Mesonephric duct
Paired organ of the urogenital system in mammals
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Through Organ (biology), Physiology and Male
During embryonic development, the mesonephric ducts form as a part of the urogenital system.
Structure
The mesonephric duct connects the primitive kidney, the mesonephros, to the cloaca. It also serves as the primordium for male urogenital structures including the epididymides, vasa deferentia, and seminal vesicles.
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Development
The mesonephric duct typically forms around the 3rd-4th week of pregnancy, before the paramesonephric duct (also known as the Müllerian duct).
In both males and females, the mesonephric duct develops into the trigone of urinary bladder, a part of the bladder wall, and also produces WNT9B, which is required for the elongation of the paramesonephric duct.
Male
In a male, it develops into a system of connected organs between the efferent ducts of the testis and the prostate, namely the epididymis, the vas deferens, and the seminal vesicle. The prostate forms from the urogenital sinus and the efferent ducts form from the mesonephric tubules.
For this, it is critical that the duct is exposed to testosterone during embryogenesis. Testosterone binds to and activates androgen receptor, affecting intracellular signals and modifying the expression of numerous genes.
In a mature male, the function of this system is to store and mature sperm, and provide accessory semen fluid.
Female
In a female, with the absence of anti-Müllerian hormone secretion by the Sertoli cells and subsequent Müllerian apoptosis, the mesonephric duct regresses, although inclusions may persist. The vestigial epoophoron arises from this duct. Also, lateral to the wall of the vagina, a Gartner's duct could develop as a remnant. Normal regression starts with the action of COUP-TFII in the Wolffian mesenchyme.
The mesenchyme of the mesonephric duct is retained in female reproductive tissue after sexual differentiation. As the mesonephric ducts regress, they undergo significant chromatin remodeling and move to surround the paramesonephric duct. Cells from the Wolffian mesenchyme occur in a layer around the inner epithelium, where the Müllerian mesenchyme cells also reside. The two can be distinguished by their gene expression profile: cells derived from the Wolffian mesenchyme expresses AR, while those from the Müllerian mesenchyme expresses AMHR2. Both types of mesenchyme become smooth muscle and fibroblasts. They appear quite evenly mixed in the oviduct, but remain clearly separated in the uterus, with the Wolffian mesenchyme settling in the mesometrial side.