Vas Deferens

This is a long, muscular tube carrying mature sperm from the epididymis to the urethra before ejaculation. The vas deferens serve as a storage reservoir for sperm until ejaculation.

1.0Development of the Gonads 

• In the embryo, a primitive gonad forms on each side from the genital ridge, a cluster of tissue near the adrenal glands. 
• This gonad develops two layers: a cortex and a medulla, which remain identical in both sexes until the sixth week of development. 
• In genetic males, between the seventh and eighth weeks, the medulla develops into a testis, while the cortex regresses. 
• At this stage, Leydig and Sertoli cells emerge, and the secretion of testosterone and Müllerian Inhibiting Substance (MIS) begins
• In genetic females, the opposite occurs: the cortex develops into an ovary, and the medulla regresses. 
• The embryonic ovary does not produce hormones during this stage. Hormonal treatments administered to the mother do not influence gonadal differentiation in humans, although they may affect ductal and genital development, as observed in some experimental animals.

2.0Embryology of the Genitalia

• In the seventh week of gestation, the embryo has both male and female primordial genital ducts.
•  In a normal female fetus, the müllerian duct system develops into uterine tubes (oviducts) and a uterus. 
• The Wolffian duct system on each side develops into the epididymis and vas deferens in the normal male fetus. 
• The external genitalia are similarly bipotential until the eighth week. 
• After that, the urogenital slit disappears, and male genitalia forms, or it remains open and female genitalia forms. 
• When the embryo has functional testes, male internal and external genitalia develop.
•  The Leydig cells of the fetal testes secrete testosterone, and the Sertoli cells secrete müllerian inhibiting substance (MIS; also called müllerian regression factor, or MRF). MIS is a 536-amino-acid homodimer that is a member of the transforming growth factor β (TGF-β) superfamily of growth factors, which includes inhibins and activins. 
• In their effects on the internal as opposed to the external genitalia, MIS and testosterone act unilaterally. 
• MIS causes regression of the müllerian ducts by apoptosis on the side on which it is secreted, and testosterone fosters the development of the vas deferens and related structures from the Wolffian ducts.
•  The testosterone metabolite dihydrotestosterone induces the formation of male external genitalia and male secondary sex characteristics.
•  MIS continues to be secreted by the Sertoli cells, reaching mean values of 48 ng/mL in plasma in 1- to 2-year-old boys.
•  After that, it declines to low levels by puberty and persists at low but detectable levels throughout life. 
• In girls, MIS is produced by granulosa cells in small follicles in the ovaries, but plasma levels are very low or undetectable until puberty. 
• After that, plasma MIS is about the same as in adult men, about 2 ng/mL. The functions of MIS after early embryonic life are unsettled, but it is probably involved in germ cell maturation in both sexes and the control of testicular descent in boys.

3.0Anatomy of the Vas Deferens

• In the tail of the epididymis, the ductus epididymis becomes less coiled and increases in diameter. Beyond this point, it is referred to as the ductus deferens or vas deferens. 
• This tube, approximately 45 cm (18 inches) long, ascends along the back of the epididymis, passes through the spermatic cord, and enters the pelvic cavity. 
• Once inside, it loops over the ureter and extends along the back of the urinary bladder. The widened end of the ductus deferens is called the ampulla.
• The inner lining of the ductus deferens is made up of pseudostratified columnar epithelium and lamina propria (areolar connective tissue). The muscular layer consists of three smooth muscle layers: an inner and outer longitudinal layer and a middle circular layer.
• Functionally, the ductus deferens transports sperm during sexual arousal from the epididymis to the urethra through peristaltic contractions of its muscular walls. Similar to the epididymis, it can store sperm for several months, after which any unused sperm are reabsorbed.

4.0Functions of the Vas Deferens

• The vas deferens are an important site for sperm storage. 
• Because the tightly packed sperm are relatively inactive and their metabolic needs are accordingly low, they can be stored in the ductus deferens for up to 2 months, even though they have no nutrient blood supply and are nourished only by simple sugars present in the tubular secretions.
• Secretions of ampulla moisten the sperm and help to keep them viable. 
• In a vasectomy, a common sterilization procedure in males, a small segment of each ductus deferens (alias vas deferens, hence the term vasectomy) is surgically removed after it passes from the testis but before it enters the inguinal canal, thus blocking the exit of sperm from the testes.