Function

The primary functions of the testes are to produce sperm (spermatogenesis) and to produce androgens, primarily testosterone. Both functions of the testicle are influenced by gonadotropic hormones produced by the anterior pituitary. Luteinizing hormone (LH) results in testosterone release. The presence of both testosterone and follicle-stimulating hormone(FSH) is needed to support spermatogenesis. Testosterone also controls testicular volume.

Spermatogenesis

Spermatogenesis occurs in the seminiferous tubules that form the bulk of each testis. The process begins at puberty, after which time sperm are produced constantly throughout a man’s life. One production cycle, from spermatogonia through formed sperm, takes approximately 64 days. A new cycle starts approximately every 16 days, although this timing is not synchronous across the seminiferous tubules. Sperm counts—the total number of sperm a man produces—slowly decline after age 35, and some studies suggest that smoking can lower sperm counts irrespective of age.

The testes work best at temperatures slightly less than core body temperature. The spermatogenesis is less efficient at lower and higher temperatures. This is presumably why the testes are located outside the body. There are a number of mechanisms to maintain the testes at the optimum temperature. The cremasteric muscle (Pic.3.), 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.

Development

There are two phases in which the testes grow substantially; namely in embryonic and pubertal age.

Embryonic

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, (such as GATA4, SOX9 and AMH), which leads to development of the male phenotype, including directing development of the early bipotential gonad down 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 (97% full-term, 70% preterm), both testes have descended by birth. In most other cases, only one testis fails to descend (cryptorchidism) and that will probably express itself within a year.

Pubertal

The testes grow in response to the start of spermatogenesis. Size depends on lytic function, sperm production (amount of spermatogenesis present in testis), interstitial fluid, and Sertoli cell fluid production. After puberty, the volume of the testes can be increased by over 500% as compared to the pre-pubertal size.Testicles are fully descended before one reaches puberty.

Anatomical structure

The testes are each approximately 4 to 5 cm in length and are housed within the scrotum. While the size of the testicle varies, it is estimated that 21.9% of men have a higher left testicle, while 27.3% of men have reported equally-positioned testicles.

During the seventh month of the developmental period of a male fetus, each testis moves through the abdominal musculature to descend into the scrotal cavity. This is called the “descent of the testis”. Cryptorchidism is the clinical term used when one or both of the testes fail to descend into the scrotum prior to birth.

In healthy European adult humans, average testicular volume is 18 cm³ per testis, with normal size ranging from 12 cm³ to 30 cm³. The average testicle size after puberty measures up to around 5 cm long, 2 cm in breadth, and 3 cm in height. 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). The size of the testicles is among the parameters of the Tanner scale for the maturity of male genitals, from stage I which represents a volume of less than 1.5 ml, to stage V which represents a testicular volume of greater than 20 ml.

Testes are surrounded by two different layers of protective connective tissue (Pic.1.):

  • Tunica vaginalis - an outter serous membrane that isan extension of the peritoneum of the abdomen and has both a parietal and a thin visceral layer.
  • Tunica albuginea - a toughprotective sheath of dense inner connective tissue layer covering the testis itself. Not only does the tunica albuginea cover the outside of the testis, italso invaginates to form septa that divide the testis into 300 to 400 structures called lobules.

Duct system:

Within the lobules, sperm develop in structures called seminiferous tubules. The tubules are lined with a layer of cells (germ cells) that from puberty into old age, develop 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 (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. (Pic.4.)

Histological structure

There are several cell types that can be found intestes:

Within the seminiferous tubules (Pic.2.):

  • germ cells developing into spermatogonia, spermatocytes, spermatids and spermatozoa through the process of spermatogenesis
  • Sertoli cells – the true epithelium of the seminiferous epithelium, critical for the support of germ cell development into spermatozoa, Sertoli cells secrete inhibin
  • peritubularmyoid cells  - surround the seminiferous tubules

Between tubules (interstitial cells):

  • Leydig cells localized between seminiferous tubules. Leydig cells produce and secrete testosterone and other androgens important for sexual development and puberty, secondary sexual characteristics like facial hair, sexual behavior and libido, supporting spermatogenesis and erectile function. Leydig cells may grow uncontrollably and form a Leydig cell tumour. These tumours are usually benign.
  • Intersticial macrophages and epithelial cells.

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 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 from self-tissue, a male 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 (auto-immune 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.

Find more about related issues

Sources

Testes ―sourced from Lumen licensed under CC BY 4.0
Testicle ―sourced from Wikipedia licensed under CC BY-SA 3.0
Seminiferous tubule ―by Nephron licensed under CC BY-SA 3.0
Appendix of testis ―by Carter licensed under CC0 1.0
Overview of the Male and Female Reproductive Systems ―sourced from Boundless licensed under CC BY-SA 4.0
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