Hair is made of two separate structures: the Hair Shaft – the part you can physically see above the scalp – and the Hair Follicle – the part that you cannot see.
The outer covering of the hair shaft is known as the Cuticle. It is made of flattened cells that overlap each other like scales. The cuticle protects the inside of the hair shaft from damage.
Read on as we break down the hair structure in the simplest way possible. We will also cover:
- How does hair grow?
- Hair growth phases
- Rate of hair growth and
- How to promote healthy hair growth
Why do we care about the hair structure?
Understanding the hair structure is key to fully knowing what your hair needs to thrive and grow well. This is a pre-requisite to understanding things like hair porosity, hair texture, curl pattern and much more.
The Hair Structure
Hair is composed of two main layers: the hair shaft and the hair follicle each with individual component as illustrated below.
The hair shaft is made up a tough protein called Keratin which is responsible for making the hair strong and flexible. The Hair shaft is further divided into 3 layers: an outer covering known as the Cuticle, an a Cortex and an inner Medulla layer,
The Cuticle is the outer covering of the hair shaft. It is a thin protective layer that contains high amounts of keratin, composed of scale-shaped cells that are layered one over the other.
The Cortex is the middle layer of the hair shaft. It is the thickest hair layer that contains long keratin chains that are responsible for hair elasticity and resistance.
Joining together the cells of the cortex is some intercellular cement rich in lipids and proteins.
This is the innermost layer of the hair shaft, composed of an amorphous, soft, oily substance. It is the most soft and fragile layer of hair.
The Hair texture is determined by the shape and structure of both the cortex and the medulla whose shapes are also determined by the shape of the hair follicle.
The hair follicle is the primary structure through which hair grows. It is further divided into an outer and inner root sheath.
The follicle is club shaped. At the end, there is a Papilla which is a network of blood vessels that are responsible for supplying hair with nutrients.
Surrounding the Papilla is the hair bulb which is the part of the follicle that actively produces hair.
The bulb is associated with the Sebaceous gland. The Sebaceous gland secrets Sebum that natural lubricates and conditions the hair. 
Hair Structure and Color
The hair follicle isn’t just responsible for hair growth, it also influences what the hair looks like.
For instance, the shape of the follicles determines the hair texture (straight, curly, wavy or coily) the Oval follicles produce curly hair while circular follicles produce straight hair.
Hair follicles also plays a role in determining the hair color, it stores melanin which is determine the hair pigment. Melanin exists in two forms: eumelanin and pheomelanin.
Genetic makeup will determine whether you have eumelanin or pheomelanin, as well as how much of each pigment you have. Presence of Pheomelanin makes hair red while an abundance of eumelanin makes hair black. A moderate amount of eumelanin makes hair brown, and very little eumelanin makes hair blonde.
How Does Hair Grow?
New cells are constantly forming in the hair bulb. These cells stick together and harden. As this happens, a full strand of hair develops from the hardened hair cells.
As the new hardened cells keep on attaching to the hair from below, it is gradually pushed up out of the skin.
The papilla, which is a network of blood vessels constantly supplies oxygen and nutrients to the hair follicle thus promoting hair growth.
The growing hair passes through the sebaceous gland which lubricates the hair making it soft and shiny.
Hair Growth Phases
Hair grows and sheds in phases. The anagen phase is the first phase, during which active growth occurs. The catagen phase, which is a much shorter phase marks a transition from the hair follicle’s active growth to the resting phase.
During the telogen phase, the hair follicle is at rest and no new growth occurs. The final hair growth phase is the exogen which is when hair sheds off.
On average, 50 hairs are lost and replaced per day. Hair loss occurs if there is more hair shed than what is replaced.
1. Anagen Phase
This is the active growth phase. During this phase, cells within the hair papilla continuously divide and form new hair fibers. The length of this phase is determined by genetics, on average it can last up-to 8 years.
The longer the hair remains in this phase, the longer the hair grows. About 85% of the hair on your head is in Anagen phase all the time.
2. Catagen phase
The catagen phase is the shortest phase lasting anywhere from 2-3 weeks. It’s called a transitional phase and begins immediately the Anagen phase comes to an end.
During this phase, hair becomes detached from the dermal papilla; it loses its blood supply and becomes pushed out of the dermal layer.
3. Telogen phase
The Telogen phase is also referred to as the resting phase which last between 1 to 4 months. The hair follicle lies dormant during this phase. 6% to 8%of hair is in this phase.
The exogen phase is also known as the shedding phase. This when the hair sheds off giving room to a whole new growth process to begin.
On average this phase lasts between 2 to 5 months.
As the growth cycle is very specific each individual hair follicle, hair does not fall out at once.
Hair growth Rate
Have you ever wondered how long hair grows in a month?
A study done by the American Academy of Dermatology showed that on average hair grows about ½ an inch per month.
As this is an average, there are great variation in the growth rate from race to race.
Read this article to learn more: How fast does hair grow?
Factors determining hair growth
Your genes determine how fast or slow your hair grows. Genes also dictate your hair color, texture and even volume.
Hair grows faster and fuller when younger and slow down with age. As we get older, the rate of new follicle development slows down.
Health & nutrition
Your body needs proper nutrition to promote nourishment and healthy follicle development. Other factors include state of mental and physical health.
For instance, some studies show that stress can lead to hair loss as it leads to a temporary condition called Telogen effluvium that is characterized by hair loss.
Exercise is also an important part of physical health that ensure continuous blood supply to your scalp thus nourishing the hair follicles.
Due to the higher testosterone level in male, their hair tends to grow faster and thicker.
Hair care practices
Poor hair practices such as excessive use of heat and chemicals may lead to slow hair growth.
Other practices that affect growth rate is they types of products you use, how well you moisturize and condition the hair and how well you protect the hair from physical damage
How to promote Healthy hair growth
Here are key things you need to considered in order to grow healthy hair
- Have a proper nourishing diet
- Stay hydrated
- Maintain physical fitness
- Moisturize and condition properly
- Use the right products and tools
- Protect your hair from physical and chemical damage
Did you know hair porosity plays a key role in hair growth? Take this simple hair porosity test to determine if your hair is HIGH, LOW or NORMAL porosity.