Board Thread:Theories and Speculation/@comment-3568954-20150210145020/@comment-945885-20150211072242

Kinkaku and Ginkaku have the markings, too, so it stands to reason that it's connected to Kurama's chakra. Especially considering that the markings become more "dramatic" when they're powering up using Kurama's chakra.

Keep in mind that the markings go back to the one-shot pilot, where Naruto was literally Nine Tails' son assuming a human form.