A hair follicle is part of the skin that grows hair by forming old cells together. Attached to the follicle is a sebaceous gland, a tiny sebum-producing gland found everywhere except on the palms, lips and soles of the feet. The greater the density of hair, the more sebaceous glands will be present. Also attached to the follicle is a small bundle of muscle fibres (arrector pili) responsible for causing the follicle to become more perpendicular to the surface of the skin, and causing the follicle to protrude slightly above the surrounding skin (piloerection). This process is familiar to us as goose bumps (or goose flesh). Stem cells are located at the junction of the arrector and the follicle, and are principally responsible for the ongoing hair production during a process known as the Anagen stage. The hair growth cycle consists of three stages:
- Growth Phase (Anagen)
- Transition or “Shrivelling” phase (Catagen)
- Resting Phase (Telogen) the old hair falls out and the Growth phase begins again. The average growth rate of healthy hair follicles on the scalp is .04 cm per day or about 1cm a month.
During the Anagen or growth phase, the cells in the root of the hair are dividing rapidly, extending the hair shaft. The matrix of the hair surrounds and feeds the hair during its’ stage of growth. During the Anagen or growth phase the hair is most receptive to Laser Hair Removal as it is rich in protein, keratinized cells and melanin. The amount of time the hair follicle stays in the Anagen phase is determined by your genetic make-up. A hair’s Anagen phase may last for several years, with Anagen hairs representing about 85% of our hair.
At the conclusion of the Anagen phase the follicle enters the Catagen or transition phase. During the Catagen phase, chemical and structural changes occur in the hair follicle causing the hair to retract and move away from the dermal papilla. The hair stops growing, and remains in this phase for only two to three weeks before moving into the next phase. Approximately 5% of our hair is in the Catagen phase at any time. The next stage is the Telogen or resting phase when the hair begins to shed while the follicle is stimulated to begin a new cycle. The length of this stage varies significantly, with some hair replaced immediately as a newly growing hair pushes the “resting” hair out of the follicle. The new hair emerges from the same opening at the surface of the skin as the old one, repeating the hair growth cycle again. Pulling out a hair in this phase will reveal a solid, hard, dry, white material at the root. Approximately 10% of our body’s hair is in the Telogen phase with 25-100 Telogen hairs being shed each day.
Two common types of Hair
Body hair that grows in most places on the human body in women and men known as “vellus hair”, these hairs are typically shorter and finer, and are often referred to as being like “peach fuzz” Terminal hair, developed or mature hair. Typically this hair is longer, coarser, thicker, and darker than vellus hair, covering body areas including the head, arm pits and pubic areas in patterns determined by gender, age and DNA genetic make-up. Interestingly, babies are born with an identifiably different third type of hair, referred to as “lanugo”, finer than mature “vellus hair” it covers nearly all the body of new borns.
To better understand Hair Growth and how Laser works to safely remove hair it is desirable to have some understanding of the structure of hair. Our DNA genetic make-up determines much of our appearance including hair colour, texture and type, hair type is commonly referenced as being either straight, curly, wiry or wavy, these attributes can modify or change substantially over time due to age, diet and climatic conditions.
At the base of the hair follicle is a large structure referred to as the papilla. The papilla mostly comprises connective tissue and a capillary loop. Cell division in the papilla is rare or infrequent.
The Hair Matrix
Surrounding the papilla is the hair matrix, a collection of epithelial cells often interspersed with the pigment producing cells. Cell division in the hair matrix is responsible for the cells that will form the major structures of the hair fibre and the inner hair root sheath. The hair matrix epithelium is one of the fastest growing cell populations in the human body, which is why some forms of chemotherapy that kill dividing cells or radiotherapy may lead to temporary hair loss of this rapidly dividing cell population. The papilla is usually egg or pear shaped with the matrix wrapped completely around it except for a short stalk-like connection to the surrounding connective tissue that provides access for the capillary.
The root sheath is composed of an external root sheath, a middle layer, and an internal cuticle that is continuous with the outermost layer of the hair fibre.
Some Examples of Growth Times
|Anagen (Growth Phase)||2-3 years (sometimes longer)||18 to 24 months||6 to 8 months|
|Catagen (shrivelling) Phase||2 to 3 weeks||6 to 10 weeks||6 to 10 weeks|
|Telogen (resting) Phase||About 3 months||About 3 months||About 5 months|
The duration of the hair growth cycle varies in different parts of the body. For eyebrows, the cycle is completed in around 4 months, while it takes the scalp 3 to 4 years to complete; this is why eyebrow hairs have a fixed length, while hairs on the head grow so long.
Indicative table of Hair growth by stage:
The duration of these phases varies from person to person and is affected by our basic genetic (DNA) make-up, age, health and gender. Different hair colour and follicle shape also affects the timings of these phases. Because laser hair removal treatment is only effective during the Anagen or growth phase when the hair is rich in protein, keratinized cells and melanin, a significant amount of hair is not removed by initial laser treatment and further laser treatments are required for the hairs that were in transition and resting phases as they progress to the Anagen growth phase and are susceptible to the laser.