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.

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.

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

An elaborate system of ducts collects urine from all nephrons, and this structure is known as the urinary collecting system (UCS). This study focused on how the UCS is formed during human embryogenesis. Fifty human embryos between the Carnegie stage (CS) 14 and CS23 were selected from the Kyoto Collection at the Congenital Anomaly Research Center of Kyoto University, Japan. Metanephroses, including the UCS, were segmented on serial digital virtual histological sections. Three-dimensional images were computationally reconstructed for morphological and quantitative analyses. A CS timeline was plotted. It consisted of the 3-D structural morphogenesis of UCS and quantification of the total amount of end-branching, average and maximum numbers of generations, deviation in the metanephros, differentiation of the urothelial epithelium in the renal pelvis, and timing of the rapid expansion of the renal pelvis. The first UCS branching generation occurred by CS16. The average branching generation reached a maximum of 8.74 ± 1.60 and was already the twelfth in CS23. The total end-branching number squared between the start and the end of the embryonic period. UCS would reach the fifteenth branching generation soon after CS23. The number of nephrons per UCS end-branch was low (0.21 ± 0.14 at CS19, 1.34 ± 0.49 at CS23), indicating that the bifid branching occurred rapidly and that the formation of nephrons followed after. The renal pelvis expanded mainly in CS23, which was earlier than that reported in a previous study. The number of nephrons connected to the UCS in the expanded group (246.0 ± 13.2) was significantly larger than that of the pre-expanded group (130.8 ± 80.1) (P < 0.05). The urothelial epithelium differentiated from the zeroth to the third generations at CS23. Differentiation may have continued up until the tenth generation to allow for renal pelvis expansion. The branching speed was not uniform. There were significantly more branching generations in the polar- than in the interpolar regions (P < 0.05). Branching speed reflects the growth orientation required to form the metanephros. Further study will be necessary to understand the renal pelvis expansion mechanism in CS23. Our CS-based timeline enabled us to map UCS formation and predict functional renal capacity after differentiation and growth.