Testicle

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Testicle

Human male reproductive system and adjacent structures
Latin	testis
Gray's	subject #258 1236
Artery	Testicular artery
Vein	Testicular vein, Pampiniform plexus
Nerve	Spermatic plexus
Lymph	Lumbar lymph nodes
Dorlands/Elsevier	t_05/12799705

The testicles, or testes from latin coi (singular testis), are the male generative glands in animals. Male mammals have two testicles, which are often contained within an extension of the abdomen called the scrotum.

1 Size and growth
2 Position
3 Function
- 3.1 Temperature regulation
4 Structure
5 Testicular size
6 Health issues
7 Consumption of testicles
8 Additional images
9 See also
10 External links

[edit] Size and growth

During puberty, the testicles grow in response to the start of spermatogenesis. Size depends on lytic function, sperm production (amount of spermatogenisis present in testis), interstitial fluid, and Sertoli cell fluid production. After puberty, the volume of the testicles can be increased by over 500% as compared to the pre-pubertal size.^{[citation needed]}

[edit] Position

It is most common for one testis to hang lower than the other. The percentage of men with a lower hanging right testis or left testis is about equal. This is due to differences in the vascular anatomical structure on the right and left sides.

Look up testes in
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[edit] Function

Like the ovaries (to which they are homologous), testicles are components of both the reproductive system (being gonads) and the endocrine system (being endocrine glands). The respective functions of the testicles are:

producing sperm (spermatozoa)
producing male sex hormones, of which testosterone is the best-known

Both functions of the testicle, sperm-forming and endocrine, are under control of gonadotropic hormones produced by the anterior pituitary:

luteinizing hormone (LH)
follicle-stimulating hormone (FSH)

[edit] Temperature regulation

In land mammals the testes are located outside of the body, suspended by the spermatic cord within the scrotum. This allows for more efficient fertile spermatogenesis in mammals, due to the fact that spermatogenic DNA polymerase beta and recombinase activities exhibit unique temperature optima, at a degree somewhat less than core body temperature (37 °C or 98.6 °F for humans).

The cremasteric muscle is part of the spermatic cord. When this muscle contracts, the cord is shortened and the testicle is moved closer up toward the body, which provides slightly more warmth to maintain optimal testicular temperature. When cooling is required, the cremasteric muscle relaxes and the testicle is lowered away from the warm body and are able to cool. This phenomenon is known as the cremasteric reflex. It also occurs in response to stress (the testicles rise up toward the body in an effort to protect them in a fight), and there are persistent reports that relaxation indicates approach of orgasm. There is a noticeable tendency to also retract during orgasm.

The testicles can also be lifted voluntarily using the pubococcygeus muscle, which partially activates related muscles. This can sometimes be triggered by tightening or sucking in the stomach or abdomen. The testicles have a tendency to perspire profusely.

Monotremes, armadillos, sloths, elephants, and rhinoceroses have low body temperatures and do not have external scrotal sacs; their testes remain within the abdomen.[1] Marine mammals also have internal testes, but it has recently been shown (eg, for dolphins) that they use elaborate vascular networks to provide the necessary temperature lowering for proper operation.

Animals other than mammals do not have externalized testicles. Birds, despite having very high core body temperatures have internal testes: it was once theorized that birds used their air sacs to cool the testes internally, but later studies revealed that birds' testes function at core body temperature.[2]

[edit] Structure

Transverse section through the left side of the scrotum and the left testis.

[edit] Duct system

Under a tough membraneous shell, the tunica albuginea, the testis contains very fine coiled tubes called the seminiferous tubules. The tubes are lined with a layer of cells that, from puberty into old-age, produce sperm cells. The sperm travel from the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly-created sperm cells mature (see spermatogenesis). The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions.

Between the seminiferous tubules are special cells called Leydig cells (or "interstitial cells") where testosterone and other androgens are formed.

[edit] Blood supply and lymphatic drainage

Blood supply and lymphatic drainage of the testes and scrotum are distinct:

The paired testicular arteries arise directly from the abdominal aorta and descend through the inguinal canal, while the scrotum and the rest of the external genitalia is supplied by the internal pudendal artery (itself a branch of the internal iliac artery).
Lymphatic drainage of the testes follows the testicular arteries back to the paraaortic lymph nodes, while lymph from the scrotum drains to the inguinal lymph nodes.

[edit] Layers

Many anatomical features of the adult testis reflect its developmental origin in the abdomen.

The layers of tissue enclosing each testicle are derived from the layers of the anterior abdominal wall. Notably, the cremasteric muscle arises from the internal oblique muscle.

[edit] The blood-testis barrier

Large molecules cannot pass from the blood into the lumen of a seminiferous tubule due to the presence of tight junctions between adjacent Sertoli cells. The spermatogonia are in the basal compartment (deep to the level of the tight junctions) and the more mature forms such as primary and secondary spermatocytes and spermatids are in the adluminal compartment.

The function of the blood-testis barrier (red highlight in diagram above) may be to prevent an auto-immune reaction. Mature sperm (and their antigens) arise long after immune tolerance is established in infancy. Therefore, since sperm are antigenically different than self tissue, a male animal can react immunologically to his own sperm. In fact, he is capable of making antibodies against them.

Injection of sperm antigens causes inflammation of the testis (autoimmune orchitis) and reduced fertility. Thus, the blood-testis barrier may reduce the likelihood that sperm proteins will induce an immune response, reducing fertility and so progeny.

[edit] Testicular size

Testicular size as a proportion of body weight varies widely. In the mammalian kingdom, there is a tendency for testicular size to correspond with multiple mates (e.g., harems, polygamy). Production of testicular output sperm and spermatic fluid is also larger in polygamous animals, possibly a spermatogenic competition for survival. Elephants are known to have the biggest testes in the animal kingdom.

In normal adult human males, testicular size ranges from the lower end of around 14 cm³ to the upper end larger than 35 cm³. Measurement in the living adult is done in two basic ways:

comparing the testicle with ellipsoids of known sizes (orchidometer).
measuring the length, depth and width with a ruler, a pair of calipers or ultrasound imaging. The volume is then calculated using the formula for the volume of an ellipsoid: 4/3 π × (length/2) × (width/2) × (depth/2).

Usually right and left testicles are about the same size.

To some extent, it is possible to change testicular size. Short of direct injury or subjecting them to adverse conditions, e.g., higher temperature than they are normally accustomed to, they can be shrunk by competing against their intrinsic hormonal function through the use of externally administered steroidal hormones. Steroids taken for muscle enhancement often have the undesired side effect of testicular shrinkage. Similarly, stimulation of testicular functions via gonadotropic-like hormones may enlarge their size. Testicles may shrink or atrophy during hormone replacement therapy.

[edit] Health issues

The testicles are well-known to be very sensitive to impact and injury.

The most prominent diseases of testicles are:

testicular cancer and other neoplasms
swelling of a testicle, caused by hydrocele testis
inflammation of the testicles, called orchitis
inflammation of the epididymis, called epididymitis
spermatic cord torsion also called testicular torsion
varicocele [3] — swollen vein from the testes, usually affecting the left testicle
anorchidism is the absence of one or both testicles.

The removal of one or both testicles is termed

orchidectomy, in medicine (where orchiectomy and orchectomy are synonymous), and
castration in general use, especially when done as punishment or torture

Testicular prostheses are available to mimic the appearance and feel of one or both testicles, when absent as from injury or as treatment for gender identity disorder. There have also been some instances of their implanting in dogs.

[edit] Consumption of testicles

See Rocky Mountain oysters.

[edit] Additional images

Testicle of a cat: 1 Extremitas capitata, 2 Extremitas caudata, 3 Margo epididymalis, 4 Margo liber, 5 Mesorchium, 6 Epididymis, 7 testicular artery and vene, 8 Ductus deferens

Testis surface

Testis cross section

The right testis, exposed by laying open the tunica vaginalis.

[edit] See also

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