Hair: Videos & Practice Problems
Hair consists of three keratinized layers: the medulla, cortex, and cuticle, with the cuticle protecting inner layers and preventing damage like split ends. Hair follicles, extending from the epidermis to dermis, contain the hair bulb and papilla, where matrix cells divide to form the hair shaft. Hair growth cycles alternate between active growth and resting phases, influenced by androgens, which can cause male pattern baldness by shortening growth cycles. Understanding hair anatomy, follicle structure, and growth dynamics is essential for comprehending conditions like alopecia and the role of keratin in hair strength and flexibility.
The Anatomy of Hair: Layers and Functions
The Anatomy of Hair: Layers and Functions
Hair is more than just a style statement—it’s a fascinating structure made of dead, keratinized cells. These cells form three concentric layers: the medulla, cortex, and cuticle.
The medulla, the innermost layer, contains soft keratin and air spaces, but it’s absent in fine hairs. Surrounding it is the cortex, a bulky layer of flattened cells that gives hair its strength and flexibility. The outermost layer, the cuticle, is like shingles on a roof, overlapping to protect the inner layers and prevent matting. It’s the most keratinized part, ensuring durability and compactness.
Ever wondered why split ends happen? The cuticle wears away at the tips, exposing the inner layers to frizz. Hair conditioners smooth the cuticle, making hair shiny and healthy-looking!
Which layer of the hair is responsible for its strength and flexibility?
Hair Follicles: The Growth Factory
Hair Follicles: The Growth Factory
Hair follicles are the tubular structures that produce hair, extending from the epidermis into the dermis. At the base lies the hair bulb, which houses the hair papilla. This papilla contains capillaries that supply nutrients and signals for hair growth.
The follicle wall has layers: the peripheral connective tissue sheath, glassy membrane, and epithelial root sheath. The epithelial root sheath has two parts: the external sheath, continuous with the epidermis, and the internal sheath, derived from matrix cells.
Hair grows as matrix cells divide rapidly near the papilla, pushing older cells upward. These cells become keratinized and die, forming the hair shaft. Stem cells from the hair bulge replenish matrix cells, ensuring continuous growth.
What is the function of the hair papilla within the hair bulb?
Hair Growth Cycles and Changes Over Time
Hair Growth Cycles and Changes Over Time
Hair growth follows cycles: an active growth phase and a resting phase. During the active phase, matrix cells divide rapidly, and hair grows. In the resting phase, the matrix becomes inactive, and the follicle shrinks. New hair pushes out the old one at the start of the next cycle.
Hair growth rates vary by region, sex, and age. Scalp follicles stay active for years, allowing long hair growth, while eyebrow follicles are active for only months. Hair growth peaks in the teen years and slows by the 40s, leading to thinning and balding.
Male pattern baldness occurs when follicles respond to androgens with shorter growth cycles, producing fine vellus hairs. Treatments aim to inhibit androgens or improve blood flow but are only partially effective.
What happens during the resting phase of the hair growth cycle?
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Hair is composed of three concentric keratinized layers: the medulla, cortex, and cuticle. The medulla is the innermost layer containing soft keratin and air spaces, but it is often absent in fine hairs. Surrounding the medulla is the cortex, which consists of flattened cells that provide hair with strength and flexibility. The outermost layer is the cuticle, made of overlapping keratinized cells resembling shingles on a roof. This layer protects the inner layers from damage and prevents matting. The cuticle is also responsible for hair's durability and compactness. Damage to the cuticle, such as from split ends, exposes the inner layers, leading to frizz and hair breakage. Hair conditioners work by smoothing the cuticle, enhancing shine and hair health.
Hair follicles are tubular structures extending from the epidermis into the dermis, responsible for producing hair. At the base of each follicle lies the hair bulb, which contains the hair papilla. The papilla houses capillaries that supply nutrients and growth signals to the follicle. Surrounding the papilla are matrix cells that divide rapidly, pushing older cells upward. These cells undergo keratinization and die, forming the hair shaft. The follicle wall consists of multiple layers, including the peripheral connective tissue sheath, glassy membrane, and epithelial root sheath. Stem cells located in the hair bulge replenish matrix cells, ensuring continuous hair growth. This complex structure allows hair to grow, renew, and maintain its integrity.
The hair growth cycle consists of two main phases: the active growth phase (anagen) and the resting phase (telogen). During anagen, matrix cells in the hair bulb divide rapidly, leading to hair elongation. This phase can last several years on the scalp, allowing hair to grow long. In the telogen phase, the matrix becomes inactive, and the follicle shrinks. Hair growth stops, and the old hair is eventually pushed out by new hair at the start of the next anagen phase. The duration of these phases varies by body region, sex, and age. For example, scalp follicles have a longer anagen phase compared to eyebrow follicles, which is why scalp hair grows longer. Changes in these cycles influence hair length and density over time.
Male pattern baldness is primarily caused by the follicles' response to androgens, particularly dihydrotestosterone (DHT). In affected follicles, androgens shorten the active growth phase (anagen) and lengthen the resting phase (telogen). This results in the production of thinner, shorter hairs called vellus hairs instead of thick terminal hairs. Over time, the follicles shrink and produce less hair, leading to visible thinning and balding. Treatments for male pattern baldness aim to inhibit androgen effects or improve blood flow to follicles, but their effectiveness is limited. Understanding the hormonal influence on hair growth cycles is essential for developing therapies for androgenic alopecia.
Keratin is a fibrous structural protein that forms the primary component of hair. In the cortex layer, keratinized cells are tightly packed and flattened, providing mechanical strength and flexibility to hair strands. The cuticle, being the most keratinized part, protects the inner layers from physical and chemical damage, maintaining hair integrity. The keratin in hair contains disulfide bonds that create cross-links between protein chains, enhancing durability and elasticity. This molecular structure allows hair to withstand stretching and environmental stress without breaking easily. Damage to keratin, such as from heat or chemicals, weakens hair and leads to issues like split ends and frizz.
