Tissue can be a skin cell.
I realize it's all in the definition of "tissue". But media reports imply it is consistent with her story that she scratched him, and the nail broke off (supposedly with enough stuff lodged on the underside of the nail to "nail" someone). How would this play out in court? The nature of this material could be key, could it not? To whether the Defense can say it just fell on the nail or the nail fell on it in the trashcan, or whether the Prosecution can say it was consistent with her story that she scratched it off during the "rape"?