3.2 Anatomy & Histology of Hair, Skin & Scalp

Anatomy & Histology of Hair, Skin & Scalp

An understanding of the anatomy and histology of the hair, skin, and scalp is essential for a professional barber. Every cut, shave, color, and chemical service interacts directly with these biological structures. A barber who understands the microscopic makeup of these tissues can prevent damage and deliver superior results.

Hair Structure: The Root and the Shaft

Hair is an integumentary structure composed of keratinized cells. It is divided into two primary parts: the hair root (located beneath the skin surface) and the hair shaft (projecting above the epidermis).

Structures of the Hair Root

The hair root is housed within the skin and relies on several specialized structures:

• Hair follicle: A tube-like pocket in the skin or scalp that contains the hair root.
• Hair bulb: A thickened, club-shaped structure that forms the base of the hair root and fits over the dermal papilla.
• Dermal papilla: A small, cone-shaped elevation at the base of the follicle. It contains the blood and nerve supply that provides nutrients for hair growth.
• Arrector pili muscle: A tiny, involuntary muscle attached to the follicle base. Cold or emotions cause it to contract, pulling the hair upright and creating "goosebumps."
• Sebaceous glands: Oil glands connected to follicles that secrete sebum, an oily substance lubricating the hair and skin.

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Three Layers of the Hair Shaft

The hair shaft is the non-living portion of the hair projecting above the skin. It consists of three distinct layers:

1. Cuticle: The outermost layer of shingle-like overlapping cells. It protects the inner structures and governs shine and porosity.
2. Cortex: The fibrous protein core middle layer containing melanin. Making up 90 percent of the hair's weight, it provides strength, elasticity, and natural color. All chemical processing takes place here.
3. Medulla: The innermost layer of round cells. It is often absent in fine or blonde hair and does not play a major role in chemical services.

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Chemical Composition and Side Bonds of the Hair

Hair is composed of approximately 90 percent keratin, a fibrous protein made of amino acids (comprising carbon, oxygen, hydrogen, nitrogen, and sulfur—the COHNS elements). Amino acids are linked by peptide bonds (end bonds) to form polypeptide chains. These chains are linked crosswise by three types of side bonds:

• Hydrogen bonds: Weak, physical side bonds easily broken by water or heat. They reform as the hair dries or cools, forming the basis of wet styling.
• Salt bonds: Weak, physical side bonds broken by changes in pH. They are vulnerable to strong acidic or alkaline solutions and reform when the normal pH is restored.
• Disulfide bonds: Strong, chemical side bonds that join the sulfur atoms of neighboring cysteine amino acids. They cannot be broken by water or heat; they are only broken by chemical reducing agents (as in permanent waving) or extreme heat.

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Anatomy and Histology of the Skin

The skin consists of two primary divisions: the epidermis and the dermis.

The Epidermis

The epidermis is the outermost, non-vascular layer of the skin. It is composed of five distinct layers:

1. Stratum corneum (horny layer): Outermost layer of dead cells that are continually shed and replaced. It acts as a waterproof barrier.
2. Stratum lucidum (clear layer): A transparent layer of clear cells found only on the palms of the hands and soles of the feet.
3. Stratum granulosum (granular layer): Composed of cells that look like granules and are filled with keratin.
4. Stratum spinosum (spiny layer): The layer where cell shedding begins. Cells have spiny appendages holding them together.
5. Stratum basale (basal or germinating layer): The deepest layer of the epidermis. Active cell division (mitosis) occurs here, and it houses melanocytes, which produce melanin.

The Dermis

The dermis (or true skin) lies beneath the epidermis and is about 25 times thicker. It consists of:

• Papillary layer: Lies directly beneath the epidermis, containing dermal papillae, tactile corpuscles (nerve endings sensitive to touch), and capillaries.
• Reticular layer: The deeper, thicker layer containing fat cells, blood vessels, lymph vessels, oil and sweat glands, and hair follicles. It provides structural support through collagen and elastin.

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Scalp Anatomy

The scalp shares the same structure as skin but has a higher density of hair follicles and sebaceous glands. The acronym SCALP helps identify its layers:

• S - Skin: Outermost layer containing sweat glands, sebaceous glands, and hair.
• C - Connective tissue: Dense subcutaneous layer containing nerves and blood vessels.
• A - Aponeurosis (epicranial aponeurosis): A broad, fibrous tendon connecting the frontalis and occipitalis muscles.
• L - Loose connective tissue: Allows the upper layers to slide easily over the skull.
• P - Pericranium: The periosteum of the skull bones.