Hair Anatomy, Mechanism & Hair Follicle: Concerns & Care
Hair is a miracle body part made of non-living keratin, rooted in living skin tissue, the hair follicle. Understanding hair anatomy, mechanism and hair follicle structure is essential for proper hair care and health.
The human body has an average of five million hair follicles that give rise to body hair. Additionally, hair has a complete mechanism for growth, nurturing, and protection, all done by hair follicle.
This article explores hair structure, functions of the hair follicle, and the hair growth cycle. It will also discuss hair loss caused by hair anatomy disorders, their identification, treatments and hair health management tips.
What Is Hair?
Hair is a thin, thread-like structure spread all over the skin of humans and some animals. It consists of a lifeless, hard protein called keratin. Hair is a crucial characteristic of mammals, known for its ability to regulate body temperature and sensory perceptions.
Why is Hair Important?
The study of hair includes its types, colors, growth, and functions. Hair anatomy plays a pivotal role in its color and functions. The main purposes of hair include:
- Hair shields the skin from harmful sun rays, dust particles, and other environmental factors.
- Hair prevents injury by forming a cushioned layer over the skin, specifically the head.
- It helps the body maintain its temperature by holding heat and allowing sweat to evaporate.
- Hair allows us to perceive movements and touch, and detect environmental changes.
Basic Hair Anatomy & Physiology
The structure of hair consists of two major parts, the hair shaft and the follicle.
- Hair Shaft: The visible outer part of hair above the skin is the hair shaft. It has three layers:
- Cuticle
- Cortex
- Medulla
- Hair Follicle: An organ within the skin that surrounds and holds the hair strands. It is composed of five basic structures:
- Hair bulb
- Dermal papilla
- Sebaceous gland
- Inner and outer root sheaths
- Arrector pili muscle
What Are The Layers Of The Hair Shaft?
The hair shaft is a non-living, cylindrical, thread-like structure that extends above the skin. Each hair strand is made of a protein, keratin. This part of the hair can be styled using various styling techniques. The functions of the three parts of the hair shaft are discussed below:
Hair Cuticle
The cuticle is the outermost layer of a hair strand and contains keratin. It is made of overlapping cells, like the scales of a fish. This layer protects the inner hair layers, i.e, cortex and medulla, and helps maintain moisture. The hair cuticle determines the texture, shine, and luster.
Cortex
The cortex is the middle and thickest layer of the hair shaft. It is also known as the main body of the hair, as the presence of keratin helps it strengthen and provides elasticity to the shaft. It also contains pigments that are responsible for the hair color.
Medulla
The medulla is an inner layer of a hair strand. It is the softest and most fragile of the hair’s layers, and its presence depends on the stage of the hair cycle.
It is made of empty or filled canal cells that are either continuous or fragmented, resulting in different forms of medulla. In some mammals, the medulla can be completely absent.
Hair Root
The hair root is the portion of a hair strand within the skin. Each hair roots deep into the skin, surrounded by a hair follicle. The hair root is the living part that secures a hair strand in its place and is surrounded by the hair follicle.
Additionally, it is responsible for transferring nutrients from the follicle to the hair’s growth center.
See Also: All Hair Types
What Is The Hair Follicle?
A hair follicle is a tubular mini-organ present within the epidermal layer of skin from which a hair strand grows. Millions of hair follicles are present in our skin. These hair follicles extend from the upper layer of skin to the dermis and subcutaneous tissue.
A hair follicle has many cells and tissues, attached to blood vessels and glands. It holds the hair structure and plays a role in hair regrowth, thermoregulation, and immunologic response.
Hair Follicle Histology: Components & Functions
The hair follicle anatomy is composed of three main regions: infundibulum, isthmus, and bulge region.
- Infundibulum: The uppermost area extending from the skin to the opening of sebaceous gland. The outer root sheath (ORS) begins from this region.
- Isthmus: Located below the infundibulum, between the sebaceous duct and bulge region, containing ORS, inner root sheath (IRS), and arrector pili muscles.
- Bulge: The lowest region which has hair follicle stem cells and the arrector pili muscle.
Below these regions are other components, i.e., hair bulb, hair matrix, and dermal papilla, which function collectively to produce and nourish hair.
- Hair Bulb
An integral component providing nutrients for growth. It is a bulb-like structure present at the base of the hair follicle that encloses the dermal hair papilla and hair matrix.
- Hair Matrix
A group of epithelial cells that divide continuously and give birth to new hair. These cells extend upwards and form keratinocytes that are responsible for the dead structure of the hair shaft.
The hair matrix also contains melanocytes that provide color to the hair shaft. In addition, it produces hair sheaths that surround and protect the shaft.
- Dermal Papilla
A network of blood vessels and connective tissues. It ensures the necessary supply of oxygen, nutrients, and growth signals to matrix cells for hair regrowth. It also provides components that determine the texture, thickness, and color of the hair shaft.
Hair matrix and dermal papilla together play a vital role during the anagen and telogen phases of the hair cycle to give rise to hair shaft and root hair sheaths.
- Sebaceous Gland
The sebaceous gland is a gland located in the dermis and connected to the hair follicle. It releases an oily substance, sebum, into the skin and hair, which keeps the hair and scalp moist, protecting them from dryness and damage.
- Arrector Pili Muscle
The arrector pili or hair erector muscles are small, smooth muscles that attach the hair follicle to the upper layer of skin. It erects and straightens the hair strand during cold weather or mental stress, causing goose bumps. In addition, it helps the body conserve heat by providing insulation to the skin.
The hair follicle, sebaceous gland, and arrector pili muscle are together named as a single unit, called the pilosebaceous unit.
- Inner Root Sheaths (IRS)
The Inner Root Sheath protects and supports the hair layers with Henle’s and Huxley’s layers. It extends from the hair matrix cells from the hair bulb to the hair strand to give it shape and texture.
- Henle’s Layer: It is the IRS’s outermost layer, made of a single sheet of cuboidal cells.
- Huxley’s Layer: Below Henle’s layer is Huxley’s layer, which consists of flat cells with grainy protoplasm.
- Outer Root Sheaths (ORS)
A single hair sheath present within the hair follicle around the IRS. It supports the hair follicle structure and extends from the dermis to the epidermis. ORS is also produced by the hair matrix and contains stem cells in the bulge area that regrow hair and repair damage in the skin.
- Follicle Stem Cells
Hair follicle stem cells, located in the bulge area of the follicle, rapidly increase hair growth during the hair cycle. They cause hair regeneration and skin healing during an injury by regrowing the damaged area in the epidermis and sebaceous glands.
See Also: Stem Cell Therapy
- Nerve Endings
The nerve endings wrap around the base of the hair bulb. When the hair moves or is pulled, these nerves activate and transfer a sensory input that indicates movement or a twist in the hair.
What Hair Is Made Of: The Chemical Composition
Keratin is the most abundant molecule in hair. Our hair consists of cells, keratinocytes that make up this protein. The substances in hair structure include:
| Components of Hair | Function | |
| Proteins(65-95%) | Keratin | Responsible for the dead hair structure and provides strength, shape, and elasticity to a hair strand. |
| Lipids(3-4%) | Fatty acids, cholesterol, squalene, and sebum | Maintains hair shine and flexibility, prevents hair dryness, and environmental damage. |
| Water(12-15%) | Water | Hydrates and provides elasticity, flexibility, and regulates overall hair health. |
| Pigments(1-2%) | Melanin | Gives color to the hair, while the shade varies with the amount and type of melanin present:Eumelanin: Black and brown shadesPheomelanin: Blonde and red shades |
| Minerals & Trace Elements | Calcium, copper, zinc, iron, silicon & others | Provides strength and support for healthy hair growth, regulates metabolism, and prevents loss. |
The Hair Elemental Composition
The elements present in the structure of hair consist of:
- Carbon (45-50%)
- Hydrogen (6-7%)
- Oxygen (22-28%)
- Sulphur (4-5%)
- Nitrogen (15-17%)
Hair Molecular Anatomy & Architecture
A hair strand mainly consists of dead keratin, a protein made of amino acids that are bonded together in complex arrangements. These amino acids include alanine, cysteine, leucine, serine, arginine, etc. The types of bonds between these amino acids and the molecular structure contribute to the shape and strength of the hair shaft.
- Disulfide Bonds
The high percentage of cysteine in keratin contributes to the formation of disulfide (S-S) bonds that strengthen and support hair.
- Helical Structure
The amino acids form a special structure, the alpha helix, through hydrogen bonding. It is a spiral-shaped structure, also known as the building block of proteins.
- Protofibrils, Microfibrils, & Macrofibrils
Two alpha helices join to form a twisted, coiled shape, called intermediate filaments or protofibrils. The bundles formed by the twisting of protofibrils are microfibrils. Eleven protofibrils in hair form one microfibril. The aggregation of microfibrils forms macrofibrils, which are aligned parallel to form a hair strand.
Hair Growth Mechanism
Hair growth is a cyclic process for producing and elongating hair strands, primarily propelled by hair follicles. The stem cells in our hair bulge continue to expand and activate follicles for regrowing hair throughout a person’s lifetime. Most hair on our body remains in the growing (anagen) phase.
Hair Growth Cycle: How Does Hair Grow?
The hair follicle goes through four essential stages, i.e., growth, transition, rest, and shedding. Here’s a quick overview of these stages:
- Anagen: The Growth Phase
During the anagen phase, the new hair grows from the epithelial stem cells present in the bulge region. The dermal hair papilla in the inferior region stimulates these cells. New cells grow and pull the existing cells outward, which stick together and harden to form a hair strand.
The inferior segment forms the anagen hair bulb growing downward after the activation of follicle stem cells. Melanocytes also supply pigments for hair color during anagen, which remains with the hair during its life.
This is the longest phase of the hair cycle.
The scalp hair remains for 2 to 7 years in the growing phase, while body hair and eyebrows remain for a few months.
- Catagen: The Transition Phase
The hair growth speed reduces during catagen due to the stoppage of blood supply to the hair. In this stage, the hair follicle starts shrinking, and the hair papilla is detached from the follicle, which supplies blood. Club hair is formed at the base of the hair follicle, a bulb-shaped keratinized structure with a white node at the end.
It is a short phase that lasts up to 2 weeks, with only 3-5% hair being in this phase at any time. The hair follicle is prepared for rest during catagen.
- Telogen: The Resting Phase
The hair follicles become inactive due to blood disconnection, and no new hair grows during telogen. It lasts for 2 to 4 months, and the hair shaft neither grows nor sheds. The hair strand is inactive and is prepared to shed.
- Exogen: The Shedding Phase
Exogen is a prolonged telogen phase during which hair gradually pulls out from the scalp. In this way, the hair starts shedding faster than normal. This phase is very important as it activates new cells in the hair follicle. About 50-100 hairs shed over a day during exogen. It is the shortest phase and is followed by anagen.
How To Keep Hair Healthy Forever?
Healthy hair enhances appearance and boosts self-confidence, making hair maintenance essential. Neglecting hair’s health makes it dry and brittle, and will let you lose your spark. Here are a few hair management tips anyone can follow:
- Wash your hair 2-3 times a week, depending upon its moisture content.
- Use mild shampoos and apply them only to the scalp.
- Apply conditioner after every wash to hydrate and lock moisture.
- Avoid vigorous washing and drying patterns that cause hair breakage and split ends.
- Protect your hair from sun exposure and heat styling tools, i.e., hair straightener or curlers.
- Take a healthy diet containing fruits, vegetables, and wheat grains that provide necessary nutrients for hair health.
- Drink plenty of water to ensure your scalp and hair get hydrated.
- Avoid applying harsh chemicals to hair, such as dyes and perms.
- Regularly massage the scalp to ensure smooth blood circulation and reactivate dormant hair follicles naturally.
- Exercise or walk regularly to reduce stress-related issues.
What Factors Affect Hair Structure & Function?
The factors that impact hair anatomy and overall health include:
- Genetics
- Environmental conditions
- Hormonal issues
- Diet
- Physical and emotional stress
- Aging
- Medication side effects
- Smoking
How Hair Anatomy Leads to Hair Loss?
Hair follicles are tiny organs responsible for hair production and growth. However, any disruptions can cause harm to them, leading to hair loss. Here are some examples of hair follicle conditions that can cause hair loss:
- Hair Follicle Miniaturization
Hair follicle miniaturization is a process in which hair follicles start shrinking and thereby resulting in the production of short, thin, and weak hair.
A male-specific hormone, Dihydrotestosterone (DHT), binds to androgen receptors in hair follicles. It causes them to shrink and reduce the blood flow to hair follicles, resulting in male or female pattern baldness, also called androgenetic alopecia. Sometimes, genetic disorders can also cause hair follicle miniaturization.
- Autoimmune Hair Loss
Autoimmune hair loss occurs when the body’s immune system accidentally attacks and damages hair follicles, primarily affecting the hair matrix cells. As a result, hair shedding produces round bald spots on the scalp. This specific hair loss condition is also known as alopecia areata. It also affects body hair, eyebrows, and eyelashes.
- Hair Loss Due To Infection
Sometimes, a bacterial or fungal infection damages hair follicles, pulling out hair strands and leading to temporary or permanent hair loss. Folliculitis is the most common among these, activating a hair loss condition with inflammation called scarring alopecia.
Tinea capitis, impetigo, and syphilis are also examples of infectious hair loss conditions.
- Mechanical Hair Loss
Mechanical hair loss, or traction alopecia, happens due to continuous tension on hair follicles by tight hairstyles, hair extensions, or accessories. It loses hair strands from the base, causing them to fall out. In addition, it also results in dry, brittle hair and split ends due to excessive breakage.
- Hair Growth Cycle Disruption
Hair growth is a natural process with hair passing through different stages from growth to shedding. Some hormonal, genetic, or environmental factors disturb the hair growth cycle, which reduces the duration of the anagen phase. It eventually results in prolonged telogen and exogen phases, causing hair to fall out scarcely.
The hair loss appears in the form of bald patches or hair thinning around the scalp.
- Chemotherapy-Induced Hair Loss
Chemotherapy is a cancer treatment that has major side effects, including hair loss. The medications used in this therapy attack and damage the follicle stem cells in the hair bulb, which initiate hair growth. It results in complete hair loss around the scalp, face, and body.
How to Identify Hair Follicle Disorders? Signs & Symptoms
Hair follicle disorders cause hair loss and thinning. See your doctor if you notice the following signs in your hair or scalp:
- Thin hairline
- Scarring
- Swelling
- Redness
- Slight bumps or pimples
- Pain and tenderness
- Bald patches
How do Doctors Test Hair Loss Conditions?
Different hair loss types have their specific signs and symptoms; however, certain tests are also performed to identify the condition. Your dermatologist will perform the following tests to diagnose the real cause of hair loss:
- Trichoscopy
- Hair Pulling test
- Blood test
- Scalp biopsy
What Are The Best Treatments For Hair Loss?
Hair loss treatments restore hair follicle health by ensuring regular blood flow, reactivating the stem cells, or transplanting new hair. The hair restoration methods recommended by surgeons include:
- Medications such as minoxidil, finasteride, or corticosteroids
- Platelet-rich plasma therapy
- Laser therapy
- Hair transplant surgery
- Immunotherapy
- Diet and stress-control
This article is medically reviewed by Medical Aesthetic Dr. Ali Khalil (PHD)
See Our Doctors & Surgeons
What is the black bulb in hair falling out?
Hair falling out during the growth (anagen) phase of the hair cycle has a black bulb at the end. While hair shedding during the exogen phase leaves a white node.
Do hair follicles grow back if pulled out?
Yes, the hair follicle grows back during the natural hair cycle if not affected by permanent hair loss.
How to tell if hair follicles are dead?
If you notice no new hair growth for an extended period, and the smoothness and shine on the scalp, it indicates that hair follicles are dead or dormant.
How many times do you need to pluck a hair before it stops growing?
Plucking hair doesn’t stop it from growing, no matter how many times it has been done. If a hair follicle is damaged due to plucking, it will heal and regrow hair after a short interval.
At what age does hair stop growing on your head?
The hair growth cycle doesn’t stop throughout the lifetime of a man or woman. However, in their 40s or 50s, it slows down and hair starts becoming thinner and weaker.
What has the thickest hair?
Asian ethnicity has the thickest hair strands, while Caucasian groups have the densest hair.
How many new hairs grow a day?
On average, a person sheds about 50 to 100 hairs per day as a normal part of the hair cycle.
