Hair Structure 101
Hair Structure 101
Author: Jenna White | San Francisco Hairstylist
April 22nd, 2022
April 22nd, 2022
Hair has been a part of the human body since the earliest days of our evolutionary history. The structure of a human hair is something most people don't often think about, but there are aspects of it that make it one of the more unique parts of your body. With this in mind, I wanted to do a post on the structure of hair. Understanding this helps lay the foundation for the things you do daily to take care of it.
Hair is mainly made up of protein known as keratin, which, in other vertebrates, makes up scales, nails, feathers, horns, claws and hooves, to give a few examples. Keratin makes up between 65 - 95% of the hair strand, and is responsible for its strength and durability. This is important because, as you likely know, your hair is not a living part of your body (many toddlers are terrified of their first haircut because they don't understand this yet, and expect it to hurt). The blood in your scalp supplies oxygen and nutrients to a hair strand, but after it leaves your scalp the strand's cells are no longer living (with the exception of melanocyte cells, which provide pigment). That's why what you do to take care of your hair matters, because you are preserving something that cannot be "brought back to life". Think of a strand of hair as being broken into four parts:
Hair is mainly made up of protein known as keratin, which, in other vertebrates, makes up scales, nails, feathers, horns, claws and hooves, to give a few examples. Keratin makes up between 65 - 95% of the hair strand, and is responsible for its strength and durability. This is important because, as you likely know, your hair is not a living part of your body (many toddlers are terrified of their first haircut because they don't understand this yet, and expect it to hurt). The blood in your scalp supplies oxygen and nutrients to a hair strand, but after it leaves your scalp the strand's cells are no longer living (with the exception of melanocyte cells, which provide pigment). That's why what you do to take care of your hair matters, because you are preserving something that cannot be "brought back to life". Think of a strand of hair as being broken into four parts:
- Bulb: the part of the hair strand that sits beneath the scalp and generates growth
- Cuticle: the outer protective layer of the strand's diameter
- Cortex: the middle layer of the strand's diameter, which holds its color
- Medulla: the center of the strand's diameter, usually only present in thicker strands of hair (people with fine hair may not have a medulla)
Bulb
The hair bulb sits beneath the scalp, and receives nutrients from blood vessels. While residing there, the strand's cells are still alive. The dermal papilla, which sits near the center of the bulb (not pictured), is believed to control the growth cycles of the hair strand (you can read more about the three growth phases of hair - anagen, catagen and telogen - here). It also produces the hair's pigment (i.e. color) via melanocyte cells. The strand grows from this bulb out of a root sheath that leads to the surface of your scalp. The shape of that sheath's tunnel attributes to your hair texture (straight versus curly), along with your strand's cuticle.
Cuticle
The outer layer of the hair strand is made up of microscopic shingles, which protect it. Healthy hair has a cuticle that is smoothed down, with shingles that are flattened against the hair strand. Damaged hair has a raised cuticle, with shingles that protrude from the surface (visual here). When a hair strand has a raised cuticle, it is more susceptible to releasing moisture, which can further contribute to a damaged state.
Cortex
The cuticle's job is to protect the inside of the hair strand, which makes up the majority of its mass. The melanocyte cells that generate the strand's pigment reside here after the strand grows beyond the scalp's surface. As you age, your body produces less melanocyte cells, which contributes to gray hair and lightening of the skin. You can read more about them in my post The Science Behind Gray Hair.
Medulla
Scientists are still trying to find the best way to define the medulla, its most basic characteristic being that it resides at the center of the hair strand. On lighter, finer hair types it may not exist, and appears to play a more prominent role in gray hair. There are three types of medulla patterns recognized in human hair: (1) continuous, (2) fragmented and (3) absent.