One of the promo material of last December Cambridge Embryonic Stem Cell Symposium was the Nature Insight: Stem Cells which was a supplement in Nature Vol. 441, No. 7097 pp1059-1102 from June , 2006. In it I found the best, brilliantly argumented and conceptualized, data-rich paper of Thomas Rando from Stanford University on Stem cells, ageing and the quest for immortality. I plan to introduce the thoughts of the paper in great details in future posts of Pimm. For today I just copy here Fig. 1: Tissue heterogeneity and stem-cell functionality for homeostasis and repair and the legend because this is the essential starting point of systemic regenerative medicine. Take an hour to think about it.