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Sources of Funding: Research funded by NIDDK U01 (DK110803). Specimens provided by the Joint MRC-Wellcome Trust Human Developmental Biology Resource (099175/Z/12/Z).

Introduction

Embryonic development of the lower urinary tract (LUT) occurs in a rapid and complex sequence where the bladder, urethra and ureters must connect in a stereotypical manner to assure proper form and function. Congenital anomalies of the kidney and urinary tract (CAKUT) are the most common human birth defects. There are limited studies of human developmental anatomy, with great reliance on animal models to infer human translation. This study examined human embryos to establish detailed information on LUT development with special focus on the ureteric bud and cloaca._x000D_

Methods

With institutional review board approval, 22 human embryos from Carnegie Stage (CS) 14 to 23 were serially sectioned at 5 μM. Representative sections were analyzed after staining with hematoxylin and eosin (H&E) for histology and immunohistochemistry (IHC) for genetic expression. ICH used antibodies to Caspase-3, p63, uroplakin, keratin 5, FoxA2, and E-cadherin. Because mouse developmental anatomy and genetic expression are well studied, we compared human embryos to equivalent stages in mouse development.

Results

We created a timeline of human LUT development (figure). Insertion of the nephric ducts (ND, also called wolffian ducts) into the cloaca occurs before CS 14 (approximately day 24), establishing a connection between the upper tract and LUT. Shortly after, the ureteric bud emerges from the posterior aspect of the ND and the ureter is joined indirectly to the urogenital sinus via the posterior-most ND segment, the common nephric duct (CND). Contrary to the "Ureteral Bud Theory" (Mackie and Stephens, 1975), the CND does not differentiate into the bladder trigone but instead undergoes apoptosis (expressing Caspase-3 on IHC) between CS 15 to 18 (days 36 to 44). Apoptosis separates the ND and ureter in close proximity to the sinus ridge, an epithelial structure located at the dorsal urogenital sinus. Simultaneously, cloacal septation completes by CS 19 (day 48). These findings are consistent with previous mouse studies._x000D_

Conclusions

This study provides an important basis for characterizing anatomic and molecular development of the human LUT. Defects in apoptosis and CND remodeling may contribute to reflux or obstruction. Further studies of human samples will be crucial to understand the CAKUT spectrum of pathology.

Funding

Research funded by NIDDK U01 (DK110803). Specimens provided by the Joint MRC-Wellcome Trust Human Developmental Biology Resource (099175/Z/12/Z).