How does hair grow? The hair growth cycle
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There is more to hair than meets the eye. Having a look at the structure of hair and dynamics of the hair growth cycle can reveal key information about whether hair is growing normally.
This article covers two main issues: the anatomy and function of each part of the hair follicle, as well as an overview of the different stages of hair growth. You'll walk away with a solid understanding of key structures of the hair follicle and the essential roles they play in maintaining the look and feel of your crowning asset.
What is a hair follicle? Hair follicle function and anatomy
The hair follicle is a constantly cycling organ that passes through several layers of scalp tissue. It can be roughly split into two parts: an upper portion which crosses the epidermis (the uppermost layer of skin) and a lower portion that extends into the layer beneath it called the dermis. This lower portion is referred to as the hair bulb.
The hair bulb contains rapidly dividing cells (matrix cells) that eventually become part of the visible hair shaft. These cells, as well as the follicle itself, are kept alive by the dermis which contains a network of blood vessels, glands and other specialized tissues.
The dermal papilla
At the base of the bulb is a group of cells called the dermal papilla. The dermal papilla provides a constant blood supply to the developing hair during the active growth phase. It also facilitates cell-to-cell communication within the hair follicle (Chen et al., 2020).
This communication is possible due to features located on the surface of cells called receptors. These receptors respond to hormones, growth factors and other molecules circulating in the blood. Androgen receptors, which respond specifically to male sex steroid hormones, are one type of receptor. They are involved in the development and progression of pattern hair loss.
When a hormone such as dihydrotestosterone (DHT) comes into contact with and attaches to its receptor, certain biochemical signals are initiated. These complex signaling pathways have the ability to influence the development of the hair follicle and movement through phases of the hair growth cycle.
The hair bulge
The cells of the dermal papilla are constantly communicating with a pool of stem cells found in a region of the hair follicle called the bulge. The bulge is located at roughly the halfway point between the upper and lower parts of the hair follicle. It supplies the follicle with unspecialized cells that are eventually incorporated into the outer layers of the bulb or inner hair shaft. The growth and movement of these cells to their final locations in the follicle are responsible for the lengthening and thickening of hair strands.
The bulge is one of the few places in adult humans where stem cells are found. These unique cells are capable of developing into almost any other kind of adult tissue (Cotsarelis, 2006).
These two processes are regulated by the dermal papilla. Another name for these cellular changes is differentiation. Differentiation is necessary to produce the complex tissues within and surrounding the hair follicle.
The inner and outer root sheaths
Examples of these specialized tissues include the inner and outer sheaths that surround the hair bulb. Rapidly dividing cells located at the core of the bulb are protected by these sheaths.
The inner sheath contains several sublayers including the cuticle, a distinctive layer of cells arranged in a scale-like pattern (Brown & Krishnamurthy, 2021). Over time, the cuticle and adjacent outer layers gradually fill with keratin, the protein that gives hair its strength and flexibility.
Meanwhile, the outer sheath surrounds the entire bulb structure. It functions as the attachment site for small muscles that control piloerection (how hairs stand-up in response to stimuli) and oil-producing glands (Brown & Krishnamurthy, 2021).
What determines hair colour and texture?
Whether hair is straight, wavy or curly depends on the direction of cells in the inner sheath as well as the shape of the hair follicle itself.
Hair colour is determined by the type and distribution of a pigment called melanin, which is produced by specialized cells called melanocytes. Both melanocytes and keratin-producing cells are the result of the differentiation process.
What is the hair growth cycle?
The hair growth cycle involves three phases: active growth (anagen), regression (catagen) and rest (telogen). Hair growth occurs as cells located within the hair follicle progress through these phases. Some clinicians will refer to a fourth phase called exogen, which simply refers to the shedding of the previous hair. The length of each of these phases depends on a number of factors including adequate nutrition and hormone levels (Houschyar et al., 2020).
In men, testosterone and dihydrotestosterone (DHT) are powerful mediators of the hair growth cycle. The binding of DHT to male sex steroid hormone (androgen) receptors on dermal papilla cells can lead to a shortened active growth phase and prolonged rest phase, which are typical of androgenetic hair loss (Chen et al., 2020).
Anagen: the active growth phase
The anagen phase is a time of active hair growth and can last anywhere from 2-7 years. This is the stage when hair lengthens noticeably at a rate of approximately half an inch per month as a result of rapid cell division and differentiation occurring within the hair bulb.
The length of time that a hair spends in the anagen phase is partially dictated by genetics and has a direct effect on the final length of the hair strand. This is one of the reasons why eyelashes and eyebrows are significantly shorter than the hair on your head.
Prior research has found that the anagen-to-catagen transition may be influenced by a group of hormones called prostaglandins, which have been shown to increase significantly before the catagen phase (Nieves & Garza, 2014).
Catagen: the regression phase
Next is the catagen phase (also known as the regression phase), which is shorter and lasts 2-3 weeks. The mature hair stops growing as cells within the hair bulb and outer sheath of the follicle begin to die.
Under a microscope, the lower part of the follicle starts to shrink and fold onto itself (Stenn & Paus, 2001). The hair bulb gradually detaches from its blood supply and cells at the base of the hair form a rounded-off, keratinized surface. At this point the hair is referred to as a club hair.
This club hair maintains contact with the dermis layer of the scalp via a structure called an epithelial strand. It is a highly specialized piece of tissue that holds the hair in place until it is eventually shed.
Telogen: the rest phase
The telogen phase of the hair growth cycle is also referred to as the rest phase and normally lasts a few months. During this stage, the club hair is slowly pushed through the upper layers of the scalp as new matrix cells within the bulb start to accumulate and form a new hair.
At the same time, the hair follicle slowly regains its original shape. The new matrix cells will eventually enter anagen and begin a new growth cycle. This regenerative process is regulated by the dermal papilla (Pantelireis & Higgins, 2018).
You may have also heard of a fourth phase called exogen. Exogen describes the actual shedding of the club hair rather than a phase associated with the follicle. It’s an indicator of the transition from telogen into early anagen (Houschyar et al., 2020).
The structure of the hair follicle and different stages of the hair growth cycle are perfectly designed to ensure the proper growth and development of mature hair strands, as well as the continuous production of new hairs. Androgens are key regulators of these processes.
But changes in the quality of hair growth or unusual shedding don’t always indicate androgenetic hair loss. For this reason, we recommend that you speak with a physician about your concerns to ensure that the root cause is correctly identified.
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