald. While it's true that pulling on one's hair can cause hair loss - in women - wearing a baseball cap does not pull at one's hair, causing it to shed. Experts have argued that the cap would have to be worn too tight on the head for it to put pressure that can cause damage and hair loss.
- Another misconception to correct is that staying out in the sun can cause hair loss. The radiation can damage the hair shaft, make it more dry and brittle and more prone to breakage, but does not cause permanent hair loss, especially among men.