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TEDMED 2012

Diane Kelly: What we didn't know about penis anatomy

April 11, 2012

We’re not done with anatomy. We know a tremendous amount about genomics, proteomics and cell biology, but as Diane Kelly makes clear at TEDMED, there are basic facts about the human body we’re still learning. Case in point: How does the mammalian erection work?

Diane Kelly - Biologist
Diane Kelly studies vertebrate anatomy, in particular the connection between the design and the function of reproductive organs. Full bio

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When I go to parties,
00:15
it doesn't usually take very long
00:16
for people to find out
00:18
that I'm a scientist and I study sex.
00:19
And then I get asked questions.
00:22
And the questions usually have a very particular format.
00:27
They start with the phrase,
00:30
"A friend told me,"
00:32
and then they end with the phrase,
00:34
"Is this true?"
00:35
And most of the time
00:37
I'm glad to say that I can answer them,
00:39
but sometimes I have to say,
00:41
"I'm really sorry,
00:43
but I don't know
00:44
because I'm not that kind of a doctor."
00:45
That is, I'm not a clinician,
00:48
I'm a comparative biologist who studies anatomy.
00:50
And my job is to look at lots of different species of animals
00:53
and try to figure out how their tissues and organs work
00:56
when everything's going right,
00:59
rather than trying to figure out
01:01
how to fix things when they go wrong,
01:03
like so many of you.
01:04
And what I do is I look for similarities and differences
01:05
in the solutions that they've evolved
01:08
for fundamental biological problems.
01:10
So today I'm here to argue
01:12
that this is not at all
01:13
an esoteric Ivory Tower activity
01:17
that we find at our universities,
01:19
but that broad study
01:21
across species, tissue types and organ systems
01:23
can produce insights
01:25
that have direct implications for human health.
01:27
And this is true both of my recent project
01:31
on sex differences in the brain,
01:33
and my more mature work
01:35
on the anatomy and function of penises.
01:37
And now you know why I'm fun at parties.
01:39
(Laughter)
01:42
So today I'm going to give you an example
01:43
drawn from my penis study
01:45
to show you how knowledge
01:47
drawn from studies of one organ system
01:49
provided insights into a very different one.
01:50
Now I'm sure as everyone in the audience already knows --
01:53
I did have to explain it to my nine-year-old late last week --
01:56
penises are structures that transfer sperm
02:00
from one individual to another.
02:04
And the slide behind me
02:05
barely scratches the surface
02:07
of how widespread they are in animals.
02:08
There's an enormous amount of anatomical variation.
02:10
You find muscular tubes, modified legs, modified fins,
02:12
as well as the mammalian fleshy, inflatable cylinder
02:16
that we're all familiar with --
02:20
or at least half of you are.
02:22
(Laughter)
02:24
And I think we see this tremendous variation
02:26
because it's a really effective solution
02:29
to a very basic biological problem,
02:32
and that is getting sperm in a position
02:34
to meet up with eggs and form zygotes.
02:37
Now the penis isn't actually required for internal fertiliztion,
02:39
but when internal fertilization evolves,
02:43
penises often follow.
02:46
And the question I get when I start talking about this most often is,
02:48
"What made you interested in this subject?"
02:52
And the answer is skeletons.
02:55
You wouldn't think that skeletons and penises
02:59
have very much to do with one another.
03:02
And that's because we tend to think of skeletons
03:03
as stiff lever systems
03:06
that produce speed or power.
03:07
And my first forays into biological research,
03:09
doing dinosaur paleontology as an undergraduate,
03:12
were really squarely in that realm.
03:15
But when I went to graduate school to study biomechanics,
03:17
I really wanted to find a dissertation project
03:20
that would expand our knowledge of skeletal function.
03:22
I tried a bunch of different stuff.
03:25
A lot of it didn't pan out.
03:27
But then one day I started thinking
03:29
about the mammalian penis.
03:31
And it's really an odd sort of structure.
03:32
Before it can be used for internal fertilization,
03:36
its mechanical behavior has to change
03:39
in a really dramatic fashion.
03:41
Most of the time it's a flexible organ.
03:42
It's easy to bend.
03:45
But before it's brought into use
03:46
during copulation
03:48
it has to become rigid,
03:50
it has to become difficult to bend.
03:51
And moreover, it has to work.
03:53
A reproductive system that fails to function
03:55
produces an individual that has no offspring,
03:58
and that individual is then kicked out of the gene pool.
04:01
And so I thought, "Here's a problem
04:05
that just cries out for a skeletal system --
04:07
not one like this one,
04:10
but one like this one --
04:13
because, functionally,
04:17
a skeleton is any system
04:19
that supports tissue and transmits forces.
04:21
And I already knew that animals like this earthworm,
04:24
indeed most animals,
04:26
don't support their tissues
04:27
by draping them over bones.
04:29
Instead they're more like reinforced water balloons.
04:30
They use a skeleton that we call a hydrostatic skeleton.
04:33
And a hydrostatic skeleton
04:37
uses two elements.
04:39
The skeletal support comes from an interaction
04:41
between a pressurized fluid
04:43
and a surrounding wall of tissue
04:45
that's held in tension and reinforced with fibrous proteins.
04:47
And the interaction is crucial.
04:51
Without both elements you have no support.
04:53
If you have fluid
04:57
with no wall to surround it
04:58
and keep pressure up,
05:00
you have a puddle.
05:01
And if you have just the wall
05:03
with no fluid inside of it to put the wall in tension,
05:05
you've got a little wet rag.
05:07
When you look at a penis in cross section,
05:09
it has a lot of the hallmarks
05:12
of a hydrostatic skeleton.
05:14
It has a central space
05:16
of spongy erectile tissue
05:18
that fills with fluid -- in this case blood --
05:19
surrounded by a wall of tissue
05:22
that's rich in a stiff structural protein called collagen.
05:24
But at the time when I started this project,
05:28
the best explanation I could find for penal erection
05:31
was that the wall surrounded these spongy tissues,
05:34
and the spongy tissues filled with blood
05:38
and pressure rose and voila! it became erect.
05:40
And that explained to me expansion --
05:43
made sense: more fluid, you get tissues that expand --
05:47
but it didn't actually explain erection.
05:51
Because there was no mechanism in this explanation
05:54
for making this structure hard to bend.
05:58
And no one had systematically looked at the wall tissue.
06:01
So I thought, wall tissue's important in skeletons.
06:03
It has to be part of the explanation.
06:06
And this was the point
06:08
at which my graduate adviser said,
06:11
"Whoa! Hold on. Slow down."
06:13
Because after about six months of me talking about this,
06:17
I think he finally figured out
06:20
that I was really serious about the penis thing.
06:21
(Laughter)
06:24
So he sat me down, and he warned me.
06:27
He was like, "Be careful going down this path.
06:30
I'm not sure this project's going to pan out."
06:32
Because he was afraid I was walking into a trap.
06:34
I was taking on a socially embarrassing question
06:37
with an answer that he thought
06:42
might not be particularly interesting.
06:44
And that was because
06:47
every hydrostatic skeleton
06:48
that we had found in nature up to that point
06:50
had the same basic elements.
06:52
It had the central fluid,
06:53
it had the surrounding wall,
06:55
and the reinforcing fibers in the wall
06:56
were arranged in crossed helices
07:00
around the long axis of the skeleton.
07:02
So the image behind me
07:04
shows a piece of tissue
07:05
in one of these cross helical skeletons
07:07
cut so that you're looking at the surface of the wall.
07:09
The arrow shows you the long axis.
07:12
And you can see two layers of fibers,
07:13
one in blue and one in yellow,
07:15
arranged in left-handed and right-handed angles.
07:17
And if you weren't just looking at a little section of the fibers,
07:20
those fibers would be going in helices
07:22
around the long axis of the skeleton --
07:25
something like a Chinese finger trap,
07:26
where you stick your fingers in and they get stuck.
07:28
And these skeletons have a particular set of behaviors,
07:30
which I'm going to demonstrate in a film.
07:33
It's a model skeleton
07:36
that I made out of a piece of cloth
07:37
that I wrapped around an inflated balloon.
07:39
The cloth's cut on the bias.
07:41
So you can see that the fibers wrap in helices,
07:43
and those fibers can reorient as the skeleton moves,
07:46
which means the skeleton's flexible.
07:50
It lengthens, shortens and bends really easily
07:52
in response to internal or external forces.
07:54
Now my adviser's concern
07:58
was what if the penile wall tissue
07:59
is just the same as any other hydrostatic skeleton.
08:01
What are you going to contribute?
08:03
What new thing are you contributing
08:05
to our knowledge of biology?
08:06
And I thought, "Yeah, he does have a really good point here."
08:08
So I spent a long, long time thinking about it.
08:11
And one thing kept bothering me,
08:13
and that's, when they're functioning,
08:15
penises don't wiggle.
08:18
(Laughter)
08:20
So something interesting had to be going on.
08:22
So I went ahead, collected wall tissue,
08:25
prepared it so it was erect,
08:28
sectioned it, put it on slides
08:30
and then stuck it under the microscope to have a look,
08:32
fully expecting to see crossed helices of collagen of some variety.
08:35
But instead I saw this.
08:40
There's an outer layer and an inner layer.
08:42
The arrow shows you the long axis of the skeleton.
08:45
I was really surprised at this.
08:48
Everyone I showed it
08:50
was really surprised at this.
08:51
Why was everyone surprised at this?
08:52
That's because we knew theoretically
08:54
that there was another way
08:57
of arranging fibers in a hydrostatic skeleton,
09:00
and that was with fibers at zero degrees
09:03
and 90 degrees to the long axis of the structure.
09:05
The thing is, no one had ever seen it before in nature.
09:09
And now I was looking at one.
09:12
Those fibers in that particular orientation
09:15
give the skeleton a very, very different behavior.
09:18
I'm going to show a model
09:21
made out of exactly the same materials.
09:22
So it'll be made of the same cotton cloth,
09:24
same balloon, same internal pressure.
09:26
But the only difference
09:30
is that the fibers are arranged differently.
09:32
And you'll see that, unlike the cross helical model,
09:34
this model resists extension and contraction
09:37
and resists bending.
09:40
Now what that tells us
09:41
is that wall tissues are doing so much more
09:42
than just covering the vascular tissues.
09:45
They're an integral part of the penile skeleton.
09:47
If the wall around the erectile tissue wasn't there,
09:51
if it wasn't reinforced in this way,
09:53
the shape would change,
09:55
but the inflated penis would not resist bending,
09:56
and erection simply wouldn't work.
09:59
It's an observation with obvious medical applications
10:01
in humans as well,
10:03
but it's also relevant in a broad sense, I think,
10:05
to the design of prosthetics, soft robots,
10:08
basically anything
10:11
where changes of shape and stiffness are important.
10:12
So to sum up:
10:15
Twenty years ago,
10:17
I had a college adviser tell me,
10:18
when I went to the college and said,
10:20
"I'm kind of interested in anatomy,"
10:22
they said, "Anatomy's a dead science."
10:23
He couldn't have been more wrong.
10:25
I really believe that we still have a lot to learn
10:27
about the normal structure and function of our bodies.
10:30
Not just about its genetics and molecular biology,
10:33
but up here in the meat end of the scale.
10:36
We've got limits on our time.
10:39
We often focus on one disease,
10:41
one model, one problem,
10:43
but my experience suggests
10:44
that we should take the time
10:46
to apply ideas broadly between systems
10:48
and just see where it takes us.
10:50
After all, if ideas about invertebrate skeletons
10:52
can give us insights
10:56
about mammalian reproductive systems,
10:57
there could be lots of other wild and productive connections
10:59
lurking out there just waiting to be found.
11:03
Thank you.
11:06
(Applause)
11:08
Translator:Timothy Covell
Reviewer:Morton Bast

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Diane Kelly - Biologist
Diane Kelly studies vertebrate anatomy, in particular the connection between the design and the function of reproductive organs.

Why you should listen

Diane A. Kelly is a Senior Research Fellow at the University of Massachusetts, Amherst. Her research interests include the evolution of copulatory systems and sexual differentiation in the nervous system. She is best known for her original work on the anatomy and function of vertebrate penises, but has also written children’s books, created exhibits for science museums, helped exhume a mastodon, and designed and published a pair of quirky science card games. Kelly holds a Ph.D. in Zoology from Duke University and an A.B. in Biological Sciences from the University of Chicago.

She blogs at Science Made Cool -- where she wrote about what it was like to give this talk.

Listen to Diane's hilarious, thoughtful StoryCollider podcast, "Death on the Road" >>

The original video is available on TED.com
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