Branching morphogenesis of the urinary collecting system in the human embryonic metanephros

Epithelial development of the urinary collecting system in the human embryo

The urinary collecting system (UCS) consists of organized ducts that collect urine from the nephrons and transport it to the ureter and bladder. Understanding the histogenesis of the UCS is critical. Thirty human embryos between the Carnegie stages (CS) 18 and 23 were selected from the Congenital Anomaly Research Center, Kyoto, Japan. Epithelia of the UCS, ureter, and bladder of each sample were randomly selected. Histological findings of the epithelia were analyzed according to the following criteria: type of epithelium, presence or absence of glycogen, percentage of migrated nuclei, percentage of cells in mitosis, and the surrounding mesenchyme. A thickened epithelium lining a narrow luminal cavity was observed in the pre-expanded pelvic specimens at CS18-CS23. At CS23, after pelvic expansion, the UCS showed a thin epithelium with a large luminal cavity mainly located on the early branches, whereas the epithelium covering the subsequent branches had medium thickness. Histological characteristics differed depending on the UCS part and sample stage. The degree of differentiation was evaluated, revealing that in CS18-CS23 pre-expanded pelvis specimens, the undifferentiated epithelium was found in the zeroth to third/fifth generation, whereas at CS23, after pelvic expansion, a differentiated epithelium covered the UCS zeroth to seventh generation. In a comparison of the urothelial epithelium between the UCS, ureter, and bladder, we found that urinary tract differentiation may be initiated in the bladder, followed by the ureter, UCS zeroth to seventh generations, and finally, UCS eighth to end generations. An understanding of the histogenesis of embryonic stage UCS can aid in the clinical management of congenital urinary tract defects and other diseases.

Human iPSC-derived renal collecting duct organoid model cystogenesis in ADPKD

In autosomal dominant polycystic kidney disease (ADPKD), renal cyst lesions predominantly arise from collecting ducts (CDs). However, relevant CD cyst models using human cells are lacking. Although previous reports have generated in vitro renal tubule cyst models from human induced pluripotent stem cells (hiPSCs), therapeutic drug candidates for ADPKD have not been identified. Here, by establishing expansion cultures of hiPSC-derived ureteric bud tip cells, an embryonic precursor that gives rise to CDs, we succeed in advancing the developmental stage of CD organoids and show that all CD organoids derived from PKD1-/- hiPSCs spontaneously develop multiple cysts, clarifying the initiation mechanisms of cystogenesis. Moreover, we identify retinoic acid receptor (RAR) agonists as candidate drugs that suppress in vitro cystogenesis and confirm the therapeutic effects on an ADPKD mouse model in vivo. Therefore, our in vitro CD cyst model contributes to understanding disease mechanisms and drug discovery for ADPKD.

Nascent nephrons during human embryonic development: Spatial distribution and relationship with urinary collecting system

The two major components of the metanephros, the urinary collecting system (UCS) and nephron, have different developmental courses. Nephron numbers vary widely between species and individuals and are determined during fetal development. Furthermore, the development of nascent nephrons may contribute to the expansion of the proximal part of the UCS. This study investigated the distribution of nascent nephrons and their interrelationship with UCS branches during human embryogenesis. We obtained samples from 31 human embryos between Carnegie stages (CSs) 19 and 23 from the Kyoto Collection at the Congenital Anomaly Research Center of Kyoto University in Japan. Serial histological sections of the metanephros with the UCS were digitalized and computationally reconstructed for morphological and quantitative analyses. The three-dimensional (3D) coordinates for the positions of all UCS branch points, end points, attachment points to nascent nephrons (APs), and renal corpuscles (RCs) were recorded and related to the developmental phase. Phases were categorized from phase 1 to phase 5 according to the histological analysis. The UCS branching continued until RCs first appeared (at CS19). End branches with attached nascent nephrons (EB-AP[+]) were observed after CS19 during the fifth generation or higher during the embryonic period. The range of end branch and EB-AP(+) generation numbers was broad, and the number of RCs increased with the embryonic stage, reaching 273.8 ± 104.2 at CS23. The number of RCs connected to the UCS exceeded the number not connected to the UCS by CS23. The 3D reconstructions revealed RCs to be distributed around end branches, close to the surface of the metanephros. The RCs connected to the UCS were located away from the surface. The APs remained near the end point, whereas connecting ducts that become renal tubules were found to elongate with maturation of the RCs. Nascent nephrons in RC phases 3-5 were preferentially attached to the end branches at CS22 and CS23. The mean generation number of EB-AP(-) was higher than that of EB-AP(+) in 19 of 22 metanephros and was statistically significant for eight metanephros at CS22 and CS23. The ratio of the deviated branching pattern was almost constant (29%). The ratio of the even branching pattern with EB-AP(+) and EB-AP(+) to the total even branching pattern increased with CS (9.2% at CS21, 19.2% at CS22, and 45.4% at CS23). Our data suggest the following: EB-AP(+) may not branch further at the tip (i.e., by tip splitting), but branching beneath the AP (lateral branching) continues throughout the embryonic stages. Our study provides valuable data that can further the understanding of the interactions between the UCS and nascent nephrons during human embryogenesis.