Every aspect of life is part of a cycle in one way or another. It’s no different with each cell in our body and one of the most obvious manifestations of how we go through our own growth as humans is the hair growth cycle.
People and animals are constantly moving through states of birth, growth, and death, with new births starting the cycle all over again. Plants sprout from seeds, mature, and later die to be replaced by newer buds. The changing of the seasons marks the passing of time and bears witness to the birth and rebirth of every living thing.
Babies develop follicles anywhere from three to five months while inside the womb. These follicles produce the first downy hairs called lanugo on the fetus. Although they may appear to be fine and colorless, they provide warmth, cushion, and protection for the body.
Lanugo hair sheds just around the seventh to eighth month of gestation and is replaced by vellus and terminal hairs which continue to grow, shed, and be replaced throughout adulthood.
Vellus hair resembles lanugo because of their softness, fine quality, and lack of pigmentation. They cover almost all areas of the body except for the palms, soles, lips, and genital areas.
During puberty, hormonal changes turn most vellus hair into terminal hair which are coarse, thicker, and pigmented. These are found mostly on the arms, legs, eyelashes, eyebrows, and scalp. They also start to grow on the groins of both males and females, and around the nose and jawbone in males.
The “Gens” of the Hair Growth Cycle
At birth, a baby will possess the most number of follicles they can ever have in their entire lifetime. These follicles continue to produce new hairs that grow and shed – each new growth is similar to the previous one, but their color and size depends on what their genes provide.
Anagen: The Growth Phase
The first phase in the hair growth cycle is called anagen – a long period of slow growth during which around 90% of hair lengthens by 300 micrometers per day, or around one centimeter every 28 days.
This goes on for a period ranging from two to seven, and sometimes up to ten years for scalp hair. Those growing in the eyebrows, underarms, and pubic area have a shorter cycle that lasts from four to seven months.
Hair growth during the anagen phase is affected by factors such as nutrition, disease, and stress. As the cells actively divide to lengthen the shaft – the visible part of the hair that lies above the epidermis – proper care of the hair from within will ensure that hair remains thick and healthy.
Catagen: The Regressing Phase
For a period of two to three weeks toward the end of anagen, cell division starts to slow down or regress as it enters the catagen phase. The root shrinks to 1/3 of its original size and the hair begins to break off from the root, pushing upward where it turns into a tough protein substance called keratin.
Telogen: The Resting Phase
This brief period is followed by the telogen or resting phase for approximately three to nine months when the fully grown hair stops growing. New hair starts to develop in the follicle and signals the start of a new anagen phase, with the cycle repeating itself every four to five years.
Exogen: The Shedding Phase
An article published in the Journal of Investigative Dermatology describes a “distinct phase” called exogen which is characterized by the shedding of the fully grown shafts at the rate of 100-150 strands daily.
Although some shedding may occur in the telogen phase, not all strands continue to the exogen phase because they can go through several cycles before falling off completely. This occurrence prevents a significant loss of hair at any one time.