With just a few donor stem cells, a team of Japanese researchers have grown functional mouse kidneys inside rats, an advance that could address the chronic global shortage of donor kidneys.

The new kidneys appeared to be functional, providing proof-of-concept validation that this approach could be used to generate human kidneys inside livestock, according to the study to be published in the forthcoming Nature Communications journal.

For patients with end-stage renal disease, kidney transplant is the only hope for regaining quality life. But many patients never undergo transplant surgery because of a chronic shortage of donor kidneys. Researchers have also been working on ways to grow healthy organs outside the human body. 

One such method, blastocyst complementation, has produced promising results in generating rat pancreas in apancreatic mutant mice. Researchers from the National Institute for Physiological Sciences in Japan decided to investigate whether the method could be used to generate functional kidneys, which would have much larger application in regenerative medicine due to the high donor demand.

Initial attempts by the team to grow rat kidneys were unsuccessful, as rat stem cells did not readily differentiate into the two main types of cells needed for kidney formation. However, when the reverse scenario was attempted, mouse stem cells efficiently differentiated inside rat blastocysts, forming the basic structures of a kidney.

After being implanted into pseudo-pregnant rats, the complemented blastocysts matured into normal foetuses. Remarkably, more than two-thirds of the rat neonates contained a pair of kidneys derived from the mouse stem cells. 

Further screening showed that all the kidneys were structurally intact, and at least half could potentially produce urine. "Our findings confirm that interspecific blastocyst complementation is a viable method for kidney generation," said Masumi Hirabayashi from the varsity. "This approach could be used to generate human stem cell-derived organs in livestock, potentially extending the lifespan and improving the quality of life of millions of people worldwide," Hirabayashi added.