3.2 Trichology: Hair Structure, Growth & Disorders

3.2 Trichology: Hair Structure, Growth & Disorders

Quick Summary: Trichology is the scientific study of hair, its structure, functions, diseases, and care. State board examination questions heavily test hair shaft morphology, side bond chemistry, the three phases of the hair growth cycle, and clinical identification of contagious scalp conditions.

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Divisions of the Hair Structure

Human hair is divided into two primary parts:

1. Hair Root: The portion of hair located beneath the surface of the skin, enclosed within the hair follicle.
2. Hair Shaft: The portion of hair that projects above the skin surface.

Anatomy of the Hair Root

The hair root consists of five key anatomical structures:

• Hair Follicle: A tube-like depression or pocket in the skin or scalp that contains the hair root. Follicles are distributed over the entire body except the palms of hands and soles of feet.
• Hair Bulb: The lowest area of the hair strand. A thickened, club-shaped structure that forms the lower expanded end of the hair root, fitting over the dermal papilla.
• Dermal Papilla: A small, cone-shaped elevation located at the base of the hair follicle that fits into the hair bulb. It contains the blood and nerve supply that provides nutrients necessary for hair growth.
• Arrector Pili Muscle: A small, involuntary muscle attached to the side of the hair follicle. Contracted by cold or fear, it causes the hair to stand erect, resulting in "goosebumps."
• Sebaceous Gland: Oil glands attached to hair follicles that secrete sebum, lubricating the hair and scalp.

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Microscopic Structure of the Hair Shaft

The hair shaft consists of three concentric layers of keratinized cells:

1. Cuticle

• Structure: Outermost layer consisting of a single overlapping layer of transparent, scale-like cells resembling shingles on a roof.
• Function: Serves as the protective barrier for the inner cortical structures. A healthy, compact cuticle creates shine and smooth texture while managing porosity (hair's ability to absorb moisture or chemicals).

2. Cortex

• Structure: Middle, fibrous protein core formed by elongated cells. Accounts for approximately 90% of the total hair weight.
• Function: Houses all natural pigment (melanin) and is responsible for hair elasticity, tensile strength, and natural color. All chemical services (permanent waving, hair relaxing, oxidation haircolor) take place within the cortex.

3. Medulla

• Structure: Innermost central core composed of soft, round cells.
• Function: Has no known functional role in cosmetology services. The medulla is frequently absent in very fine or light blonde hair, but is present in coarse hair and beard hair.

       [ HAIR SHAFT CONCENTRIC CROSS-SECTION ]

               +-----------------------+
               |        CUTICLE        |
               |  (Outer Scale Layer)  |
               | +-------------------+ |
               | |      CORTEX       | |
               | | (90% Weight/Bonds)| |
               | | +---------------+ | |
               | | |    MEDULLA    | | |
               | | | (Inner Core)  | | |
               | | +---------------+ | |
               | +-------------------+ |
               +-----------------------+

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Chemical Composition & Side Bonds of the Cortex

Hair is composed of approximately 90% protein, primarily keratin. Protein is built from chains of amino acids linked together end-to-end by peptide bonds (end bonds). Long polypeptide chains are cross-linked laterally by three types of side bonds inside the cortex:

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The Hair Growth Cycle

Hair grows in repeating cycles divided into three distinct chronological phases:

1. Anagen Phase (Active Growth Phase)

• Duration: Lasts 2 to 6 years (average 3-5 years).
• Characteristics: New hair cells are actively produced in the hair bulb. Approximately 85% to 90% of scalp hair is in the anagen phase at any given time.
• Growth Rate: Average scalp hair growth is 1/2 inch (1.25 cm) per month.

2. Catagen Phase (Transition Phase)

• Duration: Lasts 1 to 2 weeks.
• Characteristics: Mitosis stops, the hair follicle shrinks, and the root detaches from the dermal papilla, forming a bulbous structure called a club hair.

3. Telogen Phase (Resting / Shedding Phase)

• Duration: Lasts 3 to 6 months.
• Characteristics: The final resting phase where the old hair remains in the follicle until it is shed or pushed out by a new growing anagen hair. Approximately 10% to 15% of scalp hair is in telogen.

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Hair Loss & Scalp Disorders

State board exams demand clear distinction between non-contagious hair loss / scalp conditions and contagious diseases that prohibit salon service.

Hair Loss Types (Alopecia)

• Alopecia Androgenetica: Pattern baldness caused by genetic factors, aging, and hormonal sensitivity to dihydrotestosterone (DHT).
• Alopecia Areata: Autoimmune disorder causing sudden hair loss in round or oval patches on the scalp or body.
• Telogen Effluvium: Premature shedding of hair in the telogen phase triggered by severe stress, shock, illness, or childbirth.

Non-Contagious Scalp Conditions (Serviceable)

• Pityriasis Capitis Simplex: Classic dry dandruff characterized by itchy scalp and small, translucent white flakes.
• Pityriasis Steatoides: Severe waxy/greasy dandruff characterized by an accumulation of greasy, yellowish scales mixed with sebum.

Contagious Disorders (Non-Serviceable - Refer to Physician)

• Pediculosis Capitis: Infestation of the scalp with head lice. Eggs (nits) adhere tightly to hair shafts near the scalp.
• Tinea Capitis: Ringworm of the scalp caused by a fungal infection. Characterized by red papules or spots at openings of hair follicles; hair breaks off easily leaving dull stumps.