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 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.
There are two phases in which the testes grow substantially; namely in embryonic and pubertal age.
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.
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.
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:
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.):
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.)
There are several cell types that can be found intestes:
Within the seminiferous tubules (Pic.2.):
Between tubules (interstitial 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.
The pathological inability to ejaculate in males, with (orgasmic) or without (anorgasmic) orgasm.
Male diagnosis connected with male infertility characterised by the complete absence of semen.
Complete absence of sperm in the ejaculate of a man.
Male infertility diagnosis characterized by extremely low concentration of sperm in semen.
A man's inability for or persistent difficulty in achieving orgasm, despite typical sexual desire and sexual stimulation.
A class of sexual disorders defined as the subjective lack of normal ejaculation.
The inability (that lasts more than 6 months) to develop or maintain an erection of the penis during sexual activity.
A medical term which describes a diminished functional activity of the gonads – the testes and ovaries.
A condition in which a man has an unusually low ejaculate (or semen) volume.
A condition in which fertility impairment occurs spontaneously or due to an unknown cause.
A genetic condition where the primary symptom is a failure to start puberty or a failure to fully complete puberty.
The set of symptoms that result from two or more X chromosome in males.
Complete absence of sperm in the ejaculate due to testicular failure.
Absence of sperm in the ejaculate despite normal spermatogenesis, caused by an obstruction of the genital tract.
Semen with a low concentration of sperm and is a common finding in male infertility.
An inflammation of the testes, involving swelling and heavy pains.
A physical syndrome described by pain or burning sensation of the urethra or perineum during or following ejaculation.
An inflammation of the prostate gland.
The semen, which would normally be ejaculated via the urethra, is redirected to the urinary bladder.
The absence of any developmental stage of sperm cell in the testes.
Antibodies that bind to sperm, inhibiting their movement, stopping recognition and entry into the egg.
Teratospermia is a condition characterized by the presence of sperm with abnormal morphology that affects fertility in males.
Cancer that develops in the testicles.
The inability of the testicles to produce sperm or testosterone.
Emergency medical condition occurring when the spermatic cord twists and cuts off the testicle's blood supply.
A medical condition impairing the function of the thyroid.
In the case of cryptorchidism one or both testes are absent from the scrotum. It is is the most common etiologic factor of azoospermy in the adult.
An abnormal enlargement of the pampiniform venous plexus in the scrotum.
The male sex chromosomal disorder characterized by a spectrum of clinical presentations, ranging from ambiguous to normal male genitalia.
A family of genetic disorders caused by missing gene(s) in the Y chromosome.
Scrotum is an anatomical male reproductive structure that consists of a suspended sack of skin and smooth dual-chamber muscle located under the penis.
Tube structures within the testes where spermatogenesis occurs.
The duct in the testicle that carries semen from the epididymis to the ejaculatory duct.
Connect the rete testis with the initial section of the epididymis.
The cell found in interstitial tissue of testicles responsible for production of androgens - male hormones.
The cell in seminiferous epithelium responsible for nutrition and development of germ (sperm) cells.
A male reproductive cell which is able to fertilize the counterpart female gamete - the oocyte.
An undifferentiated male germ cell with self-renewing capacity representing the first stage of spermatogenesis.
A hormone, that provokes the regression of male fetal Müllerian ducts.
Steroid hormone, secreted by the ovaries, whose function is to prepare the uterus for the implantation of a fertilized ovum and to maintain pregnancy.
A type of sexual dysfunction in which a person cannot achieve orgasm despite adequate stimulation.
A not-temporary condition in which the testes diminish in size and may be accompanied by loss of function.
A medical condition whose main symptom is low sexual desire.
Testosterone levels lower than the reference ranges.
An organism has passed the usual age of onset of puberty with no physical or hormonal signs.
The condition where the testicles are abnormally enlarged.
The failure to achieve a clinical pregnancy after 12 months or more of regular unprotected sexual intercourse.
A condition refers to semen with a low concentration of sperm.
Decrease of facial and body hair in males.
The absence of sexual appetite.
Pain or burning sensation of the penis or scrotum following ejaculation.
The decreased ability of sperm cell to move progressively.
A frustration caused by a discrepancy between a person's desired and achieved sexual activity.
Diagnostic symptom of varicocele after palpation of the scrotum.