updated 3/18/98

I. Origin

The epididymis is an evolutionary innovation of mammals which functions in:

1. transport of spermatozoa from the vas efferentia to the vas deferens,
2. concentration of spermatozoa,
3. maturation of spermatozoa,
4. storage of spermatozoa and
5. secretion of energy substrates and materials coating spermatozoa.
   

The epididymis is rudimentary or absent in fish, amphibians and some reptiles. In some amphibians and reptiles, spermatozoa are stored and matured in an ampulla or large central canal of the testis. It is in mammals with external testis that the most extensive development of the epididymis has occurred. The mammalian male has reorganized the Wolfian duct into the vas efferentia, epididymis and vas deferens. To form the epididymis, the Wolfian duct enlarges and develops a functional epithelium that will be competent to mature spermatozoa under androgen control.

II. Structure

The epididymis is a single convoluted tubule extending from the vas efferentia to the vas deferens. It is surrounded by smooth muscle and lined with a tall columnar epithelium. The epithelium is made of principal and basal cells. The principle cells have stereocilia or a brush border. The basal cells are apical to the lumen and often have a clear cytoplasm. The numbers of basal cells changes with the segment observed. The epithelium is dependent on testosterone for its function and maintenance. The cell types and physiological activity of the epididymis in sperm maturation, concentration and storage are different in successive segments. Therefore, it is usually divided into three anatomical areas called the caput (head), corpus (body), and cauda (tail).

III. Functions of the Epididymis

A. Maturation of Spermatozoa

Spermatozoa recovered from the rete testes, vas efferentia or early caput epididymis are live but infertile and without coordinated motility. They are fertile and capable of motility when recovered from the lower corpus or cauda epididymis. Sperm will mature in isolated segments of the epididymis but only if they have passed through the caput first. Some of the maturation changes occurring in spermatozoa during epididymal passage are:

1. acquisition of fertility
2. development of motility
3. nuclear maturation - chromatin is condensed by increased affinity of S-S bonds of the DNA histone complex
4. water is lost from the sperm cell
5. the acrosome changes shape in some species
6. the surface charge changes
7. the permeability of the plasma membrane increases as does susceptibility to cold shock damage
8. a sialic acid containing mucopolysaccharide coating is deposited on the sperm cell (this may be a decapacitation factor to protect the spermatozoa during storage or after deposited in the uterus)
9. the cytoplasmic droplet migrates down the tail and is lost in most species
10. cellular stored lipid reserves are depleted

B. Concentration of Spermatozoa

The epididymis removes water and particulate material from sperm cells and from the fluid transporting them from the testes. Fluid production by the testes is great and 90% of that fluid is absorbed in the epididymis. If the vas efferentia or epididymis are ligated, water accumulation causes the testes to swell in size.

C. Secretory Function

1. The epididymis can convert testosterone to the potent androgen di-hydrotestosterone.
2. The energy substrate glycerolphosphorylcholine is secreted in large quantities and carried with the semen to the female where enzymes of the female reproductive tract make it available for energy.
3. Sialic acid - a part of the mucopolysaccharide coating on sperm cells - is produced in large quantities.
4. Acyl-carnitine - useful in lipid transport and in providing a source of energy for the sperm cell is secreted in abundance.
   

D. Sperm Transport Function

Spermatozoa are transported through the epididymis by fluid pressure from the testes and contraction of smooth muscle around the epididymis.

E. Storage Function

Spermatozoa are stored primarily in the cauda epididymis and may remain there for 10 to 60 days depending on the frequency of ejaculation.

IV. Hormonal Control of the Epididymis

Either hypophysectomy or castration will result in functional atrophy of the epithelium of the epididymis and loss of epididymal mass. The contained spermatozoa lose viability and eventually degenerate. These functions are restored by testosterone replacement. The corpus and cauda epididymis can utilize testosterone removed from the circulatory system. The caput, however, is dependent on testosterone from the testicular fluid which the caput readily converts (about 90%) to the highly potent di-hydrotestosterone. Testosterone is transported from the testes to the epididymis by a high affinity androgen binding protein produced by the Sertoli cells. The diagram below illustrates the fertility of spermatozoa taken from successive segments of the rabbit epididymis.

Segment Number Segment Area Fertility 1 caput 0% 2 caput 0% 3 corpus 14% 4 corpus 97% 5 cauda 99% 6 cauda 99%

IV. The Accessory Sex Glands

Sperm do not pass through these glands but are mixed with their secretion's. The accessory sex glands are dependent on the hormone testosterone or dihydrotestosterone for their development, maintenance and secretory ability.

A. Seminal vesicles

These are large lobular secretory glands which contribute a great volume of fluid to semen. The fluid dilutes sperm at ejaculation and also serves to activate motility and provide nutrients for sperm metabolism. Seminal vesicle fluid is rich in proteins, citric acid, fructose, inorganic ions and enzymes.

B. Prostate

This gland secretes a watery fluid of about pH 7.5 to 8. It is located on and around the pelvic urethra.

C. Cowper's gland

These glands are paired and secret a watery fluid which cleanses the male and female reproductive tracts before the passage of spermatozoa. They also dilute the semen and in the boar, cause coagulation of the seminal vesicle secretions.

V. Urethra and Penis

The urethra receives sperm from the vas deferens and seminal fluids from the accessory sex glands. It is surrounded by smooth muscle, whose contraction forces semen into the female at ejaculation. The penis is composed of cavernous sinuses (erectile tissue) which are engorged with blood at erection. In the bull the penis is normally held retracted by a retractor penis muscle. At ejaculation this muscle relaxes allowing the sigmoid flexure of the penis to thrust the penis forward. The free end of the penis is characterized by the glans penis and the external urethral opening. The penis (except during erection) is withdrawn into a tube-like cavity surrounded by the folds of the sheath.

VI. Erection, Emission and Ejaculation

A. Erection

Erection is the process by which the penis elongates and becomes more rigid. Both sensory input to the glans penis and psychic stimuli lead to stimulation of sympathetic, parasympathetic nerves. The sympathetic nerves lead to inhibition of vasoconstriction while the parasympathetic nerves cause dilation of blood vessels and engorgement of the corpus cavernosum. At the same time the parasympathetic nerves stimulate contraction of the ischiocavernosus muscle to lead to decreased venous outflow from the penis. There is also relaxation of the penile retractor muscle in the bull, boar, ram and stallion.

B. Emission.

Emission is the process by which sperm move up from the cauda epididymis and into the colliculus seminalis. The sperm then mix with accessory sex gland secretions before ejaculation. The process of emission is controlled by sympathetic nerves and leads to contraction of smooth muscles of the cauda epididymis, vas deferens, seminal vesicles, prostate and closure of the sphincter of the bladder.

C. Ejaculation.

Parasympathetic nerves control the contraction of striated muscles of the ischiocavernosus, bulbospongiosus and pelvic urethra. Again stimulation of these nerves can be via tactile or psychic input. In the bull and ram, the tactile sensitivity of the penis is primarily responsive to temperature. In the boar, stallion and human, the penis is primarily responsive to pressure. The contractions of the ischiocavernosus, bulbospongiosus and pelvic urethra muscles force the semen (sperm and accessory gland secretions) to move toward the penis. Rhythmic contractions of the bulbospongiosus muscle causes temporary increases in blood pressure in the corpus spongiosum which moves as waves toward the glans penis. It is these waves of increased blood pressure and the temporary occlusion of the urethra that moves the semen out of the male.