00:01
Welcome. This is a very important topic,
especially for the men in the audience,
but I will say that 10 to 20% of the women
will also have the same concerns.
00:11
So this is androgenic alopecia or hair loss
related to hormonal stimulation.
00:17
So androgenic alopecia is really due to the
androgen dihydrotestosterone.
00:23
This is one of the metabolites of
testosterone period.
00:27
The epidemiology clearly happens more
commonly in men because they have higher
levels of testosterone,
and therefore make more of the five hydroxy
testosterone. The prevalence does increase
with age.
00:39
That's because the follicles that are
responding to the metabolite of testosterone
are bombarded with progressively longer and
longer and longer amounts of of that hormone,
and therefore tend to have more hair loss
with time will effect.
00:56
Sadly, 80% of men by the age of 80.
01:00
So we should all live so long.
01:02
But at that point there will be some degree
of hair loss.
01:06
There is a lower prevalence in non-European
populations.
01:09
This actually has to do with levels of
expression of the receptor,
and not so much by levels of of expression of
the testosterone.
01:18
And as I've already mentioned,
10 to 20% of women may also be affected by
this same general pathophysiology.
01:25
So what is going on here?
There is a genetic predisposition.
01:29
This is clearly an androgen dependent trait.
01:33
It is the sensitivity or the kind of the the
predisposition is transmitted by the X
chromosome. So it's not from your dad.
01:41
Guys if your dad's bald don't worry about it
too much.
01:44
If your mom's dad was bald,
do worry.
01:47
It's related to the X chromosome because
that's where the androgen receptor is coded.
01:52
So think about this. It's all about the
androgen receptor.
01:56
So to understand how hair loss occurs we have
to understand how hair growth normally
occurs. And there are four basic phases of
hair growth.
02:05
It's not just it's constantly growing.
02:07
In fact, it's growing,
stopping, pausing and then restarting over
and over and over again over the lifespan of
an individual.
02:15
In the initial phase,
called antigen, which is the growing phase,
the hair shaft, the follicle at the base of
the hair is actively synthesizing new keratin
and making hair. In approximately 90% of all
hair follicles are going to be in this phase,
and they will be constantly synthesizing hair
for over a period of time of 2 to 6 years.
02:35
At that time point, then they go into kind of
a pause phase,
a regression phase called catagen.
02:42
It's transitory, hopefully,
where the follicle that's responsible for
synthesizing the hair degenerates or
regresses even.
02:51
And but fortunately it's brief.
02:53
Again, it's transitory.
02:55
Except if you have too great an exposure to
the dihydrotestosterone.
02:59
And at any given time,
in a typical individual who's not losing
their hair, but a typical individual such as
myself Of or are you listening?
About 1% of all your hair follicles are in
the catagen phase.
03:11
Okay. Short, transitory,
a degenerative phase.
03:16
Then you enter into the telogen,
or resting phase.
03:19
This is where the hair follicle is not
actively synthesizing hair.
03:23
The keratin to make the hair,
but is will be potentially able to be
kickstarted and restart synthesis again in
the antigen phase.
03:32
Approximately 10% of all your hair follicles
are in the telogen phase,
and this will last for 2 to 3 months.
03:38
So there's not an active regression anymore,
but it's basically a pause in in the game
plan. So you're not synthesizing the keratin
or hair at this point.
03:47
And then when the hair is no longer being
synthesized during this resting phase,
it reaches a kind of terminal position.
03:57
The epithelium around the hair root kind of
compresses the hair and you actually lose it.
04:06
So the terminal position in position in the
follicle is reached and it detaches and that
resting hair will come out.
04:14
So when you comb your hair,
even now in your youth,
as you're watching this,
you will have a certain percentage of the
hairs coming out on the comb.
04:23
Don't worry about it. That's completely
normal at this point.
04:27
After the oxygen phase,
then we can restart and we can go back into
the antigen phase. If all is well with the
hair follicle,
if it is not, if it has not been destroyed,
or if it's not being constantly maintained in
catagen, that's the next part.
04:44
So this is normal in androgenic alopecia is
when we change that ratio of antigen to
catagen. Remember, most hair follicles 90%
are going to be an antigen in most of us if
we're maintaining our hair.
04:58
And the catagen phase,
that regressive phase is only like 1% of the
hair follicles. If we shift that ratio now,
we're at risk of having significant hair
loss. Us. Here's what's happening.
05:13
So the receptor, the dihydrotestosterone
receptor,
the DHT receptor on the hair follicle is
going to be exquisitely responsible for
driving hair synthesis from that antigen into
the catagen phase.
05:29
It's going to be driving that that
degenerative regressive phase.
05:34
So DHT binds to its receptor.
05:36
If you have more receptor or more DHT,
you're going to have more hair follicles that
are going to be influenced by that.
05:44
That shortened the antigen phase from 2 to 6
years to very short.
05:48
And you have a greater percentage of cells
that are in the catagen phase.
05:53
And with that regression,
you can actually have the hair follicle
become shorter and shorter,
less robust.
05:58
And even on successive cycles it's not making
hair very well.
06:02
And then you end up with these short,
fine, hyperpigmented so-called Velux hairs.
06:08
Vellus hair.
The lecture Androgenetic Alopecia: Pathophysiology by Richard Mitchell, MD, PhD is from the course Degenerative Changes of Skin and Hair.
What percentage of hair follicles are typically in the anagen phase during normal hair growth?
How does dihydrotestosterone (DHT) alter the hair growth cycle in androgenic alopecia?
Why is maternal family history more relevant for predicting male pattern baldness?
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