Welcome to this overview of hair biology!

The information provided here really helps to understand the hair removal and hair regrowth processes. We believe that you will find it easy to follow and it is fully illustrated.

Our thanks to Dr. Kevin McElwee for the basic hair biology text and graphics, and H Mueller Design for the course design.

Do you know which organ in the body is the largest?

The skin is the largest organ! It is of primary importance to our survival but is often overlooked when examining the health and welfare of an individual.

The skin consists basically of two main layers, the epidermis (the outer layer that we can see) and the dermis (the inner layer). The skin rests on the subcutaneous tissue that consists largely of a loose mesh of collagen fibre, fat cells and muscle tissue.

This picture is taken from Microsoft’s Encarta Encyclopedia. Click on the picture or a larger view and description.

The skin houses a number of components: blood vessels, nerve endings, and of course the hair follicles – the subject of our study.


Functions of the Skin

The skin has a diverse range of functions:

  • Temperature Through its extensive blood supply and sweat glands, the human skin is able to maintain the constant temperature (about 37 oC).
  • Excretion Waste materials such as salts and water are removed from the body via the skin.
  • Vitamin formation Photochemical action on skin promotes the production of vitamin D. The skin is our primary source.
  • Sensory function Through the extensive network of sensory receptors sensations of pressure, texture, temperature and pain.
  • Pigmentation Melanin pigments protect against the excesses of ultra violet rays.
  • Protection The epidermis prevents absorption of unwanted, potentially dangerous materials
  • Immunological defense The epidermis, particularly the stratum corneum (the outer most keratinized skin layer), provides a passive defense against entry of opportunistic infecting organisms.

In short, the skin acts as a barrier and is the primary organ through which we interact with the external world.

Functions of the Hair

The hair plays a significant part in many of the skin’s properties:

  • Hair provides protection against heat loss by adjusting the hair density. Each hair has a little muscle that contracts when it is cold. This “fluffing up” the hair layer keeps the air next to the skin and so provides a heat trapping, invisible layer.
  • Hair fibre also helps with protection forming a tough barrier helping protect the epidermis from minor abrasions and/or from ultra violet light.
  • Specialized hair such as eyebrows and eyelashes protect the eyes by channeling or sweeping away fluids, dust and debris. Nasal hair plays an important role in trapping air borne foreign particles before reaching the lungs.
  • Hair fibre also increases the surface area for faster evaporation of sweat from neighbouring glands and individual hairs can aid sensory function.

Of course hair is VERY important from a cosmetic point of view – but we leave that to the television commercials to explain!

Just how many hairs to we have? On average, the total number of hair follicles for an adult human is estimated at 5 million – with 1 million on the head, of which 100,000 alone cover the scalp!

In this module we will take a closer look at the structure (morphology) of the hair follicle. The material covered in this module can be a little involved. But please persist – it is well worth the effort.

General Appearance

Imagine two layers of dough rolled out on a table top: one light-colored layer on top of a dark layer.

layer1

Now imagine that you press a hole with your finger into the top layer until you almost touch the table top.

layer2

You have have now created a ‘follicle’ in the dough. Notice that the top layer lines the hole all the way through the bottom layer.

The top layer corresponds with the epidermis layer of the skin, and the bottom layer with the dermis layer.

We have now created a ‘follicle’ in our two layer dough example. Now, if it were possible, plant a small bulb at the bottom of that follicle. Once it grows and the shaft starts to appear above the surface of the dough, we have a model of a real hair follicle. A schematic representation of a real hair follicle is shown below:
fol10

The Dermal Papilla

At the heart of each active hair follicle lies the dermal papilla – the bulb in our dough model. The dermal papilla has a healthy “pear” shape in normal hair follicles. The DP consists of a highly active group of cells responsible for the production of hair fibre.

Growth by Division
At this point we need to digress and review a few basic biology concepts. The human body is made up of cells. Cells are the building blocks we are made up.

The body grows when an individual cells divides into two identical new cells. All the cells in the body can trace their origin back to the single ovum cell in the mother. But different cells take on different characteristics to better suit them to their specific tasks. This is called ‘differentiation’.

Keep this principle in mind as we continue our study of the hair follicle.

The Dermal Papilla

We know that the way the body grows is by cells dividing. So if a hair is to grow, there must be cell division.

In the immediate vicinity of the dermal papilla there are epidermal cells (called cortical or matrix cells) that are an actively proliferating group of cells. During the growth phase (more about this in a later module) of the follicle, these epidermal cells divide rapidly. The resultant growth is pushed up the follicle and becomes the hair shaft that protrudes above the skin surface, i.e. the part we can see.

Under the influence of the dermal papilla, the epidermal cells differentiates (change) to produce the keratinized hair fibre and associated products. Click here if you want to see that schematic diagram of the hair follicle again.

This is a microscopic view of the hair papilla. You can click on the image for a larger annotated view.

The Hair Shaft

In the previous section we saw that the epidermal cells close to the papilla multiplied and became keratinized (differentiated) to form the hair shaft.

The hair shaft – the part that protrudes above the skin surface – in fact consists of several layers: the cortex and surrounding hair cuticle. The central core of the shaft is called the medulla.

The photomicrograph on the left shows the surface of a human hair as seen under an electron microscope.

The part of the hair shaft below the surface – i.e. inside the follicle – is called the root. The root is surrounded by an inner root sheath and an outer root sheath.

The Hair Root

In the previous section we saw that the part of the hair shaft below the surface – i.e. inside the follicle – is called the root. The root is surrounded by an inner root sheath and an outer root sheath.

The inner root sheath can in turn be divided into three layers the cuticle, Huxley layer and Henle layer. The inner root sheath stops at the level of the sebaceous gland to leave only the hair cortex and surrounding cuticle to protrude above the epidermis. Click here if you want to see that schematic diagram of the hair follicle again.

The Outer Root Sheath is distinct from other epidermal components of the hair follicle being continuous with the epidermis. The “bulge” region in the outer root sheath is the site at which the erector pili muscle is attached (not shown). This is the muscle that makes hair stand erect and produces “goose bumps” when you are cold.

A glassy membrane, called the basement membrane, separates the outer root sheath and the dermal sheath. The basement membrane provides a physical dividing line between epidermal and dermal cells. This physical barrier is vital to our immunological protection.

This microscope photograph is of a section through the hair root showing all the different layers. You can click on the images for a larger annotated view.

So often people are concerned when they see a few hairs in the basin or shower after a shampoo: “Eek!, I’m going bald!” While this concern is understandable (we speak of personal experience), a knowledge of the hair growth cycles would allay most of the fears. It is normal for as many as a hundred hairs to fall out per day and is part of the normal growth cycle.

But an understanding of the hair growth cycles is also vital when discussing hair removal processes. The phase of the growth cycle that a hair is in determines the effectiveness of the hair removal treatment.

Terminology

Under normal circumstances hair growth in each hair follicle occurs in a cycle. There are three main phases of the hair growth cycle:

  • anagen the active growth phase
  • catagen transition phase
  • telogen the rest phase

Anagen is the longest phase with up to 90% of follicles on a normal human scalp in this active hair growth state at any given time.

Correspondingly there are up to 10% of hair follicles in the telogen or rest phase. (That amounts to about 10 000 scalp hairs that are typically shed during this phase.)

As we mentioned, there are three main phases of the hair growth cycle; anagen, catagen and telogen with anagen further subdivided into proanagen, mesanagen and metanagen.

Anagen is the active growth phase when hair fibre is produced. Proanagen marks initiation of growth with RNA and DNA synthesis in a follicle which then quickly progresses through mesanagen to metanagen and maximum follicle length and girth. In this mature state of proliferation and differentiation the hair follicle consists of a total of eight concentric layers and melanogenesis (pigment formation) occurs within pigmented hair follicles (more about this when we discuss laser hair removal).

Anagen is followed by catagen, a period of controlled regression of the hair follicle. Ultimately the hair follicle enters telogen, when the follicle is in a so-called resting state. The next few pages will show the growth cycles schematically.

The regression of a mature anagen hair follicle

On entering catagen the dermal papilla condenses as the cells become inactive. With a lack of dermal papilla cell stimulation, the hair fibre and root sheaths stop growing.

In telogen the dermal papilla can become isolated in the dermis and the hair fibre can easily be pulled out (by combing or brushing).
anagen catagen telogen

Diagram showing a resting hair follicle returning from resting telogen to growing anagen. If the old fibre has not already fallen out it is pushed out by the new hair fibre growing underneath.
telogen mesanagen anagen

Some Statistics

The average rate of hair fibre growth is around 0.35mm a day but this rate varies depending on the site of the hair follicle and the age and sex of the individual.

The duration of the anagen growth phase for scalp hair is usually 6-10 years while telogen lasts just 30-90 days and catagen is best estimated at 14-21 days.

Normally this cycle of hair production and inactivity will continue for the duration of the individual’s life but other factors can influence and inhibit hair production. Factors may include adverse reactions to drugs and cosmetics, or as a result of scarring, tumors, radiation, the genetics of the individual, hormones and/or their immune system.

We discussed in a previous module how the body grows by cell division and that all cells trace their origin back to the single ovum cell in the mother’s womb. Gradually, as the cells divide and different cells take on different characteristics (differentiation), the foetus starts to take shape. So at which stage are the hair follicles formed? Why is it important to know?

In this module we will look at this fascinating subject and discuss some of the implications for hair removal and hair regrowth.

Fetal Development

During the development of the fetus there is frantic cell growth and division activity. It is interesting to note that the body cells later loose this ability to multiply rapidly with the exception of the dermal papilla in the hair follicle. The dermal papilla and associated epidermal cells retain this ability for most of the adult life.

It is not surprising that the establishment of a dermal papilla during embryogenesis (early stages of embryo formation) is vital to the subsequent development of all hair follicles and associated modified structures.

Just so that we have a frame of reference, the table below summarizes the stages of fetal development:

End of month | Typical changes

  1. Eyes, ears, and nose not yet visible. Backbone starts to form. Heart forms and begins beating.
  2. Eyes far apart, eyelids fused. Arms and legs almost formed. Internal organs continue to form. Bones start to develop (ossification). Fingers are formed.
  3. Eyes almost fully formed but eyelids still fused. Appendages are fully formed and nails start to develop.
  4. Face takes on human features and hair appears on head. Skin bright pink. Continued development of body systems and internal organs.
  5. Head less disproportionate to rest of body. Fine hair (laguno hair) covers the body. Skin still bright pink. Rapid development of body systems.
  6. Eyelids separate and eyelashes form. Skin still pink and wrinkled.
  7. Head and body even more proportionate. Skin wrinkled and pink.
  8. Subcutaneous fat is deposited. Skin becomes less wrinkled.
  9. More subcutaneous fat accumulates. Laguno hair is shed. Nails extend to end of fingers.

Early stages

As we saw in table in the previous section, scalp hair appears at the end of month 4 and body hair at month 5. A lot of activity must take place ‘behind the scenes’ before – a follicle must be formed and the hair needs some time to grow.

Roughly at the end of month 3 the skin is smooth (like our two layer dough model). Then little dimples start to form on the skin surface. Gradually these dimples become deeper until suddenly one day after a few weeks, the tip of a very fine hair appears in the dimple! So how did that happen?

A dermal papilla is formed

As we mentioned in the introduction, the establishment of a dermal papilla during early stages of embryo formation (embryogenesis) is vital to the subsequent development of all hair follicles and associated modified structures.
The dermal papilla is a group of specialized dermal fibroblast cells which aggregate in the dermis just below the epidermis. For humans this initial aggregation begins when the embryo is approximately 3 months old. The papilla cells at this stage are only loosely collected and present as long, spindle shaped cluster. The development of a dermal papilla in the skin marks the site (the dimple) for future development of a hair follicle.

A dermal papilla is formed

An epidermal plug, or peg, of cells develops above the dermal papilla and proliferates, growing down into the dermis to eventually link with the dermal papilla.

The dermal papilla and the epidermal plug apparently “communicate”. This results in further proliferation of epidermal matrix cells and differentiation (changing or specialization of cells) into the various sheath and hair fibre structures.

The dermal papilla develops into a more identifiable structure of rounded cells containing organelles (little factories inside each cell) vital for product synthesis. However, the cells themselves at this later stage of development are non-proliferative. The development of a hair follicle is a continuum through induction, initiation, elongation and differentiation as shown below.

Final comments

It is the dermal papilla which directs and dictates the embryonic generation of a hair follicle and it also retains this instructive ability throughout the life of the hair follicle.

Experiments have shown that removal of the dermal papilla stops hair growth. But the lower third of the dermal sheath is capable of supplying new cells for regeneration of a new dermal papilla by infiltrating and transforming at the site of the original papilla with subsequent hair follicle regrowth.

With removal of more than the lower third of a hair follicle, reformation of a dermal papilla is unable to occur and the hair follicle is effectively permanently destroyed. This is an important fact for our discussion of hair removal treatments.

The dermal papilla cells retain their embryonic functional abilities and are able to induce new hair fibre growth in mature, adult skin when implanted into previously deactivated hair follicles and in close association with outer root sheath epidermal cells.

This short course is about the biology of the hair and not hair removal.

However, we feel that it is appropriate to cover a few aspects of hair removal in the light of what we have just learned in the rest of the course. In particular we will focus on two methods:

  • electrolysis
  • laser

Electrolysis methods

Electrolysis is a time-honored method of permanent hair removal. There are two basic processes: galvanic and short-wave. In both cases a thin filament (thin wire – also commonly referred to as a needle) is inserted into the hair follicle as far as it will go. Then an electrical current is applied to the follicle.

In the galvanic process a direct current is is used. The current causes a chemical reaction in the follicle that produces sodium hydroxide. The sodium hydroxide destroys the follicle.

In the short-wave process a high frequency current is used. This current quickly produces concentrated heat at the filament tip which destroys the surrounding follicle tissue.

A third process, the blend, is a combination of galvanic and short-wave currents.

Electrolysis methods – it’s all about growth cycles!

For electrolysis to be successful, it must be possible to insert the filament so the the tip is close to the papilla area. The table below shows the follicle treatability in the different phases.

Anagen
This is the optimum phase. Here the hair is growing and when it is visible above the skin, it can be treated. There is plenty of water and salt available in the root area to form sodium hydroxide and destroy the papilla and the lower third of the root sheath. If treated correctly, there will be no regrowth from this follicle.

Catagen
This may look good because the hair is visible, but ….. As we know the hair growth has stopped, the hair is ready to shed, and the papilla has shrunk back. Follicle destruction is not possible. A follicle treated in the catagen stage will still produce another hair.

Telogen
This is the rest phase. The hair has typically been shed and is then not even visible above the skin. The papilla is located well down and protected by the plug above and cannot be destroyed.

Laser hair removal

Laser hair removal is a very new process that has only been on the market for a few years.

First of all, just what is a laser? It is simply a source of very bright light. Lasers have another important characteristic – they produce light of a single colour (wavelength for the ‘techies’ among us). By careful selection of the material used in the laser tube, it is possible to obtain the specific colour needed for different applications. In the case of hair removal red light is used.

Like short-wave electrolysis, laser hair removal also uses heat to destroy the follicle. The objective with laser hair removal is to rapidly heat up the papilla and lower third of the root sheath (to destroy the follicle) – but not to damage other structures in the skin like the nerve endings and blood vessels.

The advantage of the laser is that the laser beam can treat several follicles at the same time.

Laser hair removal – the role of melanin

We know that light produces heat – just think of the warmth of the sun’s rays! And also that black surfaces absorb light much more effectively. The key to the laser removal process is melanin. Melanin, or pigment, is a very dark, near black, substance. It is the presence of melanin in the hair, and also the skin, that gives it its colour. Being a near-black substance melanin absorbs light extremely well.

After extensive research it was found that red light was the perfect color (wavelength) for hair removal. Melanin would absorb the red light well whereas the other structures in the skin would not – the red light simply passes straight through.

We know that light produces heat – just think of the warmth of the sun’s rays! And also that black surfaces absorb light much more effectively. The key to the laser removal process is melanin. Melanin, or pigment, is a very dark, near black, substance. Being a near-black substance melanin absorbs light extremely well.

After extensive research it was found that red light was the perfect color (wavelength) for hair removal. Melanin would absorb the red light well whereas the other structures in the skin would not – the red light simply passes straight through.

But is there melanin in the hair? Yes, it is the presence of melanin in the hair, and also the skin, that gives it its colour.

In the immediate vicinity of the dermal papilla there are epidermal cells (called cortical or matrix cells). (You may want to review the module on hair structure and the microphotographs in particular). These cells proliferate and form the hair shaft. Scattered between these cells are melanocytes – these are cells that manufacture melanin giving the hair shaft its colour.

In laser hair removal the laser probe is placed on the skin surface. The pulses of red light penetrate the skin and are absorbed by the melanin in the papilla region, the lower root sheath and the hair itself. The hair follicle is then destroyed as a result of the rapid heat build up in the area.

Laser hair removal – it’s a matter of cycles!

As we showed on the previous page, the laser works well on hair follicles in the anagen or growth phase. Then there is plenty of melanin present.

However in the telogen or rest phase, the dermal papilla shrinks back and cell proliferation, including melanin production, ceases. When a follicle is treated in this phase, the hair shaft (if it has not been shed yet) will be destroyed by the laser light. But the papilla will be untouched and a new follicle will form when the anagen phase begins again.

Laser hair removal – the downside

The laser works best on people with dark hair (plenty of melanin) and pale skins. On the other hand red, blond and gray hair contain virtually no melanin and are not destroyed by the red laser beam.

Dark skin and tanned skin contain lots of melanin. If these skin types are treated with a red light laser, it could cause damage to the skin. Depending on circumstances this could be temporary white (de-pigmented) spots, or much more serious damage.

Hair removal – apparent regrowth

We saw from our table of statistics that there can be as many as 500 follicles per square centimeter but only 50 to100 hairs may be visible above the skin. Some follicles are in catagen and telogen, and some are dormant.

Suppose a particular one square centimeter area is treated, either by electrolysis or laser, and the visible hairs are cleared – i.e. those in anagen. A few weeks later the hairs grow back, or appear to do so. Why would this be so? Well, with the 50 to100 hairs cleared, there are still a good 400 that could still grow.

Hair removal technologists are often confronted by clients who have ‘regrowth’ in an area previously cleared and then the clients are very disappointed. It should be easy to see now that it will require several sessions to clear a particular area. But with each successive treatment there will be less and less follicles left to go into anagen and start growing.