Question on built rods

[JohnH0802]

I recently bought a custom made fly rod and noted that the rod ends that go into the feruls are hollow. Now I do realize that rods are hollow. My question is what is the filler that is used in factory rods to fill this in, and is it important to do? I am just starting the planning phases of building a fly rod for the first time, and want to get all of the information down so that I can do it right the first time (i.e. can't afford to make any mistakes).

Thanks,
John