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Penis, testes, and associated ducts

Key terms

Primary organs produce gametes; in this tutorial we’ll learn about the testes, which produce sperm (aka sperm cells) in a process called spermatogenesis.

The testes also produce testosterone, which regulates spermatogenesis and has extra-testicular androgenic effects (growth of the penis, musculoskeletal mass, sebaceous gland activity, etc.).

Androgenic effects, Testosterone physiology

Spermatogenesis and testosterone production are regulated via LH and FSH, which are anterior pituitary hormones.

Testicular physiology, testosterone physiology, male reproductive physiology

The accessory organs comprise the ducts and glands that facilitate gamete transport, including the penis, prostate, and other structures.

Semen is a combination of sperm cells and the supportive fluid and nutrients keep them alive during transport; sperm cells comprise only 10% of the total semen volume.

Sagittal View

We first outline the torso, penis, scrotum, and upper thigh. For anatomical context, we show the pubic symphysis anteriorly, and the rectum and sacrum, posteriorly.

The perineal membrane supports the pelvic organs and anchors the external genitalia.

The urinary bladder is immediately posterior to the pubic symphysis.

The prostate gland is inferior to the urinary bladder.

The urethra, which transports both urine and semen, can be subdivided based upon the structures it passes through:

The prostatic urethra passes through the prostate gland. The membranous urethra passes through the deep perineal pouch and perineal membrane. The spongy urethra passes through the corpus spongiosum of the penis.

The corpus spongiosum is the erectile tissue of the ventral compartment of the penis. The corpus cavernosum is the erectile tissue of the dorsal compartment.

Penis Anatomy, corpus spongiosum, corpus cavernosum, penile urethra

Ejaculation

The movement of sperm and seminal fluids from the epididymis through the penis.

Ejaculation is a two-phase reflex:

Emission occurs when sperm and seminal fluids are ushered into the proximal urethra. Expulsion occurs when the semen is projected through the urethra and the external urethral orifice.

Pathway of sperm through the reproductive tract:

The pea-sized bulbourethral glands (aka Cowper’s glands) lie within the deep perineal pouch. These glands secrete lubricating mucus into the spongy urethra, cleansing and preparing it prior to the arrival of semen.

The testes are housed in the scrotum, which is outside of the body cavity.

The testes’ location outside the body reflects the delicate nature of sperm, which are sensitive to temperature. The body core is too warm for viable sperm production, so storage outside the body keeps them cooler; if the external environment becomes too cold, muscles in the scrotum and spermatic cord contract to bring the testes closer to the body for warmth.

The testes comprise the seminiferous tubules, where the sperm cells are produced and nurtured during spermatogenesis.

Spermatogenesis is continuous during the reproductive years, and a single sperm spends about 74 days in this stage.

Immature sperm cells leave the testes and enter the epididymis for further development, a process that takes about 14 days. The epididymis is a long, coiled tube with a head that nestles over the superior and posterior surfaces of the testis; sperm can be stored here for several months until they begin to degrade and are reabsorbed.

If ejaculation occurs, mature sperm cells are ushered into the ductus deferens (you may know this structure as the vas deferens).

The ductus deferens is a long tube that ascends into the pelvis via the spermatic cord and wraps around the urinary bladder and over the ureter; its muscular lining actively transports the sperm during the emission phase of ejaculation.

On the posterior surface of the bladder, the distal end of the ducts deferens widens to form an ampulla.

The seminal vesicle is nearby; it supplies most of the seminal plasma volume.

The ampulla of the ductus deferens and the seminal vesicle drain into the ejaculatory duct.

The ejaculatory duct travels through the prostate gland and drains into the prostatic urethra. At this point, prostatic gland secretions join the sperm and seminal fluid to form semen.

Semen travels through the urethra and exits the via the external urethral orifice.

Formation of Semen

First, we outline the ampulla of the ductus deferens, the seminal vesicle, ejaculatory duct, prostate, bulbourethral gland, and the urethra.

The bulbourethral glands secrete lubricating mucus into the spongy urethra prior to the arrival of the rest of the seminal fluid (this mucus is also known as pre-ejaculate).

Sperm from the ductus deferens joins seminal fluid from the seminal vesicle in the ejaculatory duct. Seminal vesicle secretions are viscous and sugary, which protects and nourishes the sperm.

As sperm and seminal fluid pass through the urethra, prostatic secretions are added, forming semen. Prostatic secretions are alkaline and rich in enzymes that facilitate sperm survival in the vagina and uterus.

During the expulsion phase of ejaculation, semen is expelled through the external urethral orifice. Usually, but not always, ejaculation and orgasm occur close together.

Within the female reproductive tract, sperm cells can live an average of 5 days. When outside the body and exposed to air, sperm cells die within minutes; they can survive longer in warm, moist environments.

Clinical Correlations

Cryptorchidism a condition in which one or both testes do not descend into the scrotum before birth; it is associated with hormonal abnormalities and infertility.

Early in pregnancy, the testes form in the fetal abdomen and then descend into the scrotum around month 7. However, cryptorchidism occurs in approximately 3% of full-term and 30% of pre-term males; it is the most common pediatric genital abnormality.

In most cases, cryptorchidism spontaneously resolves by 6 months post-birth; in other cases, hormones and surgery are used to relocate the testis to the scrotum.

Vasectomy is a highly effective form of fertility control in which the ductus deferentes (plural of “deferens”) are surgically sealed shut. Sperm production is not inhibited, but its transport to the urethra is.

Sperm that remain in the epididymis degrade and are reabsorbed by the body, and seminal fluid production continues as normal. Vasectomy is technically reversible but is generally considered a permanent method of fertility control.

Penis, testes, and associated ducts

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