Board Thread:Questions and Answers/@comment-13442349-20130726072748/@comment-11051098-20130802205519

Good point BassGod, I never thought about that.

He lost his heart when Ulquiorra killed him with Cero Oscuras, and regained his heard in the shape of a new Hollow mask when Hollow-Zangetsu took control of his body. Then, when the new mask shattered after Ulquiorra cut off his horn and his Cero imploded, he regained his heart again.

It makes sense.