1. How did the idea of transdifferentiation arise?
In the late 1990s, sex-mismatched transplants and experiments with rodents revealed apparent transgressions of embryonic cell fates. Bone marrow cells could yield liver, muscle, neuron, and endothelium, while neurons could give rise to blood, and hepatocytes to pancreatic beta cells. Stem cells seemed to home in on injury sites, producing daughters that dedifferentiate and redifferentiate into exactly what is needed.
2. When did cell fusion enter the picture? Transdifferentiation was a logical conclusion based on phenotypes, but delving into DNA content and ploidy revealed another explanation for the apparently switching cell fates--merging genomes. In 2002, two groups showed bone marrow cells and neural stem cells, respectively, fusing with embryonic stem cells to yield embryonic stem-like cells, in mouse cell culture. Counting the chromosomes revealed that 2N had become 4N. A year later, scientists added the anticipated in vivo view: Hematopoietic stem cells ...
