Dragon Blood. Both the Blades and the 'Beards go on and on about it. The akaviri blood seal at Sky Haven Temple needs a Dragonborn's blood to be broken. Paarthurnax says "Sossedov lost mul" when you take part in the traditional greeting of two of the Dov.
Now... Bone marrow generates blood, yes? So this blood coursing through your Skyrim PlayerChar's veins comes from their marrow.
...Do Dragonborn have dragon bones? Not shapes, obviously, but tissue.