White Brindle Theory

Yes you did. I think it was your first thread and probably the one that caused me to point you here.

Lol, you're right! I still have the PM. Thanks for pointing me here again! ^_^

I don't think the markings themselves are caused by anything coming from the neural crest and travelling to the white areas. The white areas are caused by something affecting the flow of melanocytes so they don't reach those certain areas. They are simply areas the melanocytes never reached.
Melanocytes move from the neural crest to certain areas of skin where they grow and spread, normally they will fill in the skin entirely with colour producing cells but certain genes stop this happening.
For example I think tobiano allows the melanocytes to reach their proliferation areas but slows then stops the spread from those points, before the animal is 'coloured in' so to speak. If you think about what tobiano markings look like, they look like a spread of colour from specific points.
Splash might be caused by the melanocytes slowing from the start so they don't travel far from the neural crest before they are stopped from spreading (there is a window for the spread of melanocytes during embryo development).
This is just my own theory based on reseach of rat embryos with the hooded allele, which is caused by the slowing of the proliferation of melanocytes.
I wish I could make animations but me and my computer aren't tecchie enough :-/

ETA IMO white brindle would be caused by parts of the neural crest not releasing melanocytes (or releasing faulty melanocytes), leaving lines of pigmentless skin/coat.

That may be one of the causes of certain white brindles. It really makes sense for this guy.
[img]http://i872.photobucket.com/albums/ab28…]

If it was just colors like chestnut and black it would make sense for marking creation. But what about bay? How does the black on the legs stay even?
[img]http://i872.photobucket.com/albums/ab28…]
A) No markings B) Normal white markings C) Markings where black from bay is prevented from traveling to the end of the leg.

What I'm asking is why do white markings look like B where the white appears overlayed on the base color and not appear like C if the melanocytes are prevented from traveling all the way down to their destination?

Also how are there so many white marked brindles that have the white starting from the neural crest and going in the direction of creating a leg marking?

[img]http://i872.photobucket.com/albums/ab28…]
(The Bavarian filly) http://www.brindlehorses.com/jsbatteate…" onclick="window.open(this.href);return false;

These visually hint to me that the color came first and something blocked the melanocytes from attaching to the skin and hair later in development, and from the white brindles I've seen it looks like whatever causes it starts from the neural crest. The white in the one horse looks like it is clumping together and filling in the sock, there are even dark spots in the sock itself.

Also if color and markings develop simultaneously, how are patchy chimeras, who are likely caused by later embryonic fusion, with even markings explained? Wouldn't the the white markings be all over?

I'm just trying to explain what I see >.< don't take it as an attack.

I actually understand that.

Yay I made sense to someone! I know I usually don't :?

The melanocytes don't leave the neural crest in their finished form, they proliferate 'onsite'. They actually leave the neural crest as melanoblasts, melanocytes precursors.
The white markings that spread down legs might be faulty melanocytes rather than lack of as in normal markings. Someone would have to so some microscopic work on skin samples to find out what was going on. The flow of melanoblasts is on a timer, when its done its done, you won't get a second flow of something different once the flow of melanoblasts has happened.
I think if markings travelled from the neural crest these sorts of weird flowing white markings would be very common.
The markings on bay thing- like I said, the colour doesn't migrate from the neural crest in its finished form, it travels as melanoblasts and becomes melanocytes 'onsite', bay is programmed to have dark legs, the dark will only form on the legs, unless the melanoblasts never reached them when the legs will be white. The dark colour on the legs doesn't travel in its finished form.
Chmeras often seem to have even markings, or only have the patchy markings on one side of their body, I guess they must fuse incredibly early before any body parts have been sorted.

Here is a pic showing cause of hooded markings in rats, it has been found they are caused by slowing of the spread of melanoblasts, not something that happens after the malanoblasts have spread all over.

http://www.ratbehavior.org/images/Hoode…" onclick="window.open(this.href);return false;

I absolutely love trading genetics theories :D
Its always just theories and personal opinions to me, using what I have learned and understood. Probably someone who actually knows will come along soon and laught their head off LOL.

I don't think the white brindles are due to an error in melanocyte/melanoblast migration--one theory that makes more sense to me (and explains why they might be mosaics) is that there is some mutation which occurs at some point in development prior to melanocyte migration which results in a defect in pigment formation. These mutated cells migrate along just like the others and produce melanocytes covering the surface of the horse as usual, but the daughter melanocytes do not produce pigment normally and hence the areas where they migrate to appear white. The stripes you see just trace out the path of melanocyte migration taken by the cells carrying this mutation.

This thread is very similar to a previous one: [url=http://forum.equine-color.info/viewtopi… a Bird chimeric?[/url]

For your perusal: similar phenomena in humans
[url=http://archderm.ama-assn.org/cgi/reprin… R. Mosaicism in human skin Arch Dermatol 1993; 129: 1460–1470[/url] (talks about mosaicism in humans and the similar skin patterns that result)
[url=http://jmg.bmj.com/content/30/9/752.ful… C et al. Epidermal mosaicism and Blaschko's lines J Med Genet 1993; 30: 752-755[/url] (shows that stripes are due to genetically different cells, and argues why this is often missed when the wrong cell type is tested)

Thanks for the link Dogrose,
I'm wondering though if there is something that physically/chemically prevents melanocytes/melanoblats from reaching their destinations creating white markings. I can see them being dispersed on a timer, but how long does it take them to effect the skin and hair cells? And why can patterns like frame get created?

I'm also wondering what a frame overo chimera would look like, and if the stripes would go around the frame white patches if that's the cause? :random thought mostly, I honestly would be curious to see that:

Roan only effecting individual hairs seems strange since only those hairs are singled out while surrounded by colored ones. It appears that something has to be preventing those individual hairs from being colored.

Thanks for trading theories!

Nerd: At least in the case of Natal Clasi it makes sense that his white is caused by mismarked rabicano. His dam and one foal from brazil supposedly had "white brindling" but there are no pictures of either of them, inheritable rabicano makes more sense, and the pattern of his white follows that of an extensive un-dispersed rabicano.

I'm not saying that ALL white brindles are caused by messed up white markings, but certain ones really do appear to be created by this.