Three-dimensional analysis of mesonephric tubules remodeling into efferent tubules in the male mouse embryo

Motile cilia: Key developmental and functional roles in reproductive systems

BACKGROUND

Cilia are specialized microtubule-based organelles that extend from the cell surface and are classified into non-motile and motile types. The assembly and function of cilia are regulated by a complex molecular network that enables motile cilia to generate fluid flow across epithelial surfaces through coordinated beating. These motile cilia are found in the respiratory, nervous, and reproductive systems. In males, motile cilia are found in the efferent ducts and facilitate the transport of sperm from the testis to the epididymis. In females, they are mainly found in the oviducts, where they help to transport, nourish and fertilize eggs, and are also present in the endometrial epithelium.

MATERIAL-METHODS

This review compares the common factors that affect motile cilia in both male and female reproductive tracts, discusses the origin and development of multiciliated cell and cilia within the efferent ducts and oviducts, and enumerates the infertility or related reproductive diseases that may arise due to motile cilia defects.

RESULTS-DISCUSSION

In males, motile cilia in the efferent ducts create turbulence through their beating, which keeps semen suspended and prevents ductal obstruction. In females, motile cilia are distributed on the epithelia of the oviducts and the endometrium. Specifically, motile cilia in the infundibulum of the oviduct aid in capturing oocytes, while cilia in the isthmus region have been found to bind to sperm heads, facilitating the formation of the sperm reservoir. Several common factors, such as miR-34b/c and miR-449, TAp73, Gemc1, and estrogen, etc., have been shown to play crucial regulatory roles in motile cilia within the efferent ducts and oviducts, thereby further influencing fertility outcomes.

CONCLUSIONS

Pathogenic mutations that disrupt ciliary function can impair ciliogenesis or alter the structure of sperm flagella, potentially resulting in infertility. Consequently, motile cilia in both the male and female reproductive tracts are crucial for fertility. There are still numerous unresolved mysteries surrounding these cilia that merit further investigation by researchers, as they hold great significance for the clinical diagnosis and treatment of infertility and related reproductive disorders.

Mesonephric tubules expressing estrogen and androgen receptors remain in the rete ovarii of adult mice

The rete ovarii and epoophoron in females are homologous structures of the rete testis and efferent/epididymal duct in males and are derived from the developing rete cells and mesonephric tubules, respectively. Sex steroid hormones play a critical role in reproductive function for both sexes, and we recently reported expression patterns of sex steroid receptors in developing male reproductive tracts. However, their expression patterns in females remain unclear. We, therefore, investigated the three-dimensional structure and expression patterns of sex steroid receptors in the rete ovarii and epoophoron of fetal and adult female mice. In adult females, the epoophoron was not adherent to the rete ovarii. The rete ovarii had a bursa-like structure, with its extra-ovarian region protruding toward the epoophoron. A marker for mesonephric tubules, PAX2 (Paired box 2), was detected in the epoophoron and a small population of epithelial cells in the extra-ovarian rete ovarii. These epithelial cells expressed estrogen receptor and androgen receptor. During development, mesonephric tubules were adherent to the rete ovarii at first, but as the development proceeded, the continuity was lost due to the interruption of the tubule rather than separation between the tip of the tubule and rete ovarii. These findings suggest that epithelial cells, originating from the mesonephric tubules, persist even in the adult rete ovarii with maintained expressions of receptors for estrogen and androgen.

Tissue clearing and imaging approaches for in toto analysis of the reproductive system†

New microscopy techniques in combination with tissue clearing protocols and emerging analytical approaches have presented researchers with the tools to understand dynamic biological processes in a three-dimensional context. This paves the road for the exploration of new research questions in reproductive biology, for which previous techniques have provided only approximate resolution. These new methodologies now allow for contextualized analysis of far-larger volumes than was previously possible. Tissue optical clearing and three-dimensional imaging techniques posit the bridging of molecular mechanisms, macroscopic morphogenic development, and maintenance of reproductive function into one cohesive and comprehensive understanding of the biology of the reproductive system. In this review, we present a survey of the various tissue clearing techniques and imaging systems, as they have been applied to the developing and adult reproductive system. We provide an overview of tools available for analysis of experimental data, giving particular attention to the emergence of artificial intelligence-assisted methods and their applicability to image analysis. We conclude with an evaluation of how novel image analysis approaches that have been applied to other organ systems could be incorporated into future experimental evaluation of reproductive biology.

Three-dimensional analysis of junctions between efferent and epididymal ducts in the human caput epididymis

BACKGROUND

Due to the scarcity of studies using human tissues, the limited information is currently available on the gross structure of the caput epididymis in humans, at which efferent ducts connect to the epididymal duct.

OBJECTIVE

The present study investigated the three-dimensional structures of efferent and caput epididymal ducts in humans, with a focus on junctions between the former and the latter.

MATERIALS AND METHODS

We examined three sets of human efferent and caput epididymal ducts in specimens obtained from prostatic carcinoma patients. They were reconstructed from serial paraffin sections using a segmentation model created by a deep learning protocol and high-performance three-dimensional reconstruction software.

RESULTS

Serial sections and three-dimensional images of human efferent and caput epididymal ducts were combined to obtain the detailed anatomical information. When a single efferent duct was defined as a duct connecting to both the extra-testicular rete testis and epididymal duct, there were 14.7 efferent ducts with a total length of 3.0 m per specimen on average. The cranial portion of the efferent ducts joined to a single duct and terminated at the end of the epididymal duct, whereas other efferent ducts terminated independently on the side of the epididymal duct. These two types of junctions between the efferent and epididymal ducts differed in the patterns of the epithelial-type switch. The epididymal duct consisted of multiple segments, which were separated by a minimal amount of connective tissue septa or even without them. Efferent ducts occupied most of the volume of the caput epididymis.

DISCUSSION AND CONCLUSIONS

By utilizing deep learning, we reconstructed human efferent and caput epididymal ducts and revealed their precise three-dimensional structures, which differed from those of rodents in several aspects. The present results may be useful for analyzing anatomical abnormalities related to some types of male infertility.

A Single Administration of Progesterone during the Neonatal Period Shows No Structural Changes in Male Reproductive Tracts in Mice

The concentration of female-dominant steroid hormones, such as progesterone and estrogen, drops after birth in neonates. We have reported that neonatal estrogen treatment results in inflammation in the epididymis after puberty in male mice. Our recent study discovered that progesterone receptor was specifically expressed in efferent ducts just before birth in male mice. Therefore, this study aimed to reveal the impact of neonatal progesterone administration on the efferent ducts after puberty. Progesterone was subcutaneously administered to neonatal mice on their birthday in three groups: high-dose (200 mg/kg), low-dose (8 mg/kg), and control (cottonseed oil). Their testis and epididymis were collected at 12 weeks old. Semi-serial paraffin sections of these tissues were prepared and evaluated through PAS-hematoxylin staining. Efferent ducts were reconstructed into a three-dimensional structure, and their length and volume were analyzed. Spermatogenesis in the testis and epithelium of the tracts appeared normal, even in individuals administered with progesterone. There were no significant differences in the length and volume of the efferent ducts among the three groups. This study suggests that progesterone treatment in neonatal mice does not cause any structural changes in the male reproductive tracts at puberty, unlike the neonatal estrogen treatment.

Expression patterns of sex steroid receptors in developing mesonephros of the male mouse: three-dimensional analysis

The androgen pathway via androgen receptor (AR) has received the most attention for development of male reproductive tracts. The estrogen pathway through estrogen receptor (ESR1) is also a major contributor to rete testis and efferent duct formation, but the role of progesterone via progesterone receptor (PGR) has largely been overlooked. Expression patterns of these receptors in the mesonephric tubules (MTs) and Wolffian duct (WD), which differentiate into the efferent ductules and epididymis, respectively, remain unclear because of the difficulty in distinguishing each region of the tracts. This study investigated AR, ESR1, and PGR expressions in the murine mesonephros using three-dimensional (3-D) reconstruction. The receptors were localized in serial paraffin sections of the mouse testis and mesonephros by immunohistochemistry on embryonic days (E) 12.5, 15.5, and 18.5. Specific regions of the developing MTs and WD were determined by 3-D reconstruction using Amira software. AR was found first in the specific portion of the MTs near the MT-rete junction at E12.5, and the epithelial expression showed increasing strength from cranial to the caudal regions. Epithelial expression of ESR1 was found in the cranial WD and MTs near the WD first at E15.5. PGR was weakly positive only in the MTs and cranial WD starting on E15.5. This 3-D analysis suggests that gonadal androgen acts first on the MTs near the MT-rete junction but that estrogen is the first to influence MTs near the WD, while potential PGR activity is delayed and limited to the epithelium.

Hedgehog signaling regulates Wolffian duct development through the primary cilium†

Primary cilia play pivotal roles in embryonic patterning and organogenesis through transduction of the Hedgehog signaling pathway (Hh). Although mutations in Hh morphogens impair the development of the gonads and trigger male infertility, the contribution of Hh and primary cilia in the development of male reproductive ductules, including the epididymis, remains unknown. From a Pax2Cre; IFT88fl/fl knock-out mouse model, we found that primary cilia deletion is associated with imbalanced Hh signaling and morphometric changes in the Wolffian duct (WD), the embryonic precursor of the epididymis. Similar effects were observed following pharmacological blockade of primary cilia formation and Hh modulation on WD organotypic cultures. The expression of genes involved in extracellular matrix, mesenchymal-epithelial transition, canonical Hh and WD development was significantly altered after treatments. Altogether, we identified the primary cilia-dependent Hh signaling as a master regulator of genes involved in WD development. This provides new insights regarding the etiology of sexual differentiation and male infertility issues.

The murine male reproductive organ at a glance: Three-dimensional insights and virtual histology using label-free light sheet microcopy

BACKGROUND

The unique anatomy of the male reproductive organ reflects its complex function from sperm maturation to their storage for months until emission. Since light microscopy in two dimensions (2d) cannot sufficiently demonstrate its complex morphology, a comprehensive visualization is required to identify pathologic alterations in its entire anatomical context.

OBJECTIVES

Aim of this study was to use three-dimensional (3d) light sheet fluorescence microscopy (LSFM) to visualize entire murine testes in 3d, label-free and at subcellular resolution, and to assign local autofluorescence to testicular and deferent structures.

MATERIALS AND METHODS

Murine testes were fixed with four different fixatives and subsequently cleared with benzoic acid/benzyl benzoate. Hereafter, complete murine testes were scanned with LSFM with different fluorescence filter sets and subsequently embedded in paraffin for further conventional planar histology.

RESULTS

Autofluorescence signals of the murine reproductive organ allowed the unambiguous identification of the testicular anatomy from the seminiferous tubules to the vas deferens with their specific stratification independent of the used fixative. Blood vessels were visualized from the pampiniform plexus to the small capillaries of single tubules. Moreover, due to the specific intrinsic fluorescence properties of the efferent ducts and the epididymis, luminal caliber, the epithelial stratification and retronuclear cytoplasmic inclusions gave a unique insight into the interface of both morphological structures. Subsequent 2d histology confirmed the identified morphological structures.

DISCUSSION

LSFM analysis of the murine reproductive organ allows due to its intrinsic fluorescence a simple, label-free 3d assessment of its entire duct morphology, the epithelial composition and the associated blood supply in its anatomical relation.

CONCLUSION

LSFM provides the technical basis for comprehensive analyses of pathologically altered murine testes in its entirety by depicting specific autofluorescence. Thereby it facilitates mouse studies of testicular disease or their drug related alterations in more detail potentially for clinical translation assessing human testicular biopsies. This article is protected by copyright. All rights reserved.

Comparative anatomy on the development of sperm transporting pathway between the testis and mesonephros

The male genital tract is diverse among vertebrates, but its development remains unclear, especially in the rete region. In this study, we investigated the testis-mesonephros complex of rabbit, chicken, and frog (Xenopus tropicalis) by immunohistochemistry for markers such as Ad4BP/Sf-1 (gonadal somatic and rete cells in mammals) and Pax2 (mesonephric tubules), and performed a three-dimensional reconstruction. In all investigated animals, testis cords were bundled at the mesonephros side. Rete cells positive for Ad4BP/Sf-1 (rabbit) or Pax2 (chicken and frog) were clustered at the border region between the testis and mesonephros. The cluster possessed two types of cords; one connected to the testis cords and the other to the mesonephric tubules. The latter rete cords were contiguous to Bowman's capsules in rabbit and chicken but to nephrostomes in frog. In conclusion, this study showed that mammals, avian species, and frogs commonly develop the bundle between the testis cords (testis canal) and the cluster of rete cells (lateral kidney canal), indicating that these animals share basic morphogenesis in the male genital tract. The connection site between the rete cells and mesonephric tubules is suggested to have changed from the nephrostome to the Bowman's capsule during vertebrate evolution from anamniote to amniote.