User blog comment:Xilinoc/Zanpakutō & Tōjū arcs - Review/@comment-5847836-20140123021224/@comment-5398287-20140123023801

I believe Nimaiya said they were made from spirits, so the jury's still out on that one.

And yeah, now that I think about it, the fact that Muramasa could summon Hichigo because he was a Zanpakuto spirit but not control him because he was a Hollow also makes sense.