…and this is a 
One of the subjects developmental biologists are interested in is the development of pattern. There are the obvious externally visible patterns — the stripes of a zebra, leopard spots, the ordered ranks of your teeth, etc., etc., etc. — and in fact, just about everything about most multicellular organisms is about pattern. Without it, you'd be an amorphous blob.
But there are also invisible patterns that you don't normally see that are aspects of the process of assembly, the little seams and welds where disparate pieces of the organism are stitched together during development. The best known ones are compartment boundaries in insects. A fly's wing, for instance, has a normally undetectable line running across the middle of it, a line that cells respect. A cell born on the front half of the wing will multiply and expand its progeny to cover a patch on the surface, but none of its offspring cells will cross over the invisible line into the back half. Similarly, cells born on the back half will never wander into the front.
We can see these invisible lines by taking advantage of mosaicism: generate a fly wing with two genetically distinct cell types, for instance by making one type express a pigment marker and the other not, and the boundaries become apparent. There are many ways we can generate mosaics, but in Drosophila we can use somatic recombination — with low frequency, chromosomes in the fly can undergo crossing over in mitosis, not just meiosis, so sometimes the swapping of chromosome segments will turn a daughter cell that should have been heterozygous for an allele into one that is homozygous, allowing a marker allele to express itself.
(Click for larger image)
(A) The shapes of marked clones in the Drosophila wing reveal the existence of a compartment boundary. The border of each marked clone is straight where it abuts the boundary. Even when a marked clone has been genetically altered so that it grows more rapidly than the rest of the wing and is therefore very large, it respects the boundary in the same way (drawing on right). Note that the compartment boundary does not coincide with the central wing vein. (B) The pattern of expression of the engrailed gene in the wing. The compartment boundary coincides with the boundary of engrailed gene expression.
It's like a secret code written in molecules hidden to the eye until you illuminate it in just the right way to expose it. And these lines aren't just arbitrary, they're significant. The wing boundary defines the expression of important molecules that define the identity of specific structures. The posterior half of the wing is the domain of expression of a molecule called engrailed, which is part of the machinery that makes the back half a back half. We can also stain a wing for just that gene product, and also expose the hidden lines.
We can also mutate the pathway of which engrailed is part, and do interesting things to the fly wing, like turn the back half into a mirror image of the front half. So these lines actually matter for the proper development of a fly.
So you might be wondering if we have anything similar in humans…and no, we don't have strict compartment boundaries like a fly. However, we do have normally invisible lines and stripes of subtle molecular differences running across our bodies, which are occasionally exposed by human mosaicism. These are marks called the lines of Blaschko, after the investigator who first reported a common set of patterns in patients with dermatological disorders in 1901.
Don't rip off your shirt and start looking for the Blaschko lines — they're almost always invisible, remember! What happens is that sometimes people with visible dermatological problems — rashes, peculiar pigmentation, swathes of moles, that sort of thing — express the problems in a stereotypically patterned way. On the back, there are V-shaped patterns; on the abdomen and chest, S-shaped swirls; and on the limbs, longitudinal streaks.
Here is the standard arrangement:
And here are a few examples:
Note that usually there isn't a whole-body arrangement of tiger stripes everywhere — there may be a single band of peculiar skin that represents one part of the whole.
Where do these come from? The current hypothesis is that a patch of tissue that follows a Blaschko line represents a clone of cells derived from a single cell in the early embryo. These clones follow stereotypical expansion and migration patterns depending on their position in the embryo; this would suggest that a cell in the middle of the back of a tiny embryo, as it grows larger with the growing embryo, would tend to expand first upwards towards the head and then sweep backwards and around to the front. One way to think of it: imagine taking a piece of yellow clay and sandwiching it between two pieces of green clay into a block, and then pushing and stretching the clay block to make a human figurine. The yellow would make a band somewhere in the middle, all right, but it wouldn't be a simple rectilinear slice anymore — it would express a more complex border that reflected the overall flow of the medium.
What makes the lines visible in some people? The likeliest example is mosaicism, a difference between two adjacent cells in the early embryo that then appears as a genetic difference in the expanded tissues. There are a couple of ways human beings can be mosaic.
The most common example is X-chromosome inactivation in women. Women have two X-chromosomes, but men only have one; to maintain parity in the regulation of expression of X-linked genes, women completely shut down one X. Which one is shut down is entirely random. That means, of course, that all women are mosaic, with different X-chromosomes shut down in different cells. This normally makes no difference, since equivalent alleles are present on each, but occasionally an X-linked skin disorder can manifest itself in a splotchy pattern. Another familiar example is the calico fur color in female cats, caused by the random expression of a pigment gene on the feline X chromosome.
A more spectacular example is tetragametic chimerism. This rare event is the result of the fusion of two non-identical twins at an early stage of development, producing an embryo that is a kind of salt-and-pepper mix of two individuals. After the fusion, the embryo develops normally as a single individual, but genetic or molecular tests can detect the patches of different genotypes. (No scientific tests can tell whether the individual has two souls, however.)
Another way differences can arise is by somatic mutation. Mutations occur all the time, not just in the germ line; we're all a mixture of cells with slightly different mitotic histories and some of them contain novel mutations, usually not of a malign sort, or you wouldn't be reading this right now. But what can happen is that you acquire a mutation in one cell that may predispose its clone of progeny to form moles, or acquire a skin disease, or even tilt it towards going cancerous. It's a fine thing to undergo genetic screening to find that you may not carry certain alleles associated with cancer, but you aren't entirely off the hook: you may have patches of tissue in your body that are perfectly normal and functional except that they carry an enabling mutation that occurred when you were an embryo.
One final likely mechanism is epigenetic. Throughout development, genes are switched on and off by epigenetic modification of the DNA. This process can vary: epigenetic silencing doesn't have to be 0 or 100% absolute, but can differ in degree from cell to cell. It can also vary by chromosome — you're all diploid, and epigenetic modification may affect one chromosome of a pair to a different degree than the other. Since epigenetic modifications are inherited by the progeny of a cell, that means these differences can be propagated into a clonal patch…that on the skin, will likely follow the lines of Blaschko.
Don't fret over these lines; they aren't a disease or a problem or even, in most cases, at all visible. The cool thing about them is that there is a hidden map of your secret history as an individual embedded in silent patterns in your skin — you were not defined as a single, simple, discrete genetic entity at fertilization, but are the product of complicated, subtle changes and errors and shufflings and sortings of cells. We're all beautiful pointillist masterpieces.
Life Science
Medicine & Health
Comments
Posted by: MaleficVTwin
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August 12, 2010 2:09 PM
Two souls? Imagine, that would make 14 horcruxes!!!
Posted by: Amenhotepstein
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August 12, 2010 2:10 PM
Cool PZ! Although I'm a Developmental Biologist, I'd never heard of Blaschko lines. It's interesting that the ventral midline has a Blaschko line, but not the dorsal midline, where the neural tube closes. Maybe this happens too early and the line is obscured by later cell movements? But then what about Spina Bifida and Anencephaly - both defects of dorsal midline closure?
Posted by: Sven DiMilo
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August 12, 2010 2:14 PM
Cool post; cool lines*, cool mosaicism mechanisms. But this:
wtf!? I never heard of this.
*How closely do these Blashko lines map onto the skin segments innervated by the serial spinal nerves?
Posted by: Sven DiMilo
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August 12, 2010 2:16 PM
^c
I instantly suspected as much.
Posted by: James Sweet
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August 12, 2010 2:20 PM
You know, when I was growing up, and even partway into my 20s, there was a red line that would appear on my left hand from time to time, beginning near the web of skin separating my ring finger from my pinky and ending around the heel of my palm. It's probably nothing to do with this, but I can't help but notice that it pretty much exactly coordinates with one of only two Blaschko lines that pass over the palm... Makes ya wonder, eh?
I haven't seen it in years, and it never caused any pain or sensational manifestation... it would usually show up after I got out of a hot shower, and would go away after a few minutes. So I never worried about it.
Posted by: Mike in Ontario, NY
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August 12, 2010 2:21 PM
I wonder if hair growth patterns follow Blashko lines?
The two-panel female figure sorta reminds me of Maori body art.
Posted by: Emefen
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August 12, 2010 2:23 PM
If only I had you as my Embryology prof in med school... Great explanation of a concept that a lot of people just don't know about, and are always surprised when it creeps up in a dermatology lecture here or there. Thanks!
Posted by: gussnarp
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August 12, 2010 2:24 PM
This is cool stuff. But it makes me think of how cool it would be if we actually did have tiger stripes, or brindled patterns in our skin, not of moles or rashes, but just of pigment variations. That would be really cool.
Posted by: PZ Myers
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August 12, 2010 2:33 PM
Sven hasn't done much Drosophila work, then! Yeah, flies are weird: they undergo synapsis in mitosis and can have somatic crossing over.
There is no correlation between Blaschko lines, nerves, blood vessels, or acupuncture meridians. They represent very early events in morphogenesis, long before any of that later stuff develops.
Posted by: gussnarp
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August 12, 2010 2:37 PM
@PZ - But you know some woomeister somewhere will see those lines and tell you how they are proof of acupuncture, auras, chakras, or some new brand of woo they are peddling.
Posted by: irenedelse
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August 12, 2010 2:38 PM
I wonder if Blaschko's lines can be revealed in skin color patterns? Combining mosaicism with differences in the melanin-related genes carried by each set of chromosomes, shouldn't it be logically possible?
Posted by: Rorschach
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August 12, 2010 2:40 PM
There are rare reports of psoriasis spreading along Blaschko lines , although I have never seen one myself.
Posted by: djinngenie
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August 12, 2010 2:42 PM
I have a very odd pattern of hair/no hair at all on my legs (I'm female) and have always suspected it was due to mosaicization (my father is rather astonishingly hairy, and my mother is basically hairless) but now I have an actual answer. It appears to follow the blaschko lines; way way way cool! Thanks.
PS. How is it that i managed to log in here? (insert long, rambling comment about multiple previous failures.)
Posted by: Urmensch
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August 12, 2010 2:44 PM
With #6 I wondered if hair growth follows the lines.
Double crowns and other weird hair kinks run in my family. It would be interesting to know if it is due to these Blaschko lines.
Posted by: Sven DiMilo
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August 12, 2010 2:45 PM
Can't stand the smell of the damn medium, for one thing. But yeah, genetics is not exactly my bailiwick. That's why I like posts like this one.
Posted by: djinngenie
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August 12, 2010 2:47 PM
I also have striped hair. But it doesn't look like the Blaschko lines. Chimeric affects, perhaps? Or just random weirdness.
Posted by: Loreo
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August 12, 2010 2:55 PM
The cowlicks (swirled hair growth) on the back of my head seem to follow the general direction of the Blaschko lines, in that the hair on the right side grows inward and that on the left side grows upward in a little swirl. I may be making a mistake by thinking that Blaschko lines represent some sort of direction that would affect the orientation of hair follicles, though.
In any case, it seems odd that these lines are asymmetrical. Is there an understood reason for that?
Posted by: Allen
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August 12, 2010 2:57 PM
I used to work with this guy who had a strange patch of hair on his arm near the elbow. It was much bushier than normal arm hair, and it was a different color (the same as the hair on his head), and it appears to correlate to one of the Blaschko lines. That's pretty cool.
And yeah, what #7 Emefen said, good explanation for something most people probably have never heard about. Thanks PZ, I just learned something cool today.
Posted by: kilternkafuffle
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August 12, 2010 3:09 PM
Really amazing!
Does tetragametic chimerism then actually mean that some individuals have two separate genomes?? Could they then commit a crime shedding one set of DNA, and then get away with it by submitting another one for testing? Gattaca II!
Posted by: PZ Myers
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August 12, 2010 3:26 PM
Be careful about overinterpreting. Blaschko's lines are set up very early, and other pattern forming mechanisms can kick in later to completely override them.
Posted by: Bronze Dog
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August 12, 2010 3:28 PM
Great, now all our little podlings will be wanting to have their lines tattooed.
You're probably too late: In an episode of Naruto (magical ninja anime for those not familiar), Jiraya looked at Naruto's hair whorl to tell him his his "chakra network" was a right-rotation type.
In another game I watched an Let's Play of called Fragile Dreams, one of your companion's random speaking comments was about the direction of Seto's hair whorl.
That seems to tell me that the most visible lines might have associated woo already in the public consciousness of Japan. It wouldn't be surprising if someone drew up an acupuncture-esque chart based on seeing someone's skin condition spread along them.
Posted by: Katharine
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August 12, 2010 3:33 PM
Four questions:
1) Has anything similar to the engrailed experiment been carried out in any larger animals - e.g. mammals?
2) This is probably overinterpreting, but is there any significance to the observation that there seems to be a pattern of changes in the curvature of Blaschko's lines about what is known as the 'milk lines' - the lines on which nipples develop (normal and supernumerary)?
3) What's with that line down the center of the front chest and abdomen but no dividing line down the back?
4) Is there a convenient 3D visualization of Blaschko's lines on a human body on the internet?
Posted by: Athena
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August 12, 2010 3:44 PM
"swathes of moles...in a stereotypically patterned way." Bingo! That explains the crazy layout of moles on my back and sides. I've always wondered why they were so symmetrical. None of the dermatologists could ever explain it. Thanks, PZ.
Posted by: MadScientist
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August 12, 2010 3:49 PM
The pattern on the back reminds me of the musculature pattern of a fish ... it's too long ago that I've done human anatomy so I can't recall what the muscle pattern looks like on the back. Time to dig up Gray's Anatomy.
Posted by: jensrandrup1
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August 12, 2010 3:50 PM
Off topic, I know, but for anyone interested here is a link to the (finally) released video of PZ at the AAI Copenhagen 2010 Conference.
http://www.youtube.com/user/AteistiskSelskab#p/u/3/gyEISfS15g4
Posted by: SteveM
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August 12, 2010 4:02 PM
A recent crime documentary was covering the "discovery" of chimerism and one of the pictures they showed of a known chimera had essentially a checkerboard pattern of light and dark skin over his abdomen.
Posted by: herlathing
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August 12, 2010 4:05 PM
Interesting. Perhaps it explains the "Christmas Tree" pattern seen in pityriasis rosea?
Posted by: koyote_ken
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August 12, 2010 4:14 PM
This is OBVIOUSLY the work of a designer!! Where's Casey Luskin when you need him?!?
However, put a wig on that chick and she's HOT!
Posted by: Antiochus Epiphanes
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August 12, 2010 4:14 PM
Top-notch post, PZ! Daddy like!
Posted by: Brownian, Most Vicious & Petty of Pharyngulites
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August 12, 2010 4:23 PM
So do they taste like marble rye too, or just look like one?
Posted by: Mystyk
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August 12, 2010 4:27 PM
My eldest niece has vitiligo, causing her to go from white skin to even whiter skin in various patches. PZ, do you know of any research that links vitiligo to these Blaschko lines?
Posted by: Tulse
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August 12, 2010 4:27 PM
Those would be some damn funky tattoos.
Posted by: David Marjanović
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August 12, 2010 4:29 PM
On both sides together, they remind me a lot of the good old vertebrate W-shaped muscle blocks (see comment 24). But I'm surprised they manifest in the skin.
That would make sense, but when PZ says they don't line up...
Yes. In other words, they're two half people, two half bodies.
In principle, yes. IIRC, there are known cases.
Because the neural-crest cells come from the back and spread towards the midline on the other side. Pigment cells are descendants of neural-crest cells...
Posted by: https://me.yahoo.com/a/Bc3ZXfB0xYMLqTpmIpdWNS9hqQaW#b25a7
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August 12, 2010 4:30 PM
My daughter has mosaic Turner's Syndrome. Interactive MRI has shown her brain functions differently than both those with full TS and the "normal" human brain.
She'll be participating in another study at Stanford next month.
Science is so cool!!
Posted by: Scented Nectar
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August 12, 2010 4:41 PM
Does anyone know if this might be connected to the nonpregnancy type of melasma?
Posted by: Betelgeuse
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August 12, 2010 4:44 PM
Great to have learnt something new and cool today PZ, thanks!
@kilternkafuffle & David Marjanovic
In cases of chimerism, I presume though that the cells will be a true mosaic of one genotype and the other; or do the different genotype halves follow fixed patterns of distribution?
The case of leaving one set behind with a bunch of cells, and then submitting something else for analysis would surely be a matter of pure chance if the former was true, right?
Posted by: Glen Davidson
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August 12, 2010 4:45 PM
I'd guess that some of those lines more or less follow the segmentations that are a part of our development.
Like flies, because they're segmented, and we're segmented.
Glen Davidson
Posted by: Betelgeuse
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August 12, 2010 4:47 PM
..not that it could be done on purpose, even otherwise.
Posted by: ouranosaurus
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August 12, 2010 5:00 PM
Re: tetragametic chimerism and crime. I'm not sure if there have ever been crime-related problems, but the CBC up here in Canada ran a news special a few years back about a couple of people who ran into major problems with child protection and/or organ donation because of it. IIRC, one woman needed a kidney but DNA tests showed she was the aunt of her sons (odd, because she'd, you know, been there when they were born) another woman had a DNA test due to a complicated child custody battle, turned out to apparently not be the mother, and almost lost her kids before doctors figured out what was going on.
Posted by: cuco3
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August 12, 2010 5:01 PM
I'm sure I remember chiamaerism cropping up in an episode of CSI. Needless to say, Grissom figured it out. :-)
Posted by: Franklin Percival
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August 12, 2010 5:06 PM
Thanks PZ, fascinating stuff. A few weeks ago a young mate of mine invited me over for a meal and to meet her new boyfriend. He and I got chatting when I asked him what he considered his genetic heritage to be, because although he appeared to be black African, there was evident variation in skin colour on his arms where he had very much lighter skin tiled with his facial dark black African.
He was delighted to be asked and stated that his Mum was mostly Malay while Dad was mostly Ghanaian, though there were elements of various Europeans known to be in the mix. He asked if I knew of an explanation for the colour change and I did not.
Serendipitously, I sign in this pm and PZ Myers has offered possible answers to the question which I had been trying in the back of my mind to relate to the lymphatic systems.
Posted by: Brownian, Most Vicious & Petty of Pharyngulites
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August 12, 2010 5:08 PM
Yup.
Posted by: tigerlily
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August 12, 2010 5:20 PM
Need Help Sorry for the long post.
This is off topic but I'm arguing with a creationist online. I'm not much of a scientist.
She posted this:
The biggest stumbling block to evolution as Darwin presented it is that both male and female had to evolve at the same rate and in the same way so as to be able to reproduce compatibly. The statistical chances of that happening explodes the bounds of believability. As one statistician I heard explained it, it would be a millions times more likely that a tornado, striking a garbage dump, would mix up the garbage and leave behind a functioning Boeing 747
I mentioned speciation etc. And that a the statistician was commenting on a straw man argument.
She came back with. ....
You misunderstand, the point is that the very first mutation that went from asexual to sexual reproduction (as presented by the theory of evolution) would have had to have both compatible male and female evolving simultaneously, same time, same place, etc.
Can anyone give me a simple explanation to use and preferably a link to an article or something(not from an atheist blog) to refer to.
Thanks for any help or links...
Posted by: rturpin
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August 12, 2010 5:28 PM
Curiously, these lines don't seem to demarcate dermatomes. I would expect some relationship. Care to comment, PZ?
Posted by: PZ Myers
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August 12, 2010 5:53 PM
No, these don't line up with myomeres. These cell dispersals are thought to occur very early, well before segmentation. Smear the cells out to make a ectodermal sheet, then later the sheet gets partitioned. Timing, timing, timing.
Posted by: Antiochus Epiphanes
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August 12, 2010 6:02 PM
This is probably not the appropriate thread to discuss this, but what the hell. Sex evolved very shortly after the evolution of eukaryotes (or along with eukarya), and initially did not require separate sexual classes at all (male or female)...cuz they were just unicellular protoeukaryotes. In all probability, they were haploid (one set of chromosomes) and reproduced primarily asexual. Every once in a while, two of these haploid cells would merge to briefly form a diploid cell, which would then likely undergo meiosis (and importantly recombination) giving rise to four haploid daughters.
I can't think of any online sources, but Nick Lane wrote an excellent chapter on the evolution of sex in "Life Ascending".
A cool and fairly recent paper about this would be
Glansdorff et al. 2009. The Conflict Between Horizontal Gene Transfer and the Safeguard of Identity: Origin of Meiotic Sexuality. Journal of Molecular Evolution 69:470–480.
It's a little abstruse to the average lay person, but if you can get access to it, you could skim it and get the main point without too much brainhurt.
Posted by: tigerlily
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August 12, 2010 6:06 PM
@Antiochus. Thanks! I appreciate your time!
Now back to our regularly scheduled programming...
Posted by: Antiochus Epiphanes
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August 12, 2010 6:07 PM
So in short, tigerlily...your opponent has set up a ridiculous strawman. There are lots of sexual organisms today that don't have separate sexual glasses (like male or female) and these seem to do just fine.
Stick to discussing single-celled organisms, because that is where the conversation on the origins of sex needs to be.
Must fetch daughter from daycare...
Posted by: Antiochus Epiphanes
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August 12, 2010 6:08 PM
^Ummm...that's supposed to read sexual "classes"...now where did I leave my sexual glasses?
Posted by: Glen Davidson
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August 12, 2010 6:14 PM
Really. Wow, it's all so complex that there just had to be a designer. Because, you know, ignore all of the marks of evolution in the complexity, and you will just see complexity.
Seriously, though, thanks for the explanation, and the interesting phenomenon.
Glen Davidson
Posted by: https://www.google.com/accounts/o8/id?id=AItOawmDXQQZsY4sQiOIs_r8zwGdjJJ0s1Foff4
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August 12, 2010 6:26 PM
This will throw a wrench into the reliability of genetic screening (which PZ hints at). In particular when trying to identify genetic risk factors that may affect your kids based on the genotype of the somatic tissue in the parents. In other words, one can no longer assume that the genotype of (a small part of) the soma (e.g. a buccal smear) is the same as in the germline.
Posted by: lenoxuss
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August 12, 2010 7:01 PM
Regarding origin of sex: Although their existence is not absolutely-necessary evidence that sex evolved among non-sexual organisms, we shouldn't forget that there exist today many species which reproduce both sexually and asexually.
So there isn't a "no-reproduction zone" to cross; it's rather more like a species gradually changing its diet by eating more and more of B and less of A and eventually discarding A and specializing in B. (I'm thinking of pandas and their reliance on bamboo in defiance of their carnivorous ancestry, for instance.)
Also… just because sexual reproduction is the end result (well, current result) of the interaction between gametes doesn't mean that's how the process began (indeed, the process almost certainly couldn't have begun that way). Here, Wikipedia describes some just-so stories that fail to rise to the rigor and sophistication of Goddidit.
Although I can't find it now, I swear I once saw an Uncommon Descent post that not only made this "sex can't evolve" argument, but actually argued that evolution required the independent development of sexes for each species. Yeah, like that guy said!
Posted by: Stina
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August 12, 2010 7:11 PM
Cool subject, and a very nice post, PZ. Bookmarked!
Posted by: beltlinebilly
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August 12, 2010 7:44 PM
This comment is in no way scientific but "way cool", dude. Biology was one of my favourite courses in High School (I didn't need to study, it just came to me so easily) and genetics and the expression of genes was and still is fascinating. Now I've been exposed to new information for which I give you many thanks. Again, WAY COOL!!!
Posted by: jenbphillips
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August 12, 2010 7:50 PM
tigerlily,
see here for PZ's valiant but no doubt futile effort to disabuse Ray Comfort of similarly silly notions. There are some links and some keywords to google, if you like.
Good luck!
Nice review of Blaschko's lines, PZ! It's been a while since I thought deeply about early embryogenesis, and this was a lovely read.
Posted by: Keith
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August 12, 2010 7:59 PM
Her case finally turned in her favour because she was pregnant with another child and had medical documentation proving she wasn't a surrogate. When she gave birth the infant's DNA was immediately sent off for testing, showed the same situation as that of her other children, and finally proving what was suspected by some of the medical professionals to be going on.
Posted by: Keith
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August 12, 2010 8:09 PM
What some researchers have been pointing out is that we don't really know how common the condition actually is (even tetragametic: it may be rare, but no one knows how much), simply because until recently with genetic testing, no one could point out that part of a person's body was genetically different from another part, and even today the vast, vast majority of people don't get any kind of genetic testing. And aside from criminals or paternity suits, where an oral swab or blood test might be compared to semen, skin cells, or the genetics of a child, those who do usually only get tested from one place in the body so there's no way of seeing if there might be an internal difference.
Posted by: amphiox
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August 12, 2010 8:23 PM
I seem the recall this as the plot of one of the episodes of one of the CSI shows, or maybe it was Bones.
Posted by: amphiox
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August 12, 2010 8:27 PM
re: tigerlily
It's fascinating how creationists recycle the same discredited arguments again and again. Intellectually stagnant, never changing, never adapting.
I guess at least they practice what they preach. Their ideas never evolve!
Posted by: Cactus Wren
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August 12, 2010 9:21 PM
The only really annoying part of that CSI episode is where Grissom described the character's condition to the character, ending with something like, "Your brother died -- but his genes live on in you". NO! There were two embryos, and neither of them died.
That line, I think was the writers' concession to woo: to the notion that an embryo is a "person", that there must necessarily be some continuity of identity from zygote to born child. The notion inherent to Grissom's line (it may be significant that the character was raised Catholic) is that one embryo == one person, and the other embryo only left behind "genes". What actually happened is that two embryos became one person, and it peeves me to hear Grissom imply that that's not the case.
Posted by: Paul Burnett
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August 12, 2010 10:35 PM
One of my favorite lines is the perineal raphe - see http://en.wikipedia.org/wiki/Perineal_raphe.
Posted by: tigerlily
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August 12, 2010 11:23 PM
@jenbphillips
Thanks. Tried to search before jumping on the thread. I have it bookmarked for future reference. Doubt I will make any dent with rational arguments. Learned alot on the blog. Thanks PZ.
Posted by: tigerlily
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August 12, 2010 11:28 PM
@amphiox#59
They keep saying the same thing, it's just a theory etc. The male female reproduction thing was new to me but it's hard arguing real facts with fake creationist facts. Don't think i made a dent but I tried.
Posted by: Ing: PhD Trollologist
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August 12, 2010 11:52 PM
If no one brought it up, sex doesn't necessarily mean male/female to start with. If i remember from my Evo class, the idea is early on you had just germ cells produced with no gender. As with most traits like this you had a bell curve of small micro-gametes that were fast moving but low in fuel for development, medium sized, and fat high in fuel but slow moving. If all were in a vacuum the medium sized would be superior, but in this condition micro and macro gametes formed a pressure that selected for the other. Specialization was superior and the extremes were selected for at the expense of the middle range, giving us male/female sex cells.
So sex started as cells making 1/2 gene spore cells to meet up with others and get a new offspring that had the benefit of crossing over and became gendered due to that evolution. Male/Female don't evolve separately, as long as one arises the other will be selected for as well.
Posted by: timrowledge, Ersatz Haderach
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August 13, 2010 1:22 AM
Those would be the ones they used to (still do?) advertise in the back of comics, right? "X-ray glasses - see through clothes!"Posted by: David Marjanović
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August 13, 2010 5:57 AM
People, get used to saying Sophophora instead of Drosophila. <broad grin>
The former, though the pieces of the mosaic aren't necessarily always the same size.
Segmentation isn't a property of organisms, it's one of organ systems. Part of our musculature is segmented, that of arthropods is not; arthropod skin is segmented, ours is not; arthropod extremities line up with the segments of the skin, ours don't care about the myomeres or the vertebrae at all...
Yes.
Or what they advertise right now on ScienceBlogs, in the right sidebar, at least in Austria: "Get the naked-scanner for your cellphone!" For some time, they even advertized it as illegal.
Posted by: TheCalmOne
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August 13, 2010 6:05 AM
Oh wow! My daughter has a faint but distinct line down her tummy where the skin is noticeably darker on one side. I always wondered why that is - thought it was something to do with her complicated ancestry: British (English and Welsh) on my side, Portuguese, Spanish and African on her mother's side (ex-wife devil woman is Brazilian).
Posted by: Peter Ashby
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August 13, 2010 6:21 AM
There are other invisible lines. Your mandible (lower jaw) looks all of a piece, but if you label the cells in different branchial arches (the gill like pouches in the 'neck' of embryos from pharyngula stages one) they populate different parts of the jaw bone which is formed by contributions from more than one arch.
If you have ever had to learn the tortuous paths of the cranial nerves and try and make sense of why they innervate the structures they do the answer is that nerves from a rhombomere (hindbrain segment opposite an arch) innervate structures formed by cells in the arches. Since those cells migrate all over the face, neck and throat the nerves go on strange routes, pass right by one muscle and innervate another one.
There are many aspects of adult anatomy that only make any sort of sense when you learn the embryology. I never understood brain anatomy until I learned how the brain was formed. Keeping the bends, folds and growing overs along with alar and basal plates in mind and it all falls into place.
Posted by: KG
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August 13, 2010 6:58 AM
Nice post PZ. Suggests new possibilities for body modification - surely there must be something you could do to bring out the lines?
Posted by: Twopints
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August 13, 2010 7:12 AM
Don't forget that Alan Turing did a lot of work on patternation.
"The Chemical Basis of Morphogenesis" 1952,
Most people just think of him in connection with early computing, but he was very much a polymath.
Posted by: Icaria
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August 13, 2010 9:03 AM
...Fuck you!
Posted by: Mike Crichton
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August 13, 2010 11:28 AM
Would you mind fuzzing out the genitals in the baby picture? It would really suck for one of your readers to get busted for "kiddy porn" because of some over zealous prosecutor.
Posted by: hafoccynn
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August 13, 2010 2:16 PM
Great post!
I have actually seen my Blaschko's Lines: when I was about 17 I had pityriasis rosea and it formed in this pattern. It was incredibly itchy, and my dermatologist at the time could think of nothing other than how interesting it was that it formed in this cool pattern.
---sue h
Posted by: Owlmirror
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August 13, 2010 8:28 PM
Actually, PZ posted something a while back about that, pointing out that the theory that Turing proposed was simple, beautiful... and wrong in its simplicity.
http://scienceblogs.com/pharyngula/2007/06/segmentation_genes_evolved_und.php
See in particular the cited paragraph by Sean B. Carroll.
Posted by: monado
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August 13, 2010 8:34 PM
So do hair whorls follow Blaschko's lines? If you have a double crown, does that mean that your Blaschko's lines are coming from both directions?
Posted by: bobdaboi
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August 14, 2010 12:07 PM
How does a "cell in the middle of the back of a tiny embryo" know where the middle, end, front and back of itself are?
Posted by: great.american.satan
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August 14, 2010 2:42 PM
Hair following lines? http://en.wikipedia.org/wiki/Hair_whorl
I couldn't find what I wanted on the subject, but it struck me that the whorl on the crown of the head in most people is probably formed by this. The link at the bottom of the pathetic wiki article talks about an association between direction of whorl and handedness, which makes me think... Connection between these lines and handedness? Food for uninformed speculation.
BTW, I am either having déjà vu hardcore, or tigerlily has used almost this exact question to get attention in a thread on pharyngula before. Which is sad, but it's nice to be trolling with politesse instead of cretinism.
Lastly, I want to say - as an artist - that I feel many human skin abnormalities (albinism, vitiligo, port-wine stain, birth marks) could be interesting, even attractive, if represented right. These clinical photographs make it look like so much dirt and not very pleasant. But I bet a tiger-striped lady could look positively cute if photographed by a skilled artist or viewed in natural light.
Posted by: Sven DiMilo
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December 10, 2010 11:31 PM
I think I see a homunculus in there.
Posted by: Sven DiMilo
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December 10, 2010 11:37 PM
Briefly: chemical concentration gradients, intercellular signalling, and consequent gene-regulation patterns.
Posted by: las-vegas-seo
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July 19, 2011 8:49 PM
I never realized there were physical patterns in our bodies like these Blaschko's lines you explained. It really makes me think though. I wonder if we have a similar type pattern for our emotional makeup - something to ponder for sure.
Posted by: murfomurf
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July 23, 2011 10:08 AM
Aha- when I did embryology way back in 1970, no one mentioned Blaschko's lines. I wondered then, and still do, how the very early cell divisions "knew" when they could stop replicating and start differentiating. Then I thought: How does this one cell know it is going to be the left half of a brain and the skin going wherever? etc Where is the pattern? It can't be external to the cells, but what is it written in and how does the RNA read it off to start the differentiating? Do Blascho's lines correspond with this early pattern?; and, how does the pattern get the timing right to fit all the differentiations and size increases into around 40 weeks of pregnancy? Blah, blah- I wondered a lot. I assume that cell divisions and differentiations also carry on inside the skin capsule of the embryo and also get dragged around by groups of cells migrating, eg. the migrations of neuronal axons & dendrites so they lead from the correct spinal innervation pathway to the sensory-motor areas of the brain, and how the internal neurons migrate "forward" from more "primitive" brain areas towards the cortex to give the white and grey matter. Do you know if these early cell divisions occur along B's lines as well? When might they cease having their influence on the migrated cells? [I'm fishing for clues on my pet theory about autistic brain development]. Another aspect that struck me is the colour patterning in cats you mentioned- I wondered whether their pigmentation also carried through other areas of their inner structures- I think I recall seeing patches of colour inside a cat opened for surgery, but that might be a dream! However, there are peculiar patterns that SEEM to go through cats such that you get 2 patches of colour on a pink nose and also, faint traces of tufts of grey fur at the end of their pale orange tails, which I am tempted to conclude might be related! Anyway- thanks for the info and I'm afraid I'm off wondering again!
Posted by: kittykittykatana
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September 13, 2011 12:34 PM
I was always fascinated by the natural occurrence if patterns in nature! Ever sense I was young I could see the patterns amongst the cellular liquid that was in my eye when I squinted. To me its the beauty of nature.
Katana H.
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