Scientists at the University of Southern California developed advanced kidney assembloids by merging separate organoids encompassing nephrons and collecting ducts into complex structures. Transplanted into mice, both human and mouse assembloids matured beyond embryonic stages, exhibiting kidney-like functions such as filtration, protein uptake, and hormone secretion. This breakthrough provides unprecedented high-fidelity in vivo models for studying complex kidney diseases and is a step toward synthetic kidney bioengineering for transplantation.