Hair: Videos & Practice Problems
Hair is made of dead, keratinized cells arranged in three concentric layers: the medulla, cortex, and cuticle. The medulla is the inner core and may be absent in fine hairs. The cortex forms most of the shaft and provides strength and flexibility. The cuticle is the outer protective layer of overlapping cells that helps prevent matting and protects the inner layers; when it wears away, split ends can develop.
Hair is produced by a hair follicle, a tubular structure extending from the epidermis into the dermis. At its base, the hair bulb surrounds the hair papilla, where capillaries provide nutrients and growth signals. Rapidly dividing matrix cells near the papilla become keratinized to form the hair shaft, while stem cells in the hair bulge help replenish these cells and support continued growth.
Hair growth occurs in cycles with an active growth phase and a resting phase. During active growth, matrix cells divide and the shaft lengthens; during rest, follicle activity declines until a new hair begins the next cycle. Growth rate varies by body region, sex, and age. In male pattern baldness, follicles respond to androgens by shortening the growth cycle and producing finer vellus hairs.
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 consists of three main concentric layers: the medulla, cortex, and cuticle. The medulla is the innermost layer, containing soft keratin and air spaces, but it may be absent in fine hairs. Surrounding the medulla is the cortex, which forms the bulk of the hair shaft. The cortex is made of flattened cells that provide strength and flexibility to the hair. The outermost layer is the cuticle, composed of highly keratinized, overlapping cells that protect the inner layers from damage and prevent matting. The cuticle also plays a key role in hair appearance, as its condition affects shine and smoothness. Damage to the cuticle, such as wearing away at the tips, leads to split ends and frizz.
Hair follicles are tubular structures extending from the epidermis into the dermis, responsible for producing hair. At the base of each follicle is the hair bulb, which surrounds the hair papilla containing capillaries that supply nutrients and growth signals. Rapidly dividing matrix cells near the papilla keratinize and are pushed upward to form the hair shaft. The follicle wall has multiple layers, including the peripheral connective tissue sheath 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 in cycles and respond to physiological changes.
Hair growth occurs in cycles consisting of an active growth phase (anagen) and a resting phase (telogen). During the anagen phase, matrix cells divide rapidly, producing new hair that grows longer. In the telogen phase, the matrix becomes inactive, and the follicle shrinks. When a new growth cycle begins, a new hair pushes out the old one. The length of these phases varies by body region, sex, and age, which explains differences in hair length and density. For example, scalp follicles have a long anagen phase lasting years, allowing hair to grow long, while eyebrow follicles have a shorter anagen phase lasting months, resulting in shorter hair.
Male pattern baldness is caused by hair follicles responding to androgens, particularly dihydrotestosterone (DHT), which shortens the hair growth cycle. This results in follicles producing finer, shorter hairs called vellus hairs instead of thick terminal hairs. Over time, the active growth phase becomes shorter, and the resting phase lengthens, leading to thinning hair and eventual hair loss. Treatments for male pattern baldness aim to inhibit androgen effects or improve blood flow to follicles, but they are only partially effective. Understanding the hormonal influence on follicle cycling is key to managing this condition.
Split ends occur when the hair cuticle, the outermost protective layer, wears away at the tips of the hair shaft. The cuticle is made of overlapping keratinized cells that shield the inner cortex and medulla from damage and prevent hair strands from matting. When the cuticle is damaged or eroded, the inner layers become exposed, leading to frizz and splitting. Hair conditioners help smooth the cuticle, restoring its protective function and improving hair shine and health. Proper hair care minimizes cuticle damage and reduces the occurrence of split ends.
