Hair Growth Cycle

The process of hair growth initiates within the hair follicle, which shifts from a dormant to an active state influenced by hormones and external factors. Hair grows for a period before halting growth due to a signal, leading to the degradation of the inner sheath, the hair being pushed to the surface, weakening its connection to the root, and eventually shedding. After a pause, hair growth begins anew, perpetuating this cycle known as the hair cycle.

Hair cycles through stages of growth in phases: anagen (active growth), catagen (regression), and telogen (resting). Following a robust growth phase, the hair enters a brief regression before moving into a resting phase, after which the hair sheds, making way for new growth. It’s typical to lose about 100 strands daily due to this cycle.

Hair Structure

Hair Formation

During embryonic development, hair forms as lifeless extensions from follicles, which are expansions of mesenchymal cells into the dermis or subdermal tissue.

Follicles, varying in depth across the body, comprise three primary sections: the lower, middle, and upper parts. The lower section, the most dynamic in hair formation, includes the dermal papilla, matrix, and root sheaths.

  • Dermal Papilla: An egg-shaped structure that initiates hair growth signals via cytokines to follicular cells.
  • Matrix: Surrounding the papilla, these rapidly dividing cells are crucial for hair production.
  • Inner and Outer Root Sheaths: Support structures for the growing hair, with the outer sheath’s full function yet to be understood.

Layers of Hair

  • Medulla: The innermost layer, partially keratinized, and not always present in every hair type.
  • Cortex: Contains tightly packed cells that contribute to hair’s mechanical properties and color.
  • Cuticle: The outer layer, consisting of flattened, keratinized cells overlapping like scales, protects the hair.

Physical Properties of Hair

  • Flexibility: Allows hair to alter shape and return to its original form, with the cortex’s keratin filaments playing a key role.
  • Static Electricity: Can cause hair frizz, especially under dry conditions, mitigated by moisturizers and shampoos.
  • Moisture Content: Hair’s ability to absorb moisture affects its combability and appearance.
  • Porosity: Indicates how well hair can retain moisture, impacting its health and styling.

Hair Cycle Revisited

The hair cycle, with its anagen, catagen, and telogen phases, regulates hair growth, transition, and shedding. This cycle includes specialized sub-phases like proanagen and metanagen, emphasizing the complex nature of hair growth and renewal.

Hair Development

From prenatal development with the presence of lanugo hairs to postnatal changes leading to vellus and terminal hairs, human hair undergoes significant transformations. This evolution continues with age, influenced by hormonal changes, marking the dynamic and responsive nature of hair growth throughout life.