The (from ''testes'') or '''ballock''' is the male
Generative Gland in
Animal s. This article will concentrate on mammalian testicles unless otherwise noted.
Like the
Ovaries (to which they are
Homologous ), testicles are components of both the
Reproductive System (being
Gonad s) and the
Endocrine System (being
Endocrine Gland s). The respective functions of the testicles are:
- producing Sperm (spermatozoa)
- producing male Sex Hormone s, 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 :
Male
Mammal s have two testicles(although the average man statistically has but one testicle), which are often contained within an extension of the
Abdomen called the
Scrotum . In mammals with external testicles it is most common for one testis to hang lower than the other. It is estimated that in about 85% of men the lower hanging testicle is the left one. This is due to differences in the vascular anatomical structure on the right and left sides.
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).
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 .
, 2
Spermatids , 3
Spermatocytes , 4
Spermatogonia , 5
Sertoli Cell , 6
Myofibroblasts , 7
Leydig Cell s, 8
Capillaries ]]
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 Cell s (or "interstitial cells") where
Testosterone and other
Androgen s are formed.
Blood supply and
Lymphatic Drainage of the testes and scrotum are distinct:
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 .
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
Antigen s) 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.
The testes work best at temperatures slightly less than core body temperature (37
°C or 98.6
°F for humans). The
Spermatogenesis is less efficient at lower and higher temperatures. There are a number of mechansism to maintain the testes at the optimum temperature.
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 is 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.
There are two phases in which the testicles grow substantially, namely in embryonic and pubertal age.
During mammalian development, the gonads are at first capable of becoming either adjacent to the developing kidneys. At about week 6, epithelial
Sex Cords develop within the forming testes and incorporate the
Germ Cell s as they migrate into the gonads. In males, certain
Y Chromosome genes, particularly
SRY , control development of the male phenotype, including conversion of the early bipotential gonad into testes. In males, the sex cords fully invade the developing gonads.
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. In humans the average testicle size after puberty measures up to be 2 inches long, 0.8 inch in breadth, and 1.2 inches in diameter (5 x 2 x 3 cm).
The basal condition for mammals is to have internal testicles. Only the mammals with internal testes, such as the rhinoceros.
Marine boreotherian mammals such as whales and dolphins, also have internal testes, but it has recently been shown (e.g., for dolphins) that they use elaborate vascular networks to provide the necessary temperature lowering for optimum function. As external testes would increase drag, many boreotherian aquatic mammals have internal testes which are kept cool by special circulatory systems that cool the arterial blood going to the testes by placing the arteries near veins bringing cooled venous blood from the skin.
There are several hypotheses why most boreotherian mammals have external testes which operate best at a temperature that is slightly less than the core body temperature, e.g. that it is stuck with enzymes evolved in a colder temperature due to external testes evolving for different, that it the lower temperature of the testes simply is more efficient for sperm production.
The classic hypothesis is that cooler temperature of the testes allows for more efficient fertile
Spermatogenesis . In other words, there are no possible enzymes operating at normal core body temperature that are as efficient as the ones evolved, at least no ones appearing in our evolution so far.
The early mammals had lower body temperatures and thus their testes worked efficiently within their body. However it is argued that boreotherian mammals have higher body temperatures than the other mammals and had to develop external testicles to keep them cool. It is argued that those mammals with internal testicles, such as the monotremes, armadillos, sloths, elephants, and rhinoceroses, have a lower core body temperatures than those mammals with external testicles.
However, the question remains why birds despite having very high core body temperatures have internal testes and did not evolve external testes.http://www.biolreprod.org/cgi/reprint/56/6/1570.pdf BIOLOGY OF REPRODUCTION 56, 1570-1575 (1997)- Determination of Testis Temperature Rhythms and Effects of Constant Light on Testicular Function in the Domestic Fowl (Gallus domesticus) It was once theorized that birds used their
Air Sac s to cool the testes internally, but later studies revealed that birds' testes are able to function at core body temperature.
. It has been suggested that the ancestor of the boreoeutherian mammals was a small mammal that required very large testes (perhaps rather like those of a
Hamster ) for
Sperm Competition and thus had to place its testes outside the body.
{Link without Title} This led to enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta and recombinase activities evolving a unique temperature optimum, slightly less than core body temperature. When the boreoeutherian mammals then diversified into forms that were larger and/or did not require intense sperm competition they were stuck with enzymes that operated best at cooler temperatures and had to keep their testicles outside the body.
One argument for the evolution of external testes is that it protects the testes from abdominal cavity pressure changes caused by jumping and galloping.
Newscientist.com - bumpy-lifestyle-led-to-external-testes
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. The testicles of the
Right Whale are likely to be the largest of any animal, each weighing around 500 kg (1,100 lbs).
The testicles are well-known to be very sensitive to
Impact And Injury .
The most prominent diseases of testicles are:
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, or as a catch-all term for Orchidectomy in a Veterinary context.
- Gelding in the specifically equine sense.
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.
See
Rocky Mountain Oysters .
