Unit
- Hair follicle: The hair follicle is a complex structure embedded in the dermis, consisting of the hair bulb, dermal papilla, and sebaceous gland. Each follicle produces a hair shaft that undergoes cyclical growth and rest phases known as the hair growth cycle.
- Anagen phase: This active growth phase lasts 2 to 7 years for scalp hair, during which cells divide rapidly in the hair bulb. Approximately 85 to 90 percent of scalp hairs are in anagen at any given time, producing continuous elongation.
- Catagen phase: A short transitional period lasting about 2 to 3 weeks where hair growth stops and the follicle shrinks. Catagen marks the end of active growth and prepares the follicle for the resting phase.
- Telogen phase: The resting phase lasts around 3 months, during which the hair remains in the follicle but does not grow. At the end of telogen, the hair is shed and a new anagen hair begins to form.
- Exogen phase: Sometimes considered a separate phase, exogen is the active shedding of telogen hairs. Normal daily shedding ranges from 50 to 100 hairs, a process that is often unnoticed until disruption occurs.
- Scalp hair density: The average human scalp contains about 100,000 to 150,000 hair follicles. Hair density varies by genetics, ethnicity, and age, with gradual thinning a natural part of aging for many individuals.
- Sebaceous gland: Attached to each follicle, this gland produces sebum that lubricates the hair and scalp. Sebum helps maintain the skin barrier and influences microbial balance, but excess can contribute to conditions like dandruff.
- Blood supply: The dermal papilla at the base of the follicle receives nutrients and oxygen through a rich capillary network. Disruptions to blood flow from stress or illness can affect hair growth and lead to temporary shedding.
- Androgenetic alopecia: This common form of hair loss results from genetic predisposition and sensitivity to dihydrotestosterone (DHT). It typically follows a pattern of thinning at the crown and temples in men, while women experience diffuse thinning. Early intervention with topical minoxidil or oral finasteride may slow progression.
- Telogen effluvium: A reversible condition where a sudden shock physically or emotionally shifts large numbers of anagen hairs into telogen, causing diffuse shedding about 2 to 3 months later. Common triggers include childbirth, surgery, severe infection, or crash diets. Hair typically regrows once the underlying cause resolves.
- Alopecia areata: An autoimmune disorder characterized by well defined patches of non scarring hair loss on the scalp or body. The immune system attacks hair follicles, causing hair to fall out. Severity ranges from single patches to total scalp or body hair loss. Treatment includes corticosteroids, topical immunotherapy, and JAK inhibitors.
- Folliculitis: Inflammation of the hair follicle commonly caused by bacterial or fungal infections, though mechanical irritation or excessive sweating can also trigger it. Symptoms include small red bumps or pustules around follicles, often on the scalp, face, or body. Treatment typically involves topical or oral antibiotics and proper hygiene.
Terminology
- Anagen
- The active growth phase of hair lasting 2 to 7 years.
- Catagen
- The short transitional phase between growth and rest.
- Telogen
- The resting phase before shedding.
- Follicle
- The skin structure that produces hair.
Quick Revision
- Hair grows in cycles: anagen, catagen, telogen, and exogen.
- 90 percent of scalp hairs are in anagen at any time.
- Normal shedding is 50 to 100 hairs daily.
- Sebaceous glands produce sebum for lubrication.