THE HAIR GROWTH CYCLE

Science isn’t everyone’s favourite subject, but when it comes to scalp cooling the more you can understand about biology and science, the easier it will be to understand what is going on to save those hairs. Knowledge is power after all. There are a few technical terms in this one, but it’s well worth the read, plus we have added lovely diagrams to make it all a bit clearer!

The hair follicle is a complex mini organ, which carries out the complicated task of growing hair. What you need to know to understand how chemotherapy effects the hair, is that this growth happens in cycles.

There are four parts to the life cycle of hair.

First up is the growth or anagen phase. This is where the hair is physically growing, typically around 1cm a month, though this can vary from person to person. This stage usually lasts around 3 to 6 years. The papilla (the blood supply) within the follicle feeds the hair matrix which is rapidly dividing creating new hair.

Second is the catagen or transitional phase. This is when the follicle renews itself. The follicle shrinks to about one sixth of its full size and the papilla detaches meaning there is no more blood supply allowing the hair to grow. The shrinking follicle forces the hair upwards, so though it isn’t actively growing, the hair will appear to get longer. This stage takes about 2 weeks and hair transforms into club hairs – a hard keratinised tissue creates a bulge or club shaped lump at the bottom of the hair.

Then is the telogen or resting phase. The follicle becomes dormant for around 1 to 4 months. The club hair that has been formed keeps the hair in the follicle for several months, but the hair is no longer in anyway alive or growing.

Finally, the exogen or shedding phase. At this point the dead hair is actively shed from the scalp. Most healthy people will shed around 100 hairs a day.

After all of this has happened a new anagen stage begins and a new hair begins to form from the matrix and the papilla reattaches and the cycle continues.

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In a healthy person more than 80% of your follicles are actively growing and rapidly dividing in the anagen stage at any one time, which is a why hair is so easily damaged by chemotherapy. So, what does the chemo do to the hair? Well, cancers are cells that are dividing rapidly and uncontrollably without stopping. Chemotherapy targets these fast dividing cells, but it can’t tell the difference between a bad dividing cell, cancer, and a completely healthy one, like your hair. In fact, hair follicle matrix keratinocytes, the cells that divide super quick resulting in your hair growing, can potentially divide faster than the malignant cells that chemo is supposed to target. Speedy!

As we know, most chemotherapy drugs that can cause alopecia are administered to the body via the blood stream. The blood which is infused with these drugs races around your body, allowing the chemo drugs to attack any cancerous cells, but also those handy healthy fast dividing cells too.
A really important part of the hair follicle is the hair papilla. This is the blood supply that supports hair growth, but this means that during chemo the papilla functions as each follicle’s chemo main line, flooding the hair matrix (the bit growing the hair cells) with chemo infused blood. Hence why if you receive chemotherapy without scalp cooling, you are on a one-way street to fast and extensive hair loss. This is because the damage from the chemotherapy effectively kills the hairs, causing a sudden catagen phase to begin. Hence why between days 14 to 21 after your first chemo session your hair will begin to shed, as the catagen phase has suddenly finished and the telogen and exogen phases begin.

So how does scalp cooling stop this sudden catagen phase from happening?

If you have seen the animation video on our website, you will understand how scalp cooling works (if you haven’t, have a look now). Scalp cooling works in two ways – vasoconstriction; blood vessels, specifically in this case the papilla, narrow because of the cold and reduce the blood flow to around 40%. This means there is less of the chemo infused blood reaching the follicle.
Secondly the follicle becomes temporarily dormant because of the cold. The hair matrix stops dividing and producing hair, meaning that any chemo drug that does reach the follicle through the narrowed papilla doesn’t recognise fast dividing cells and bypasses it.

This means that the hair that you retain during chemotherapy continues in its anagen phase, rapidly dividing and growing as it is supposed to. The hair that you shed will have received more chemotherapy than it can tolerate and has gone into a sudden catagen phase. This is why when you wash or brush your hair, it looks like you are losing a lot, but it is because you are liberating some of the telogen and exogen hairs. If you are gentle and follow our hair care guidance you won’t be pulling out hairs in the anagen phase.

Scalp cooling also has additional benefits that are for longer term protection of the follicles.

You may have heard of persistent chemotherapy induced alopecia. This can occur in around 15% of people receiving a drug called docetaxel. It is believed that the docetaxel reaching the hair follicles damages the stem cells within the follicle. Stem cells are usually inactive but quickly activate to trigger a new anagen phase within a follicle. If the stem cells are damaged, they do not activate, meaning that the follicle cannot be triggered to grow a new hair. Scalp cooling has been found to prevent this damage from happening. This why we always encourage people to use scalp cooling when receiving docetaxel.

What we also know is that scalp cooling results in faster healthier and stronger regrowth in comparison to hair growth in those who chose not to cold cap. The significantly lower quantities of chemo drugs that reach the follicles while cold capping means that the follicles can go back in to their anagen phases quickly. For some people this can happen before chemo treatment is even finished. For more information on regrowth you can see the Hope and Tata studies here.  

So that’s the life cycle of a hair. Hopefully now you have a better idea of what is happening as a result of both chemo and cold capping.