Brain cells lost to Alzheimers
Complete live hearts
Billions of live red blood cells
Live effective sperm
Some are already in use, others are at proof-of-concept stage and animal models. The last people to die of old age may have already been born.
Added: Myelin nerve sheathing (MS and Cerebral Palsy lose this sheath) (10/2011)
Added: Working pituitary glands in mice. (11/2011)