Hair 103: Anatomy of a hair fiber

Hair fibers are composed of many layers with the three most commonly discussed layers being the medulla, the cortex, and the cuticle.


The central medulla is the only living layer of the hair fiber and contributes to the thickness of each hair fiber.  The medulla does not extend the entire length of the hair fiber and with age, shortens and thins, leading to fine hairs.

The cortex is the middle layer composed of keratins and melanin and maintains the integrity of the hair fiber from root to tip. The cuticle contains melanin, a pigment that gives hair it’s color. A separate article describes how melanin can make hair colors from blonde to red to black. The cortex is what holds the negative charge of hair and allows the cuticle to adhere. The cortex is important for maintaining the moisture content of the hair fiber.

The cuticle, commonly targeted by hair products, is the outermost layer of the hair fiber. Under high magnification the scales of the cuticle resembles overlapping shingles on a roof. This layer is susceptible to everyday elements such as heat, sun, and chemicals. These added injuries cause mechanical damage and can alter the pH of the hair. These changes can alter the cuticle causing the individual scales to swell, soften, and open. This allows increased friction which complicates the hair fibers with tangling, knotting, reduced slip of the hair, and increased breakage.

Image of cuticle: Healthy on the left and Damaged on the right.





Hair 101: Hair follicles and their life cycle

We are born with roughly 100,000 follicles on our scalp. We cannot regrow or generate new follicles after birth so what we are born with is what we have. Follicles create hair from stem cells that sit near the follicle and produce a new hair bulb (the white part located at the root end of a shed hair and is released with the hair). Over time, we lose follicles through factors such as age, hormone, or genetic follicular miniaturization or scarring disorders that destroy the abovementioned stem cells.

Blondes have roughly 20% more follicles than brunettes and redheads have about 20% fewer follicles. Asians and African Americans have fewer follicles and lower hair density than Caucasians. The follicles on the scalps of Caucasians and Asians are straight. The follicles curl and are on a greater angle on African American scalps. This curled follicle is the reason follicles are more difficult to harvest for hair transplants in African American patients.

Follicles cycle through three natural cycles, a growth phase, a rest phase, and a fall out phase, medically referred to as anagen, catagen, and telogen phases, respectively.  Each follicle individually cycles through these three phases independently of each other. Of those ~100 000 follicles, 85-90% are in anagen, <1% are in catagen, and 10-15% are in the natural fall out phase, telogen.  Follicles stay in the anagen growth phase for 2-6 years, catagen for 2-3 weeks, and telogen for 3 months before the hair is released from the follicle. Applying these numbers, you will shed  on average 100-200 hairs/day.  If these hairs are collected, it can appear as a large amount of hair but be reassured that a fresh new anagen hair helps to push out that old telogen hair and a new follicle cycle begins.

Anagen follicles grow scalp hairs at a rate of 0.35cm/day or roughly 1cm/month. This rate may increase with medications or pregnancy and decrease with age, nutrition, medications, and illnesses. Follicles only create hair in anagen growth phases.  Therefore, hairs only grow in anagen phase, so follicles in the catagen and telogen phase only rest and fall out, respectively.  The maximum length is greatest at the end of anagen phase with the exception of breakage which can shorten hairs. Proper hair care can help maintain length retention. Moisture regimens can be found at

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