Scientists are trying to grow miniature kidneys in the lab, but current models don’t fully mimic the real organ’s complex structure or function. In this study, researchers created new “kidney progenitor assembloids” (KPAs) using mouse and human cells. These KPAs can organize themselves into structures that closely resemble real kidneys, with developing nephrons connecting to a central collecting system—just as they do in the body. The resulting tissues are more mature, more complex, and able to perform several key kidney functions both in lab dishes and after being transplanted into animals. When used to model polycystic kidney disease, the KPAs developed cysts and other hallmark features of the disorder, offering valuable insight into how different kidney cell types interact during disease. This new platform could greatly improve disease research and advance future kidney regeneration therapies.