Interspecific blastocyst complementation is a potential strategy for growing human kidneys in animal hosts. Masumi Hirabayashi and colleagues now provide evidence that this approach is viable by using it to grow mouse kidneys in rats.

“The process of interspecific blastocyst complementation includes knocking out a gene responsible for renal development in the host animal, maintaining heterogeneous mutant lines to produce homozygous mutant blastocysts, and injecting these blastocysts with exogenous pluripotent stem cells that generate a kidney in the developmental niche,” explains Hirabayashi.

Here, the researchers used CRISPR–Cas9 in rats to knock out Sall1, which encodes a protein that is essential for ureteric bud invasion in kidney development in mice. Like Sall1–/– mice, Sall1–/– rats were anephric and thus a suitable host for generating mouse kidneys. Mouse embryonic stem cells were injected into blastocysts from intercrossed Sall1+/– rats and transferred into pseudo-pregnant rat uteri. Of the 18 Sall1 neonates generated, 12 had a pair of kidneys derived from mouse embryonic stem cells. Furthermore, at postnatal day 0 (P0) there was patency between the ureter and the bladder in some mouse kidneys grown in rats, indicating their potential to excrete urine.

On previous work, Hirabayashi notes that “our initial attempts to grow rat kidneys in mice proved unsuccessful, as rat stem cells did not readily differentiate into the two main types of cells needed for kidney formation”. By contrast, at P0 in mouse kidneys grown in rats, all the renal components that derive from metanephric mesenchyme were made entirely from mouse embryonic stem cells. However, the collecting ducts, glomerular capillaries and cortical blood vessels comprised a mixture of mouse and rat cells.

“Next we must regenerate blood vessels and collecting ducts in the kidneys with embryonic stem cells using the same blastocyst complementation strategy,” says Hirabayashi. “Then, we will apply interspecific blastocyst complementation to large domestic species.”