The Hair Growth Cycle

Each hair has a life cycle in three distinct phases: anagen, catagen, and telogen. These phases occur concurrently in individual hairs, preventing hair from all falling out at once.

• Anagen (growing): Anagen is the active growth phase, which can last 1,000 days or more without interruption. On average, hair grows an average of ½ inch per month, or 6 inches in a year. As humans age, the anagen phase may shorten, producing shorter, finer hairs that may result in the appearance of thinning. How long a person’s hair grows depends on genetics and varies between genders. The Anagen phase occurs in several stages. Stages 1 through 5 are known as proanagen, and stage 6 is known as metanagen.
• In stage 1, cells at the bottom of the epithelial sac (the “secondary germ”) begin mitotic activity (cell division).
  In stage 2, the lower part of the follicle grows down and encloses part of the dermal papilla. Cells in the dermal papilla enlarge and become separated by an extracellular matrix. At the same time, the inner root sheath appears as a keratinized structure overlying the matrix.
  In stage 3, the keratinizing inner root sheath takes on a conical shape and the cortex starts to differentiate. Tyrosinase activity and melanogenesis begin the process of melanin production in melanocytes located in the matrix.
  In stage 4, cortical cells begin to become pigmented, while the cortex continues to keratinize.
  In stage 5, the hair shaft penetrates the inner root sheath at the sebaceous duct.
  In stage 6, the follicle is fully developed.
  Healthy hair growth is also contingent on proper nutrition and overall good health. Hormones also play an important role in determining the length of the growth cycle with thyroid  hormone speeding up growth in resting hair follicles and androgens (male hormones) affecting hair growth and thickness. Oestrogen slows hair growth but makes the anagen phase last longer, which is why many pregnant women report longer, lusher hair.
  • Catagen (intermediate): Melanization stops just prior to the catagen phase, which occurs when mitosis in the matrix decreases and then stops. This short phase lasts for approximately 10 days. During this time, keratinization of the hair shaft continues and the terminal portion of the hair becomes club shaped. This club remains unpigmented.¹ The base of the follicle and the club hair continue to move upward while the inner root sheath disintegrates. The vitreous (or “glassy”) membrane that is the basement membrane of the outer root sheath, thickens during this phase, while the dermal papilla loses its blood supply and extracellular matrix.
  • Telogen (resting): Telogen lasts for some 100 days during which the club is held in the epithelial sac. The dermal papilla, now devoid of its blood supply, appears as a tightly packed ball of cells. As a new hair grows it may push out the old hair, causing it to fall painlessly. At any given time, up to one in ten hairs on a human head is in the telogen phase.² It’s perfectly normal to lose up to 150 hairs each day through shedding without worrying about hair fall being excessive.
  1 Dawber, Rodney, Diseases of the Hair and Scalp, p. 7.
  2 P&G Beauty & Grooming, The Hair Growth Cycle, http://www.pgbeautygroomingscience.com/the-hair-growth-cycle.html