Board Thread:General Discussion/@comment-24382631-20140110234616/@comment-1268838-20141206164008

The immense wound inflicted on Aizen by Gin would already have been far beyond the capacity of a Hollow's ability to regenerate. Similar damage would have outright killed Ulquiorra, for example. But Aizen was able to recover from even that; I don't think it is farfetched to think that he could just grow his head back, or something similar. The Hogyoku's defenses are far stronger.