Hair! Cross-section diagrams and representations of human scalp hair.

Top diagram: Lengthwise cross-section of human scalp hair follicle (or “bulb”), displaying point of growth
Bottom diagram: Cross-section of human scalp hair, taken from follicle region. Visible hair does not contain the medullary region or fibrous tissue sheath.
Charles Darwin: Androgenic hair [beard] and male-pattern baldness of scalp hair.
Female photographs: Types of scalp hair.

Human scalp hair, as shown here, is two separate structures - a collection of dead filamentous cells, divided into roughly three regions, that extends beyond the epidermis of the scalp, and the “root”, or “bulb”, which can be seen when the hair is pulled out. This is also known as the hair follicle, and is where the hair is created.

There are multiple types of hair on the human, each with its own cellular makeup, but all following basically the same construction pattern. The three primary layers are the cuticle (a few layers of smooth, flat, thin cells, layered like roof shingles), cortex (the roughly rod-shaped keratin cell bundles, just under the cuticle), and the medulla (the innermost disorganized layer that is an open region in the center of the hair - this region is not present in all hair types).

Hair growth:

Have you ever noticed that your hair doesn’t all grow at exactly the same rate, and sometimes doesn’t get longer than a certain length? It probably has to do with the length of your growth cycle! Hair growth for each strand on your head can last for 2-7 years, and each stage of growth is happening simultaneously - that is, not every strand is shedding, growing, or resting at the same time. If that were to happen, we’d moult all at once, like birds, and go completely bald every time our hair finished its growth cycle!

Anagen phase: This is the “growth” phase, beginning in the hair follicle, and lasts between two and six years, as determined by genetics. During this phase, the cells in the follicle divide, die, and get pushed out into the visible strand. At any one time, between 80-85% of the follicles on the head are in the anagen phase.

Catagen phase: Catagen is the shortest phase of hair growth; it rarely takes longer than three weeks to complete. This is a transitional phase. When chemical signals from the body signify that it’s time to move past the anagen phase, the follicle begins to shrink and cut the hair shaft off from its blood flow. When the follicle reaches about 1/6 its original size, it is “replenished”, as its generative cells are replaced and it’s not actively creating new hair.

Telogen phase: The resting phase - the hair and follicle remain dormant for between 1 and 4 months, while the follicle regenerates. When the follicle is ready to produce hair again, it pushes the dead hair out of the scalp with the new strand, in the process known as “shedding”.

Sources:

[Wikipedia]

A Treatise on the Diseases of the Skin. Dr. Henry W. Stelwagon, 1923

Images:

A Treatise on the Diseases of the Skin. Dr. Henry W. Stelwagon, 1923

[Young woman modeling: Head, framed in flowing hair]

Charles Darwin by John Collier, 1883.

Ėti︠u︡d golovki by Sergei Mikhailovich Prokudin-Gorskii, c.1905-1915

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