What are the three parts of a hair follicle?

Hair is made up of different layers and structures.

It is the tough protein keratin that forms the strength of your hair, nails and the outer layer of skin. Keratin is a large molecule made up of smaller units called amino acids, which join together to form a chain.

The Hair Root

The epidermis, as shown in the diagram below, is the outermost layer of the skin. Each hair arises from an indentation on the epidermis. Hair is made up of two parts, the hair follicle and the hair shaft.

Hair Follicle

The hair follicle is the point from which the hair grows.

• The terminal part of the hair follicle within the skin is called a hair bulb.
• The hair bulb is the structure formed by actively growing cells.
• These cells produce the long, fine and cylindrically-shaped hair fibres.
• The hair bulb has special cells which produce the pigment that gives hair its colour.
• The pigment is called melanin and the cells producing it are known as melanocytes.
• Androgens, which are receptors for the male hormones, are located on the cells of this structure.
• The dermal papilla that feeds from the bloodstream provides nourishment to form new hair and is found at the base of the hair bulb.
• Within the skin, internal and external root sheaths cover the hair follicles.
• The outer rooting of a hair follicle has a continuous growth cycle along with the epidermis.
• Adjacent to the hair follicles are glands.
• The most important one of these glands is the sebaceous gland, as it produces and secretes the natural oils which lubricate the hair.

Hair Shaft

The part of the hair seen above the skin is called the hair shaft.

• The hair shaft is made up of dead cells that have turned into keratin and binding material, together with small amounts of water.
• This structure explains why we do not feel any pain whilst our hair is being cut.
• The hair shaft is formed of three layers:
  • The medulla – the deepest layer of the hair shaft, only seen in large and thick hairs.
  • The cortex – the middle layer of the hair shaft which provides the strength, colour and texture of a hair fibre.
  • The cuticle – the outer layer of the hair shaft is thin and colourless. It serves as protection to the cortex.

Hair grows from hair follicles situated within the fatty layer of the scalp. Contrary to the popular belief that hair grows as single strands, hair follicles actually grow in groups of 1-4 hairs called “follicular units”.

At the base of each hair follicle is a hair bulb where the growth mechanisms for producing hair occurs. Hair follicles get their nourishment from the blood vessels within the dermis. The cells divide and develop to produce the hair shaft. While the hair is still developing underneath the epidermis, it maintains a soft form. Once the pushes past the epidermis, its outside layer hardens into keratin.

Parts of the Hair

Dermal Papillae

The dermal papilla is responsible for regulating the hair cycle and hair growth, and is also comprised of androgen receptors that are sensitive to the presence of DHT.

Matrix

The matrix surrounds the dermal papillae and contains all the active cells needed for hair growth and for the development of the different parts of the hair, particularly the outer root sheath, the inner root sheath and the hair shaft. Combined, the matrix and the dermal papillae make up the hair bulb.

Outer Root Sheath

The outer root sheath, or trichelemma, is the outermost part of the hair and is keratinized. It covers the entire hair follicle inside the dermis and then transitions through to the epidermis, providing the hair follicle with an opening from which to surface from.

Inner Root Sheath

The inner root sheath is comprised of three parts: the Henley layer, Huxley layer, and cuticle. The Henley’s and Huxley’s layers are capsular layers that anchor onto each other with the purpose of stabilizing the hair. The cuticle, which is the innermost part that it closest to the hair shaft, is made from dead hardened cells and give the hair shaft added protection. This, together with the capsular layers that make up the Henley’s and Huxley’s layers, secures the hair and allows it to grow in length.

Hair Shaft

The hair shaft is the solitary part of the hair follicle that fully exits the surface of the skin. The hair shaft is made up of three layers: the medulla, cortex, and the cuticle.

The medulla is described as an unsystematic and unstructured area located in the innermost region of the hair shaft and is not always present.

The cortex, in contrast to the medulla, is highly structured and organized. The cortex is made up of keratin and is responsible for giving hair its strength and durability, as well as its water uptake. The cortex also contains melanin and determines the color of hair based on the number, distribution and types of melanin granules present.

The cuticle is the hair’s outer protective layer and is connected to the internal root sheath. It is a complex structure with a single molecular layer of lipids that helps hair repel water.

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