Hair Science

Hair is far more complex than it appears on the surface. We all know that it not only plays a vital role in the appearance of both men and women, but it also helps to transmit sensory information as well as create gender identification.

The Origins of Hair

By week 22, a developing fetus has all of its hair follicles formed. At this stage of life there are about 5 million hair follicles on the body. There are a total of one million on the head, with one hundred thousand of those follicles residing on the scalp. This is the largest number of hair follicles a human will ever have, since we do not generate new hair follicles anytime during the course of our lives. Most people will notice that the density of scalp hair is reduced as they grow from childhood to adulthood. The reason: Our scalps expand as we grow.

The Science of Hair Follicles and Growth Cycles

Hair Follicles

Hair comprises two distinct structures: first, the follicle itself, which resides within the skin, and second, the shaft, the visible portion above the scalp.

The hair follicle represents a tunnel-like segment of the epidermis extending into the dermis, composed of various layers each serving distinct functions. At the follicle’s base lies the papilla, housing capillaries—tiny blood vessels that provide nourishment to the surrounding cells. The bulb, encompassing the base of the hair and enveloping the papilla, constitutes the living segment of the hair. Remarkably, the bulb’s cells divide every 23 to 72 hours, a pace surpassing that of any other cell in the human body.

Two sheaths—the inner and outer sheath—envelop the follicle, serving as protective barriers and shaping the developing hair shaft. The inner sheath traces the path of the hair shaft and terminates beneath the opening of a sebaceous (oil) gland, occasionally accompanied by an apocrine (scent) gland. The outer sheath extends to the gland, with an erector pili muscle attaching below the gland to a fibrous layer encircling the outer sheath. Contraction of this muscle causes the hair to stand upright, stimulating the sebaceous gland to secrete oil.

The sebaceous gland is pivotal as it produces sebum, a substance that conditions both the hair and skin. Sebum production increases post-puberty but declines with age, with women exhibiting less sebum production than men as they mature.

Hair Shafts

The hair shaft consists of a rigid protein called keratin and comprises three layers. Notably, this protein is devoid of vitality, rendering the visible hair a non-living structure. The medulla constitutes the innermost layer, followed by the cortex and the outermost layer, the cuticle. The cortex predominates in the hair shaft composition. The cuticle forms a tightly arranged structure of overlapping, scale-like elements. Both the cortex and medulla house the pigment responsible for the hair’s coloration.

So my only confusion on this page is that while the setup looks great essentially Anagen, catagen, and Telogen is the hair growth cycle and it’s set up as if the hair growth cycle is its own thing separate and apart from anagen, etc.

Hair Growth Cycle

Hair growth on the scalp typically progresses at a rate of approximately 0.3 to 0.4 millimeters per day, or around 6 inches annually. Unlike many other mammals, human hair growth and shedding follow a non-seasonal and non-cyclical pattern. At any given time, a random selection of hairs is in one of three distinct stages of growth and shedding: anagen, catagen, and telogen.


Anagen represents the active phase of hair growth. During this period, the cells at the hair’s root divide rapidly, leading to the formation of new hair that pushes the club hair (hair that has ceased growth or exited the anagen phase) upward within the follicle and eventually out.

Throughout this phase, hair grows at an approximate rate of 1 centimeter every 28 days. Scalp hair typically remains in this active growth phase for a duration ranging from two to six years. Notably, individuals with difficulty growing their hair beyond a certain length tend to possess a shorter active growth phase, whereas those with notably long hair boast an extended active growth phase. In contrast, hair on the arms, legs, eyelashes, and eyebrows undergoes a brief active growth phase, typically spanning 30 to 45 days, which accounts for their comparatively shorter length compared to scalp hair.


The catagen phase serves as a transitional stage, with roughly 3% of all hairs in this phase at any given time. This stage persists for approximately two to three weeks.


Telogen marks the resting phase, encompassing approximately 6% to 8% of all hairs. For scalp hairs, this phase extends for approximately 100 days, whereas hairs on the eyebrows, eyelashes, arms, and legs exhibit longer telogen phases. Throughout this stage, the hair follicle remains entirely dormant, and the club hair becomes fully developed. Plucking a hair during this phase reveals a solid, firm, dry, white material at the root. Typically, individuals shed between 25 to 100 telogen hairs per day as part of the normal hair shedding process.

Types of Hair Loss

The word “alopecia” is the medical term for hair loss. Alopecia does not refer to one specific hair loss disease — any form of hair loss is an alopecia. The word alopecia is Latin…

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