Testicle


A testicle, also called testis is the male gonad in all gonochoric animals, including humans, and is homologous to the ovary, which is the female gonad. Its primary functions are the production of sperm and the secretion of androgens, primarily testosterone.
The release of testosterone is regulated by luteinizing hormone from the anterior pituitary gland. Sperm production is controlled by follicle-stimulating hormone from the anterior pituitary gland and by testosterone produced within the gonads.

Structure

Appearance

Male humans have two testicles of similar size contained within the scrotum, which is an extension of the abdominal wall. Scrotal asymmetry, in which one testicle extends farther down into the scrotum than the other, is common. This is because of the differences in the vasculature's anatomy. For 85% of men, the right testis hangs lower than the left one.

Measurement and volume

The volume of the testicle can be estimated by palpating it and comparing it to ellipsoids of known sizes. Another method is to use calipers, a ruler, or an ultrasound image to obtain the three measurements of the x, y, and z axes. These measurements can then be used to calculate the volume, using the formula for the volume of an ellipsoid:
However, the most accurate calculation of actual testicular volume is gained from the formula:
An average adult testicle measures up to. The Tanner scale, which is used to assess the maturity of the male genitalia, assigns a maturity stage to the calculated volume ranging from stage I, a volume of less than 1.5 cm3; to stage V, a volume greater than 20 cm3. Normal volume is 15 to 25 cm3; the average is 18 cm3 per testis.
The number of spermatozoa an adult human male produces is directly proportional to testicular volume, as larger testicles contain more seminiferous tubules and Sertoli cells as a result. As such, men with larger testicles produce on average more sperm cells in each ejaculate, as testicular volume is positively correlated with semen profiles.

Internal structure

Duct system

The testes are covered by a tough fibrous shell called the tunica albuginea. Under the tunica albuginea, the testes contain very fine-coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells that develop from puberty through old age into sperm cells. The developing sperm travel through 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. The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions.

Primary cell types

Within the seminiferous tubules, the germ cells develop into spermatogonia, spermatocytes, spermatids and spermatozoa through the process of spermatogenesis. The gametes contain DNA for fertilization of an ovum. Sertoli cells are the true epithelium of the seminiferous epithelium, critical for the support of germ cell development into spermatozoa. Sertoli cells secrete inhibin, a hormone that plays a crucial role in regulating spermatogenesis by inhibiting the secretion of FSH from the pituitary gland, thus helping to regulate sperm production. Peritubular myoid cells surround the seminiferous tubules.
Between the seminiferous tubulestubules are interstitial cells called Leydig cells that produce and secrete testosterone and other androgens important for puberty, sexual behavior, and libido. Testosterone controls testicular volume.
Immature Leydig cells and interstitial macrophages and epithelial cells are also present.

Blood supply and lymphatic drainage

The testis has three sources of arterial blood supply: the testicular artery, the cremasteric artery, and the artery to the ductus deferens. 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.
  • The testis has collateral blood supply from the cremasteric artery, and the artery to the ductus deferens. Therefore, if the testicular artery is ligated, e.g., during a Fowler-Stevens orchiopexy for a high undescended testis, the testis will usually survive on these other blood supplies.
  • 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.

    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. The cremasteric muscle arises from the internal oblique muscle.

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 occupy the basal compartment and the more mature forms, such as primary and secondary spermatocytes and spermatids, occupy the adluminal compartment.
The function of the blood–testis barrier may be to prevent an auto-immune reaction. Mature sperm emerge significantly after immune tolerance is set in infancy. Since sperm are antigenically different from self-tissue, a male animal can react immunologically to his own sperm. The male can make antibodies against them.
Injection of sperm antigens causes inflammation of the testis and reduced fertility. The blood–testis barrier may reduce the likelihood that sperm proteins will induce an immune response.

Temperature regulation and responses

in 1926 proposed that testicles were external due to spermatogenesis being enhanced at temperatures slightly less than core body temperature outside the body. The spermatogenesis is less efficient at lower and higher temperatures than 33 °C. Because the testes are located outside the body, the smooth tissue of the scrotum can move them closer or further away from the body. The temperature of the testes is maintained at 34.4 °C, a little below body temperature, as temperatures above 36.7 °C impede spermatogenesis. There are a number of mechanisms to maintain the testes at the optimum temperature.
The cremasteric muscle covers the testicles and the spermatic cord. When this muscle contracts, the cord shortens and the testicles move 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 testicles lower away from the warm body and are able to cool. Contraction also occurs in response to physical stress, such as blunt trauma; the testicles withdraw and the scrotum shrinks very close to the body in an effort to protect them.
The cremasteric reflex will reflexively raise the testicles. The testicles can also be lifted voluntarily using the pubococcygeus muscle, which partially activates related muscles.

Gene and protein expression

The human genome includes approximately 20,000 protein coding genes: 80% of these genes are expressed in adult testes. The testes have the highest fraction of tissue type-specific genes compared to other organs and tissues. About 1000 of them are highly specific for the testes, and about 2,200 show an elevated pattern of expression. A majority of these genes encode for proteins that are expressed in the seminiferous tubules and have functions related to spermatogenesis. Sperm cells express proteins that result in the development of flagella; these same proteins are expressed in the female in cells lining the fallopian tube and cause the development of cilia. Sperm cell flagella and fallopian tube cilia are homologous structures. The testis-specific proteins that show the highest level of expression are protamines.

Development

There are two phases in which the testes grow substantially. These are the embryonic and pubertal phases.
During mammalian development, the gonads are at first capable of becoming either ovaries or testes. In humans, starting at about week 4, the gonadal rudiments are present within the intermediate mesoderm adjacent to the developing kidneys. At about week 6, sex cords develop within the forming testes. These are made up of early Sertoli cells that surround and nurture the germ cells that migrate into the gonads shortly before sex determination begins. In males, the sex-specific gene SRY that is found on the Y chromosome initiates sex determination by downstream regulation of sex-determining factors, which lead to development of the male phenotype, including directing development of the early bipotential gonad toward the male path of development.
Testes follow the path of descent, from high in the posterior fetal abdomen to the inguinal ring and beyond to the inguinal canal and into the scrotum. In most cases, both testes have descended by birth. In most other cases, only one testis fails to descend. This is called cryptorchidism. In most cases of cryptorchidism, the issue will mostly resolve itself within the first half year of life. However, if the testes do not descend far enough into the scrotum, surgical anchoring in the scrotum is required due to risks of infertility and testicular cancer.
The testes grow in response to the start of spermatogenesis. Size depends on lytic function, sperm production, interstitial fluid, and Sertoli cell fluid production. The testicles are fully descended before the male reaches puberty.

Clinical significance

Protection and injury

  • The testicles are very sensitive to impact and injury. The pain involved travels up from each testicle into the abdominal cavity, via the spermatic plexus, which is the primary nerve of each testicle. This will cause pain in the hip and the back. The pain usually fades within a few minutes.
  • Testicular torsion is a medical emergency. This is because the longer it takes to access medical intervention with respect to extending ischemia, the higher the chance that the testicle will be lost. There is a 90% chance to save the testicle if de-torsion surgery is performed within six hours of testicular torsion onset.
  • Testicular rupture is severe trauma affecting the tunica albuginea.
  • Penetrating injuries to the scrotum may cause castration, or physical separation or destruction of the testes, possibly along with part or all of the penis, which results in total sterility if the testicles are not reattached quickly. In an effort to avoid severe infection, ample application of saline and bacitracin help remove debris and foreign objects from the wound.
  • Jockstraps support and protect the testicles.