Background

Humans are born with approximately one million nephrons, the functional units of the kidney, which will be used throughout their lifetime. These functional units are formed only during the fetal developmental stage, and their formation ceases completely after birth. The total number of nephrons in the two kidneys of an adult varies from 200,000 to 2,000,000, up to a 10-fold difference between individuals. If the number of nephrons falls below the standard range during the fetal period due to environmental or genetic factors, leading to oligonephronia, the risk of developing chronic kidney disease or hypertension increases in adulthood. This is the prevailing view in the medical community. Research to elucidate the detailed mechanisms of fetal kidney development has primarily relied on animal models such as mice. However, due to differences in kidney structure between humans and animals, there are limitations in directly applying the results of animal experiments to patients. Existing single-cell analysis techniques can distinguish between cell types but do not show the spatial information of individual cells or their interactions with surrounding cells. To clearly understand the developmental process of a highly organized organ like the kidney, a precise map is needed that simultaneously analyzes the spatial structure of cells and the signaling network between cells.

Key Findings

A joint research team from the University of Pennsylvania School of Medicine and the Children's Hospital of Philadelphia (CHOP) obtained more than 700,000 cells from fetal kidney tissue between 12.5 and 20.5 weeks of gestation. They combined single-cell RNA sequencing (scRNA-seq) and spatial transcriptomics (ST) to create the first map of cell gene expression and spatial location. Analysis revealed that kidney development is guided not only by cell-specific genetic programs but also by soluble signals from neighboring cells. In particular, insulin-like growth factor 2 (IGF2) was found to be a key signal in maintaining and proliferating immature kidney progenitor cells (NPCs). The mechanism involves IGF2 secreted by surrounding cells binding to NPC receptors, thereby preserving the stem cell properties. This was demonstrated in an experiment in which IGF2 supply was blocked in the culture medium, causing NPCs to lose their stem cell ability and differentiate rapidly. Furthermore, analysis of a large population genetics database confirmed that IGF2 gene variants are associated with adult kidney size and nephron number. This is noteworthy in that it elucidates the genetic factors that regulate the proliferation limit of stem cells.

Significance and Prospects

This research lays the foundation for overcoming the limitations of existing developmental research that relies on animal models. In particular, by revealing the cell-to-cell signaling network during the fetal period, it provides a breakthrough for organoid technology, which aims to create mini-kidneys in the laboratory. Existing organoids have the disadvantages of having a small number of functional units and a simple internal structure. By reproducing the cell arrangement and IGF2 concentration changes revealed by ST, a more sophisticated artificial kidney can be constructed. However, there are still many points to be addressed before this can be linked to the development of organs for transplantation or actual treatment. Fetal kidney formation involves various factors, including the immune relationship with the mother, blood vessel formation, and blood flow. It is difficult to reproduce all of these complex biological responses in a planar culture environment by simply adding individual factors. In the future, the combination with an engineering platform that can increase the simulation accuracy, such as a microfluidic chip, is considered a key task. If research continues to overcome this barrier, it is expected that it will be possible to provide customized cell therapies for patients with intractable kidney diseases.