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The Male Reproductive System

tracing maternal ancestry. However it has been recently discovered that mitochondrial DNA can be
recombinant.

Spermatozoa are produced in the seminiferous tubules of the testes in a process called
spermatogenesis. Round cells called spermatogonia divide and differentiate eventually to become
spermatozoa. During copulation the vagina is inseminated, the spermatozoa move through chemotaxis
(see glossary) to the ovum inside a Fallopian tube or the uterus.

Sperm Pathway

Spermatogenesis takes place inside a male’s testes, specifically in the walls of the seminiferous
tubules. The epididymis is a tortuously coiled structure topping the testis, it receives immature sperm
from the testis and stores it for several days. When ejaculation occurs, sperm is forcefully expelled
from the tail of the epididymis into the ductus deferens. Sperm travels through the ductus deferens and
up the spermatic cord into the pelvic cavity, over the ureter to the prostate behind the bladder. Here, the
vas deferens joins with the seminal vesicle to form the ejaculatory duct, which passes through the
prostate and empties into the urethra. Upon the sperm's exit from the testes, into the vas deferens,
muscular movements take over. When ejaculation occurs, rhythmic muscle movements of peristalsis
propel the sperm forward. This continues throughout the remainder of the sperm's journey through the
male reproductive system.

Sperm cells become even more active when they begin to interact with the fertilizing layer of an
egg cell. They swim faster and their tail movements become more forceful and erratic. This behavior is
called "hyper activation."

A recent discovery links hyper activation to a sudden influx of calcium ions into the tails. The
whip-like tail (flagellum) of the sperm is studded with ion channels formed by proteins called CatSper.
These channels are selective, allowing only calcium ion to pass. The opening of CatSper channels is
responsible for the influx of calcium. The sudden rise in calcium levels causes the flagellum to form
deeper bends, propelling the sperm more forcefully through the viscous environment.

The sperm use their tails to push themselves into the epididymis, where they complete their
development. It takes sperm about 4 to 6 weeks to travel through the epididymis. The sperm then move
to the vas deferens, or sperm duct. The seminal vesicles and prostate gland produce a whitish fluid
called seminal fluid, which mixes with sperm to form semen when a male is sexually stimulated.

The penis, which usually hangs limp, becomes hard when a male is sexually excited. Tissues in the
penis fill with blood and it becomes stiff and erect (an erection). The rigidity of the erect penis makes it
easier to insert into the female's vagina during sexual intercourse, and the extended length allows it to
reach deeper into the female's oviduct, the passage from the ovaries to the outside of the body (allowing
a shorter travel distance for the spermatozoa).

When the erect penis is stimulated to orgasm, muscles around the reproductive organs contract and
force the semen through the duct system and urethra. Semen is pushed out of the male's body through
his urethra - ejaculation. The speed of the semen is about 70 mph when ejaculation comes and it can
contain 100 to 600 million sperm cells. When the male ejaculates during intercourse, semen is
deposited into the fornix at the base of the female's vagina, near the cervix. From the fornix, the sperm
make their way up through the cervix and move through the uterus with help from uterine contractions.

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