“Because it offended conservatively minded people,” Pia said. “Particularly here in the United States, limitations were placed on what could and could not be done in stem cell research with government funds.”

“Well said,” Yamamoto commented. “Here’s a harder question. Let’s say that the research on embryonic stem cells had been allowed to proceed unimpeded. Can anyone say what the major problem would have been if the research had advanced to a point of using the stem cells to treat patients?”

None of the students moved.

“Let me give you a hint,” Yamamoto said. “I’m referring to an immunological problem.”

“Rejection!” Lesley called out, her eyes lighting up.

“Exactly. Rejection, meaning that any use of such embryological stem cells would have elicited some degree of rejection reaction. Some techniques would have reduced this problem but not completely eliminated it.”

All three students nodded. Everything that Yamamoto was saying they had heard before. “Now, can anyone define ‘induced pluripotent stem cells’ in contrast to embryological stem cells? These are the cells that Dr. Rothman and I have been working with exclusively.”

“They are pluripotent stem cells made from mature cells, usually a fibroblast and not egg cells,” Pia explained. “They are ‘induced’ by particular proteins to revert back from being a mature fibroblast to being a stem cell.”

“Exactly,” Yamamoto said. “And isn’t it a marvel that it works? For a long time one of the tenets of biological science was that cellular differentiation was a one-way street, meaning the process could never revert. But people should have known that this particular tenet was false. After all, it was known that certain animals could regrow body parts, like starfish and salamanders. Also cancer should have been a hint that the process of differentiation could go in the opposite direction, as many cancers are composed of immature cells that arise in organs populated by mature cells.”

Pia found herself glancing at her watch and sitting up straighter in her seat. She wanted to speed up the review session but didn’t know how. She inwardly groaned when Lesley piped up with a question: “How exactly are the cells changed back from mature to immature?”

“The same way that everything else is accomplished in the cell,” Yamamoto said. “By switching on and off genes. Remember, every eukaryotic cell—that is, a cell with a nucleus—contains a copy of an organism’s entire genome. Meaning that every nucleated cell has all the information necessary not only to build itself but to build the entire body. How this works is a process called gene expression, meaning the turning on and off of genes in a kind of molecular ballet. I know you learned all this in your genetics courses in college and during your first two years here at Columbia. Anyway, cellular maturation proceeds by a sequential switching on and off of the appropriate genes. It used to be thought that genes functioned by producing specific proteins, sorta one gene for one protein. But now we know it’s far more complicated, as there are significantly fewer genes than originally thought. For the cell to go in the opposite direction of maturation, the sequence has to be reversed. Are you all with me so far?”