Hair Anatomy

Hair Anatomy

Hair is actually a part of the skin—one of its appendages—which appears as a thin, thread-like projection emerging from the scalp. As mentioned earlier, the skin is made up of different layers. The outermost layer is the epidermis, and the middle layer is called the dermis. Throughout the skin, the epidermis invaginates into the dermis in certain areas, forming a tubular sac known as a follicle, where the hair is produced and grows before eventually emerging from the skin. The visible part of the hair that extends out of the skin is called the hair shaft or hair shaft proper.

The primary function of hair is protection. For example, scalp hair protects the skull. Eyebrows prevent sweat from the forehead from entering the eyes, and eyelashes act like a canopy, shielding the eyes from sunlight. Hair does not contain nerve fibers, and therefore, it has no sensation.

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In general, each strand of hair consists of two main parts: the shaft and the root. The shaft is the visible part of the hair that extends above the skin’s surface, while the root lies beneath the skin and anchors the hair within the scalp.

Figure 1: Hair Anatomy

Arrector pili muscle [1]

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A muscle called the arrector pili is attached to the fibrous layer surrounding the outer root sheath. When this muscle contracts, it causes the hair to stand upright. This action also stimulates the sebaceous gland to secrete its contents. The arrector pili muscle, often simply referred to as the pili muscle, is involuntary and not under conscious control. It connects to the central part of the hair follicle and holds the hair at an angle relative to the epidermis. In response to cold, this muscle causes the hair to stand on end.

Sebaceous gland

The sebaceous glands, also known as oil glands, are directly connected to the upper part of the hair follicle. These glands produce the oils necessary to protect and soften both the hair and the scalp. The sebaceous gland is a vital gland that secretes sebum, an oily substance. After puberty, the body produces more sebum, but as age increases, sebum production gradually decreases. These glands are distributed across the entire skin surface of the body.

Hair Structure

The hair shaft consists of three main layers: the medulla, cortex, and cuticle. These layers are illustrated in Figure 2.

Figure 2: Anatomy of Hair Layers

Hair Cuticle (Protective Outer Layer)

• The hair cuticle is a scale-like outer layer that forms the outermost part of the hair shaft.
• Cuticle scales are layered like fish scales, forming a thin, transparent, and protective covering over the hair shaft that safeguards the inner structures of the hair.
• The primary function of the cuticle is to protect the hair from environmental damage, including mechanical stress and other external aggressors.
• The cuticle is the most important factor contributing to the hair’s natural shine. Therefore, if its scale-like structure becomes damaged, the hair may appear dull, dry, and lackluster.

Cortex (Hair Shaft Core)

• The middle layer and the main structural component of the hair is called the cortex.
• It contains rope-like protein fibers that are protected by the cuticle. If the cuticle becomes damaged, the cortex is left exposed and vulnerable—leading to moisture loss and issues such as split ends.
• The thickness of the hair is determined by this layer.
• Approximately 90% of the hair’s weight is made up of the cortex.
• This layer contains keratin fibers, pigments, and dead cells, which contribute to the hair’s strength and elasticity.

Medulla (Hair Core)

• The medulla, also known as the hair core, is considered the innermost part of the hair.
• The primary function of the medulla is to nourish the hair.
• This layer is present only in thick hair; fine or vellus hairs do not have a medulla.

Hair Follicle

The hair follicle is a crucial structure located at the root area of hair growth. Its primary function is to nurture, grow, and maintain hair strands. Follicles are tubular sac-like structures in which hair strands develop and become anchored. They play a fundamental role in the growth and overall health of hair.

Variations in hair follicles are responsible for different hair types and colors. Hair follicles, composed of a complex structure of skin cells, form the hair bulb. The hair roots develop from this bulb. Nourishment of the hair root is provided through a protrusion called the papilla. Both the hair bulb and root receive blood supply, which delivers essential nutrients. This highlights the critical importance of proper blood circulation in the scalp for healthy hair growth.

Typically, a healthy individual without any medical conditions or disorders causing hair loss or thinning can have approximately 100,000 hair follicles. This number may vary slightly from person to person based on hair density and genetic factors. However, in general, it is estimated that a normal, healthy human has around 100,000 hair follicles.

Figure 3) Main Parts of the Hair

Hair Follicle Structure

1. Hair Bulge [2]

The bulge is an area located adjacent to the arrector pili muscle. This region contains a significant population of stem cells. Interestingly, the presence of the bulge enables the formation of new follicles through these stem cells. It also plays an important role in enhancing the healing process of scalp scars as well as wounds caused by irritation or acne.

2. Hair Shaft [3]

Another important part that makes up the hair follicle structure is the hair cup. As seen in images of the hair follicle, the cup is the area where the follicle is formed and where hair growth occurs.

3. Papilla

At the base of the follicle, there is a small bulge containing capillaries—tiny blood vessels—that nourish the cells. The dermal papilla is an essential part of the hair follicle structure. The papilla’s network of capillaries plays a crucial role in supplying nutrients to the follicle. Proper nourishment and blood circulation provided by the papilla are vital for healthy hair growth. Hair health and development largely depend on this important anatomical component.

4. Hair Bulb

The hair bulb [4] is the bottom part surrounding the dermal papilla [5] and constitutes the living portion of the hair. Hair bulb cells divide every 23 to 72 hours, a rate significantly faster than that of most other cells in the body.

5. Sensory Receptors

Sensory receptors are located in the hair follicle area. These receptors provide information about the position and direction of the hair. Typically, the structure and orientation of hair depend on these sensory receptors, which transmit essential signals.

6. Hair Root Sheath

The hair root sheath constitutes the most important part of the hair follicle structure. It consists of two layers: an inner sheath and an outer sheath, each with specific functions. Hair growth and overall hair health depend largely on these layers.

Figure 4) Hair Follicle Structure

Hair Anatomy Table

[1] Arrector pili

[2] Bulge

[3] CUP(Infundibulum)

[4] Bulb

[5] Dermal papillae