We recently described a two-step process of urethral plate canalization and urethral fold fusion to form the human penile urethra. Canalization (“opening zipper”) opens the solid urethral plate into a groove, and fusion (“closing zipper”) closes the urethral groove to form the penile urethra. We hypothesize that failure of canalization and/or fusion during human urethral formation can lead to hypospadias. Herein, we use scanning electron microscopy (SEM) and analysis of transverse serial sections to better characterize development of the human fetal penile urethra as contrasted to the development of the human fetal clitoris. Eighteen 7-13 week human fetal external genitalia specimens were analyzed by SEM, and fifteen additional human fetal specimens were sectioned for histologic analysis. SEM images demonstrate canalization of the urethral/vestibular plate in the developing male and female external genitalia, respectively, followed by proximal to distal fusion of the urethral folds in males only. The fusion process during penile development occurs sequentially in multiple layers and through the interlacing of epidermal “cords”. Complex epithelial organization is also noted at the site of active canalization. The demarcation between the epidermis of the shaft and the glans becomes distinct during development, and the epithelial tag at the distal tip of the penile and clitoral glans regresses as development progresses. In summary, SEM analysis of human fetal specimens supports the two-zipper hypothesis of formation of the penile urethra. The opening zipper progresses from proximal to distal along the shaft of the penis and clitoris into the glans in identical fashion in both sexes. The closing zipper mechanism is active only in males and is not a single process but rather a series of layered fusion events, uniquely different from the simple fusion of two epithelial surfaces as occurs in formation of the palate and neural tube.

Purpose We characterized the early gestation development of the female external genitalia using optical projection tomography to visualize anatomical structures at high resolution. Materials and Methods First and early second trimester human female fetal external genitalia were collected with consent after voluntary termination. Specimens labeled with anti-E-Cadherin antibody underwent analysis with optical projection tomography. Histological sections were immunostained for androgen receptor, 5α-reductase, Ki67 for proliferation and Caspase 3 for apoptosis. Results Three-dimensional reconstructions demonstrated proximal to distal canalization of the epithelial vestibular plate and formation of a vestibular groove, which remained open. Ki67 was observed throughout with greatest density in the dorsal vestibular plate at the level of the opening groove. Staining for Caspase 3 was minimal in all sections. Androgen receptor staining was seen throughout the mesenchyme and in the apical epithelium of the dorsal vestibular groove. Throughout the epithelium and epidermis 5α-reductase staining was observed. Conclusions Early development of the external genitalia in the female is analogous to that in the male, demonstrating a similar opening zipper driving canalization of the vestibular plate with localized epithelial proliferation in the absence of significant apoptosis. Thus we hypothesize that the mechanism underlying the opening zipper must be androgen independent and the absence of androgen driven urethral fusion characterizes the normal development of the human clitoris.

Objective Transcatheter aortic valve replacement is established therapy for high-risk and inoperable patients with severe aortic stenosis, but questions remain regarding long-term durability. Valve design impacts durability. Increased leaflet stresses in surgical bioprostheses have been correlated with degeneration; however, transcatheter valve leaflet stresses are unknown. From 2007–2014, a majority of US patients received first-generation balloon-expandable transcatheter valves. Our goal was to determine stent and leaflet stresses in this valve design using finite element analyses. Methods 26mm Edwards Sapien (Edwards Lifesciences, Inc, Irvine, CA) underwent high-resolution micro-computed tomography scanning to develop precise 3D geometry of leaflets, stent, and dacron. Stent was modeled using 3D elements and leaflets using shell elements. Stent material properties were based on stainless steel, while those for leaflets were obtained from surgical bioprostheses. Non-cylindrical Sapien geometry was also simulated. Pressure loading to 80 and 120mmHg was performed using ABAQUS, finite element software (Dassault Systemes, Waltham, MA). Results At 80mmHg, maximum principal stresses on Sapien leaflets were 1.31MPa. Peak leaflet stress was observed at commissural tips where leaflets connected to the stent. Maximum principal stresses for stent was 188.91MPa, and located at stent tips where leaflet commissures were attached. Non-cylindrical geometry increased peak principal leaflet stresses by 16%. Conclusions Using exact geometry from high-resolution scans, 26mm Sapien showed that peak stresses for both stent and leaflets were present at commissural tips where leaflets were attached. These regions would be prone to initiate leaflet degeneration. Understanding stresses in first-generation transcatheter valves allow comparison to future designs for relative durability.