The Wolffian ducts (WDs, also known as mesonephric ducts) are paired embryonic structures that serve as progenitors of the male internal genitalia. They develop in the male and female embryos but are only maintained in males by testosterone.[1] The WDs then give rise to the epididymis, vas deferens, and seminal vesicles. Even though the formation of the WDs is dependent on testosterone, the dynamic and complex interplay among androgens, growth factors as well as genes in the duct epithelium and mesenchyme is critical for proper formation of the male reproductive tract and system.[1]

The WDs develop from the intermediate mesoderm in craniocaudal succession. They extend from the pronephric duct near the future forelimb buds and develop caudally to the cloaca.[1] The WDs stimulates the growth of mesonephric tubules of the mesonephric mesenchyme, which extend to the epithelial cells of the gonads in males and females. The WDs form four to six pairs of cranial mesonephric tubules that are primarily caudal and do not fuse with the WDs.[2] The ureteric bud stems from the WDs posteriorly and gives rise to the pronephros, mesonephros, and metanephros of the kidney primordia by interacting with the metanephric mesenchyme. The pronephros and mesonephros degenerate after their development in females, but in males, the mesonephric tubules become the precursor to epididymal ducts and efferent ducts.[2]

Many genes and growth factors are crucial for the proper formation of the WDs. Pax2 and Pax8 are known for inducing WD formation, and Lim1 is necessary for WD extension.[3][4] Pax2, Pax8, Emx2, and Lim1 are expressed in the condensed nephrite cord and are necessary for tubulogenesis and WD development.[5] WT-1 and SIX1 also express in the nephrogenic mesenchymal condensation, and rodents without the WT-1 or SIX-1 expression have been shown to lack caudal MTs.[6][7] Mice with a null Emx2 have regular WD formation until the ducts degenerate, resulting in the failure of the kidney and reproductive tract to form.[8] Mice with a null Gata3 mutation also have defects with WD initiation.[9] FGF8, which is signaled by the intermediate mesoderm, is also essential, and not expressing it results in the nonexistence of the cranial mesonephros and mesonephric tubules (MTs). During mesonephric development, the mesenchyme expresses FGFR1 while the epithelium expresses FGFR2, thus maintaining the WD and mesonephric mesenchyme.[10] FGFR2 is said to sustain the caudal portion of the WD by managing cell proliferation.[11] Absence of forkhead transcription factors, FOXC1 and FOXC2, and SHH results in supernumerary MT formation, which suggests these genes have suppressive effects on MT development.[12][13] Retinoic acid signaling is also critical in WD development, as compound null mutations in retinoid acid receptors α and γ can cause agenesis or dysplasia of the epididymis, vas deferens, and seminal vesicles.[14]

The epithelium of the WDs also expresses WNT9b, and WNT7b is signaled, starting from E9.5. The lack of WNT9b expression correlates with the absence of MTs and epididymis at birth, and β-catenin-dependent canonical WNT signaling induces MT formation in WNT9b null mice.[15] During the formation of the metanephric kidney, WNT9b attenuation influences the epithelial cell polarity and increases the diameter of the tubules.[15]

Stabilization and Elongation of the WDs

In males, the mesonephros serves as the precursor to the male reproductive tract, whereas, in females, the mesonephros regresses. After gonadal sex differentiation, the testis produces testosterone, and locally-generated, not systemic, androgens, are required for WD stabilization.[16] However, some studies suggest androgens transported through systemic circulation is enough to prevent WD regression.[5] Androgens act on the androgen receptor (AR), and growth factors, such as FGF and epidermal growth factor (EGF), mediate androgen functions in the prostate and WD.[17]

After the WD has stabilized, the WD elongates, which is dependent on the expression of androgens and growth factor signaling. Inhba is a paracrine factor that controls the coiling of the epithelium in the anterior WD, and Pkd1 appears to be involved in signaling transduction of growth factors and cytoskeleton dynamics.[18][19]

Differentiation of the Regions of the WDs

Researchers show that the expression of some homeobox genes in specific regions is critical for WD differentiation into the epididymis, vas deferens, and seminal vesicles.[1] The Drosophila Abdominal B-related homeobox genes are important in distinguishing tissue boundaries between these anatomic structures in mice. In male mice, the epididymis expresses Hoxa9, Hoxa10, Hoxd10, and Hoxd9 while the vas deferens express Hoxa9, Hoxd9, Hoxa10, Hoxd10, and Hoxa11. Hoxa13 and Hoxd13 also express via the caudal part of the WD and seminal vesicles. Hoxa10 mutations can cause WD anterior homeotic transformation, whereby the distal epididymis and proximal vas deferens display morphological qualities of more anterior segments. Research has demonstrated Hoxa11 mutations to cause a homeotic transformation of the vas deferens to an epididymis-like phenotype. Hoxd13 mutations could lead to decreased size and cleating of the seminal vesicles.[1]