Sorry Flipo...the streaks only "appear" to be across the grain....note that the ribs are book faced and appear to be off quartered which would produce the ringing pattern in the wood. However the grain is still running tail to head. Just like the streaking.
Some woods like say padauk when sliced up will reveal white streaked pockets. Many think this is sapwood. It is not.
Although rare, many any other woods will show pockets of darker streaking, BRW being one. Especially stump woods and woods that have started to spalt. THis may be from mineral staining or "pitch flow" or just the nature of the beast.
Looking closely at the pics one can see that the streaking follows the book matched length wise grain pattern. It appears to be a very thin streak in the wood as the kurf from booking the sides was enough to alter the mirror image of the streaks almost to the point of not being recognized as the same streak.
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Me conclusions be ...1) the tree grew that way, or 2)fungus from spalting/starting to rot have discoloured the grain. It is natural just like the birthmark on my sweeties...never mind.
As to removal.... me only be guessing and personally wouldn't even bother, you know me being a Mr. Natural kinda guy.
Then there are others that wood say its the mark of the beast and that you should pack it and all its parts up and send them off to some one like me for proper disposal. ( Be sure to include the $500. disposal fee)
blessings