An in vitro construct useful for toxicity testing is provided, comprising: a three-dimensional (3D) scaffold comprising silk fibroin and having a crosslinked porous matrix; and stem cells adherent to the 3D scaffold. In some embodiments, the stem cells adherent to the 3D scaffold maintain stable mitochondrial DNA in long term culture. In some embodiments, the stem cells are urine stem cells.

Provided is a method for isolating a ureteric bud tip cell from cells, a tissue, or an organoid comprising the ureteric bud tip cell, comprising the following steps of contacting the cells, tissue, or organoid comprising the ureteric bud tip cell with a very low density lipoprotein receptor (VLDL-R) binding agent, and isolating the ureteric bud tip cell using the binding agent as an indicator.