Hair Structure

Hair has two distinct parts: the hair follicle and the hair shaft.

Hair Follicle: The hair follicle is a tiny pit located in the fat of the scalp from which the hair grows and is recognized as a separate entity within the skin.¹ The hair follicle is divided into two regions, the Hair Bulb and the Mid-Follicle Region.

The Hair Bulb: The hair bulb is situated inside the follicle and contains the hair matrix with actively dividing and growing cells that are rich in RNA, as well as the dermal papilla. The matrix is a group of epithelial cells combined with pigment-producing melanocytes. Cells in the matrix are some of the fastest growing cells in the human body.²The cells are several layers deep with an active turnover rate – researchers believe that each matrix cell divides every 23 to 72 hours.³ Active cell division takes place in the lower bulb and the upper bulb that is adjacent to the dermal papilla.

The dermal papilla is the vascularized, growing part of the hair made up of the connective tissue sheath and the vitreous membrane. The lower part of the dermal papilla connects to the fibrous root sheath. The cells surrounding the dermal papilla are precursors of the hair fiber.⁴ The size of the papilla is directly related to the size of the hair that is produced. In terminal follicles in the anagen phase, the dermal papilla is attached to a basal plate of connective tissue via a stalk containing papillary blood vessels. Researchers believe that the dermal papilla may determine the cyclical growth of each hair follicle.⁵ The papilla is considered the “communication link” between the hair follicle and the rest of the body.⁶ Capillaries allow blood and nutrients to pass through the papilla. If the papilla is destroyed, no further hairs will grow from that follicle.

As the cells grow out of the matrix, they push previously formed cells up toward the surface of the hair and scalp in a process of layering, hardening, and keratinizing. As the cells reach the upper part of the bulb, then arrange in layers: the outer three layers become the inner root sheath that lines the inside of the follicle, while the remaining three layers of cells become the hair’s cortex, medulla, and cuticle.

• Inner Root Sheath: The inner root sheath consists of the Henle layer (a single layer of cubical cells with clear flattened nuclei)⁸, Huxley layer (up to two layers of horny, flattened, nucleated cells)⁹, and the internal cuticle, which is one cell thick. The cuticle of the inner root sheath is interlocked with the hair cuticle via overlapping shingles, or cuticle cells, firmly anchoring the growing hair in the follicle. All three layers are formed from the matrix cells in the hair bulb. All three layers undergo differentiation at the same time, but at different rates with the Henle layer first, followed by the Huxley layer, then the cuticle. Thus, complete hardening and differentiation of the inner root sheath occurs before the layers of developing hair.¹⁰
• Outer Root Sheath: The outer root sheath surrounds the inner root sheath where the sebaceous duct enters the hair follicle. All of the cell layers within the outer root sheath are produced by the hair matrix.¹¹ However, the outer root sheath itself is not a product of the hair matrix. It consists of a sleeve of cells structured like the surface epidermis. The Outer Root Sheath is divided into two parts: a short lower part surrounding the outer part of the bulb (approximately one or two cells thick), and the upper part that reaches from the neck of the bulb to the sebaceous duct.¹² Although the exact function of the outer root sheath isn’t fully known, the outward migration of its cells may help facilitate the final movement of the terminal part of the hair at the end of the catagen phase.

Most terminal hair follicles are attached to arrector pili muscles, which are controlled by sympathetic nerves. They contract involuntarily under stress and allow the hair to “stand on end” as goosebumps. The arrector pili muscle is attached to the follicle below the sebaceous duct.

• Connective Tissue Sheath: The connective tissue sheath is a continuous thin layer of tissue that surrounds the base of the hair follicle, the sebaceous glands, and the papillary layer of the dermis.¹³
• The Vitreous Membrane: The vitreous or “glassy” membrane is the basement membrane of the outer root sheath that separates the outer root sheath from the connective tissue sheath. The vitreous membrane thickens during the catagen phase of hair growth.
• The Fibrous Root Sheath: Surrounding the vitreous layer is the fibrous root sheath consisting of bundles of thickened collagen. This is the outermost layer of the hair follicle that covers the hair follicle from the dermal papilla at the bottom to the papillary dermis above.
• The Mid-Follicle Region: Above the hair bulb is the mid-follicle region. This is where the growing cells die and harden and are pushed out of the skin by the growing cells below.

Hair Shaft

The hair shaft can be seen above the skin or scalp and consists mainly of dead cells, binding material, and water. It is made up of three layers: the cortex, medulla, and cuticle.

• Cortex: The cortex makes up the main bulk of the hair. It is at the very center of the hair shaft made up of long, vertical strands of low-sulfur keratin and are compressed into larger bundles called macrofibrils that are held together by sulphur-rich keratin.¹⁴ This combination lends the hair its super strength (a single hair can support around 100 grams – 3.5 ounces – without breaking). The cortex also contains pigment granules called melanin that is formed in special pigment-producing cells called melanocytes during the growth phase. There are two forms of melanin: eumelanin and phaeomelanin.

Eumelanin: Eumelanin exists as oval-shaped granules that give black and brown hair their dark pigment. These granules are hard in consistency and have sharply defined edges. The more Eumelanin that is present, the darker the hair will appear.

Phaeomelanin: This is a light pigment found in blondes and redheads. Phaeomelanin exists as smaller, partly oval/partly rod-shaped granules.

Differences in hair color depend on which type of melanin and how much is present in the cortex. Gray hair contains only a few melanin granules spread throughout the cortex while perfectly white hair has none. The graying process occurs when melanin production in the hair bulb gradually tapers off. Premature graying (in the 20s or 30s) usually happens as a result of a medical condition  or a particular gene.

• Medulla: Some, though not all hair shafts, contain a medulla, a hollow core that helps regulate body temperature. The actual structure includes a cortex-like framework of spongy keratin that supports thin shells of amorphous material. Scientists believe the medulla is a “throw back” to the time when our ancestors needed this extra insulation.¹⁶
• Cuticle: The cuticle is the outer protective layer of the hair-shaft that resembles tiles on a roof. It is made up of some 6 to 10 overlapping layers of long cells which circumvent the shaft, with their free margains pointing toward the hair tip. The cuticle contains three major layers: a cystine-rich A-layer, the exocuticle and the endocuticle. When the transparent cuticle cells lay flat, they reflect light and give hair its luster. Chemical and mechanical damage (sun exposure, brushing, heated tools) can cause the cuticle to lift, exposing the cortex beneath. This “weathering” weakens the hair shaft and can lead to splitting and breaking. A heavily damaged cuticle can become porous and cause hair to become excessively dry since the cortex can no longer retain the needed amount of moisture. Modern hair conditioners can smooth over the damaged cuticle to make hair appear healthier. In additional, panthenol (vitamin B5) can actually penetrate the scalp and hair shaft, providing much-needed moisture.

Covering the cuticle itself is a thin layer called the epicuticle. It is believed to be a lipid-containing surface membrane that may be chemically associated with the intercellular binding material.¹⁷

1 Dr. John Gray, The World Of Hair, p. 12

2 Wikipedia.org, Hair Follicle, http://en.wikipedia.org/wiki/Hair_follicle

3 Dawber, Rodney, Diseases of the Hair and Scalp, p. 23.

4 Dawber, Rodney, Diseases of the Hair and Scalp, p. 23.

5 Dawber, Rodney, Diseases of the Hair and Scalp, p. 32.

6 The International Association of Trichologists, The Hair and Scalp, p. 40.

7 The International Association of Trichologists, The Hair and Scalp, p. 40.

8 Wikipedia.org, Henle’s layer, http://en.wikipedia.org/wiki/Henle%27s_layer

9 Wikipedia.org, Huxley’s layer, http://en.wikipedia.org/wiki/Huxley%27s_layer

10 Dawber, Rodney, Diseases of the Hair and Scalp, p. 27-28.

11 Dawber, Rodney, Diseases of the Hair and Scalp, p. 23.

12 Dawber, Rodney, Diseases of the Hair and Scalp, p. 29.

13 The International Association of Trichologists, The Hair and Scalp, p. 45.

14 P&G Beauty & Grooming, Hair Structure, http://www.pgbeautygroomingscience.com/hair-structure.html

15 P&G Beauty & Grooming, Hair Color, http://www.pgbeautygroomingscience.com/hair-color1.html

16 P&G Beauty & Grooming, Hair Structure, http://www.pgbeautygroomingscience.com/hair-structure.html

17 Dawber, Rodney, Diseases of the Hair and Scalp, p. 35.