You really must read this week-long series from Will Saletan in Slate. It’s horrifying, but it’s the truth, and we need to know what’s going on.

The basic point? That the current bill sponsored by Hatch and Feinstein would prohibt procreative cloning but permit it for research, but would mandate that cloned embryos not be preserved after 14 days of existence. The "problem?" that scientific research shows that the cells derived from embryos after that point are far more useful for treatment than those from before – precisely because differentiation has begun to occur, so one can get the specific cells related to specific systems that you want. From embryos, in some studies, as old as several weeks old.

Four years ago, a team led by John Gearhart, one of the field’s top researchers, published a study of cells "derived and cultured from 5-, 6-, 7-, and 11-week postfertilization primordial germ cells." The derived cells, unlike hES cell lines from embryos before 14 days, caused no tumors when they were injected into mice. Gearhart’s team found that the derived cells "may be useful … as a resource for cellular transplantation therapies."

{snip}

Transplantation forged ahead, but differentiation lagged. Until scientists could grow the necessary tissues in the lab, they would have to enlist nature. Six to seven weeks of embryonic development seemed to do the trick. In 2003, Israeli researchers published a study showing that "when human and pig kidney precursors are obtained from 7- to 8-week human or 3.5- to 4-week pig gestation and transplanted into immunodeficient mice, they survive, grow and undergo complete nephrogenesis, forming a functional organ able to produce urine. Embryonic renal cells of earlier origin fail to mature into the desired professional cell fate." The authors wrote, "Our data pinpoint a window of human and pig embryogenesis that may be optimal for transplantation in humans."

From the second part, today:

And maybe that’s why pro-lifers missed the biggest in vivo differentiation story since then, which involved neither hES cells nor adult stem-cell therapy. Four months ago, Japanese researchers reported, "Anatomically complicated organs such as the kidney and lung, which are comprised of several different cell types and have a sophisticated 3-dimensional organization and cellular communication, have proven more refractory to stem cell-based regenerative techniques." But the researchers brought good news: They had figured out how to beat the problem. They had demonstrated a way to grow human adult bone marrow stem cells into kidney tissue: by putting the cells in embryonic rats.

Read the whole thing to get the complexity of the science. Tomorrow, he gets to the ethics. We’ll see.