The growth of human hair occurs everywhere on the body except for the soles of the feet, the inside of the mouth, the lips, the backs of the ears, the 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 keratin 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.
The first three phases — anagen, catagen, and telogen — cover the growth and maturation of hair and the activity of the hair follicles that produce individual hairs. During the final, or exogen, phase, “old” hair sheds, though usually, a new hair is getting ready to take its place.
Each phase has its own timeline, which can be affected by age, nutrition, and overall health. That means there are steps you can take along the way to help ensure that your hair follows a healthy growth cycle
1. Anagen: Growing phase
The stages of hair growth begin with the anagen phase. It’s the longest phase, lasting about 3 to 5 years for the hairs on your head, though for some people a single hair could continue growing for 7 or more years.
Fortunately, the anagen phase differs with different types of hair. For example, the anagen phase for eyebrow hairs and pubic hairs is much shorter than the phase for your scalp hairs.
During the anagen phase, your hair follicles are pushing out hairs that will continue to grow until they’re cut or until they reach the end of their lifespan and fall out. At any time, about 90 percentTrusted Source of the hairs on your head are in the anagen phase.
2. Catagen: Transition phase
The catagen phase starts when the anagen phase ends, and tends to last about 10 days or so. During this chapter, hair follicles shrink and hair growth slows. The hair also separates from the bottom of the hair follicle, yet remains in place during its final days of growing.
Only about 5 percent of the hairs on your head are in the catagen phase at any given time.
3. Telogen: Resting phase
The telogen phase typically lasts around 3 months. An estimated 10 to 15 percent of your scalp hairs are in this phase.
Hairs don’t grow during the telogen phase, but they don’t usually fall out either. The telogen phase is also when new hairs start to form in follicles that have just released hairs during the catagen phase.
Some health experts consider the telogen phase the shedding phase, as well, but many scientists have divided this stage into two parts: the telogen and exogen stages.
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4. Exogen: Shedding phase
The exogen phase is essentially an extension or a part of the telogen stage of hair growth. During the exogen phase, hair is shed from the scalp, often helped along by washing and brushing. Losing 50 to 100 hairs per day during the exogen phase is normal.
During the exogen phase, which can last about 2 to 5 months, new hairs are growing in the follicles as old hairs fall away.
The hair follicle is a tunnel-like segment of the epidermis that extends down into the dermis. The structure contains several layers that all have separate functions. At the base of the follicle is the papilla, which contains capillaries, or tiny blood vessels that nourish the cells. The living part of the hair is the very bottom part surrounding the papilla, called the bulb. The cells of the bulb divide every 23 to 72 hours, remarkably faster than any other cell in the body.
Two sheaths, an inner and outer sheath, surround the follicle. These structures protect and form the growing hair shaft. The inner sheath follows the hair shaft and ends below the opening of a sebaceous (oil) gland, and sometimes an apocrine (scent) gland. The outer sheath continues all the way up to the gland. A muscle called an erector pili muscle attaches below the gland to a fibrous layer around the outer sheath. When this muscle contracts, it causes the hair to stand up which also causes the sebaceous gland to secrete oil.
The sebaceous gland is vital because it produces sebum, which conditions the hair and skin. After puberty our body produces more sebum but as we age we begin to make less sebum. Women have far less sebum production than men do as they age.
The hair shaft is made of a hard protein called keratin and is made in three layers. This protein is actually dead, so the hair that you see is not a living structure. The inner layer is the medulla. The second layer is the cortex and the outer layer is the cuticle. The cortex makes up the majority of the hair shaft. The cuticle is a tightly formed structure made of shingle-like overlapping scales. It is both the cortex and the medulla that holds the hair’s pigment, giving it its color.