The Male Reproductive System

The main role of the organs and glands of the male reproductive tract is to manufacture fertile male gametes (pl. spermatozoa, sperm; sing. spermatozoon) contained in the fluid semen and deliver it into the female reproductive tract during copulation.

The male reproductive tract is adapted for the:

• Production, storage, and nutrition of spermatozoa.
• Transport into the female reproductive tract.
• Production of male reproductive hormones called androgens (e.g., testosterone) and other hormones or substances, such as inhibin, oestrogen, and several proteins.

The male reproductive system of mammals generally consists of three components:

1. The primary sex organs or gonads:
   • Two testes (testicles) – the sperm production centres.
2. Accessory sex organs and glands, including the:
   • Epididymis – the sperm’s final maturation and storage centre.
   • Ductus deferens.
   • Spermatic cords.
   • Ampullary glands.
   • Vesicular glands.
   • Prostate gland.
   • Bulbourethral glands.
3. The copulatory organ:
   • Penis.

Spermatozoa are produced in the testes, which are housed together with the epididymis in a pouch called the scrotum behind the penis. The scrotum provides a favourable environment for the production and maturation of spermatozoa. The accessory sex organs and the copulatory organ assist the spermatozoa in reaching the ovum (egg) of the female in an environment conducive to fertilisation of the ovum.

Schematic diagram comparing the reproductive anatomy of the boar, stallion, ram, and bull

 

 

Testes

The testes are paired organs that vary in shape, size, and location between species. Despite these differences, the testes of each of these species have the same fundamental structure. Each testis is suspended within a distinct lobe of the scrotum by the spermatic cord, which contains blood vessels, nerves, lymphatics, and the ductus deferens.
Internal anatomy of the testis

Each testis consists of a mass of coiled seminiferous tubules surrounded by the tunica albuginea, a dense fibrous capsule. A few fibrous septa, also known as trabeculae, pass inward from the tunica albuginea, dividing the testis into lobules and providing support for the seminiferous tubules and interstitial tissue that produces testosterone. Spermatogenesis, the formation of spermatozoa, takes place within the seminiferous tubules. The numerous seminiferous tubules deliver sperm to the rete testis, a network of tubules that drains into the efferent ductules. The efferent ductules fuse into one epididymal duct.

The connective tissue between the seminiferous tubules contains the interstitial cells (Leydig cells). The interstitial cells secrete the male hormone testosterone Sustentacular cells (Sertoli cells) within the seminiferous tubules envelop developing spermatozoa and their precursors. The sustentacular cells nourish the developing sperm.

 

Reproductive Functions of the Testes:

• The testes are the primary reproductive organs in the male that produce spermatozoa and testosterone.
• The testes produce inhibin, oestrogens, and a variety of proteins all important for the functioning of the spermatozoa.
• The testes also produce rete fluid which originates mainly from the seminiferous tubules. Rete fluid serves as a vehicle in which spermatozoa are suspended and removed from the testes during ejaculation, and it contains synthetic products that nourish the developing sperm.

As previously mentioned, the testes are composed of numerous thin, tightly coiled tubules called seminiferous tubules; sperm cells are produced within the tubule walls. In addition, within the walls of the tubules are numerous randomly dispersed cells known as Sertoli cells, which serve to support and nourish immature sperm cells by providing them with nutrients and blood products. As the young germ cells develop, the Sertoli cells help transport them from the seminiferous tubule’s outer surface to the central channel.

Sperm cells are continually being produced by the testes, but not all areas of the seminiferous tubules produce sperm cells at the same time.

Spermatogenesis
(Image credit: LibreTexts™ Biology)

 

The immature cells (called spermatogonia) are all derived from stem cells in the seminiferous tubules’ outer wall. The stem cells almost entirely consist of nuclear material. Mitosis, a process of cell duplication, is the initial step in stem cell multiplication. Half of these new cells become future sperm cells, while the other half remain as stem cells, ensuring a constant supply of germ cells. Spermatogonia destined to mature into sperm cells are referred to as primary sperm cells. These cells migrate from the outer portion of the seminiferous tubule to the centre and attach themselves to the Sertoli cells. The primary sperm cells then develop by increasing their cytoplasm (substances outside the nucleus) and organelles (structures within the cytoplasm). After a period of rest, the primary sperm cells divide into secondary sperm cells. During this process of cell division, the nuclear material is divided. In the nucleus of primary sperm cells, there is a diploid number of chromosomes (2n); in secondary sperm cells, however, there is only a haploid number of chromosomes (n), as in the egg. When the egg and sperm combine and their chromosomes combine, the characteristics of both individuals combine, and the new organism begins to develop.

Before a secondary sperm cell can fertilise an egg, it must undergo certain morphological changes during the maturation process. The nuclear material becomes more compact and oval-shaped; this region develops into the sperm’s head. The head is partially covered by a cap known as the acrosome, which is essential for sperm entry into the egg. The tail is attached to the opposite end of the head. The tail is derived from the cytoplasm of the secondary sperm cell. The middle piece contains a mitochondrion which supplies energy for the movement of the spermatozoon.
Structure of a mature spermatozoon

Once sperm has reached maturity, they are transported through the long seminiferous tubules and stored in the epididymis of the testes until they are ready to leave the male body.

 

Excurrent Duct System

The excurrent duct system consists of the epididymis and ductus deferens.

The epididymis consists of the long, tortuous epididymal duct, which connects the efferent ductules of the testis to the ductus deferens. Along the long axis of the testis, it appears as a firm, arching appendage. The epididymis is where spermatozoa mature before being expelled through ejaculation. Before spermatozoa can fertilise ova, they must undergo a period of maturation (usually 10 – 15 days) in the epididymis. The epididymis is arbitrarily divided into:

• A head (or caput) into which the efferent ductules empty.
• A body (or corpus) that lies along the long axis of the testis.
• A tail (or cauda) that is directly attached to the testis and the adjacent vaginal tunic by ligaments. The epididymal tail duct becomes the ductus deferens, which transports sperm from the testis to the urethra.

During ejaculation, the ductus deferens (formerly vas deferens) undergoes peristaltic contractions, propelling spermatozoa from the epididymis to the urethra.

The epididymis and ductus deferens enable the final maturation, storage, and delivery of newly formed spermatozoa to the pelvic urethra.

 

Scrotum 

The scrotum is a two-lobed cutaneous sac that conforms in size and shape to the testes it contains.
The scrotum consists of four layers:

1. Skin:

The scrotal skin is thin, elastic, and relatively devoid of hair (except in some breeds of sheep, in which fleece covers the scrotum).

2. Tunica dartos:

The tunica dartos is a layer of fibroelastic tissue and smooth muscle fibers immediately beneath the skin. The muscle fibers of the tunica dartos contract in response to exposure to cold and assist in holding the testes against the abdominal wall. The tunica dartos sends a sheet of connective tissue into the median plane between the two testes to form the scrotal septum, which divides the scrotum into two compartments, one for each testis.

3. Scrotal fascia:

Several layers of fascia lie beneath the tunica dartos and are not easily separated. The internal abdominal oblique muscle contributes a muscle slip that covers the spermatic cord. This is the cremaster muscle, which also aids in bringing the testicle closer to the body wall in response to low environmental temperatures or as a protective reflex.

4. Parietal tunica vaginalis.

 

Accessory Sex Glands

The male accessory sex glands produce the majority of ejaculate or sperm-transporting semen. Semen provides favourable conditions for sperm nutrition and buffers the natural acidity of the female genital tract. Ampulla of the ductus deferens, vesicular gland, prostate gland, and bulbourethral gland are the accessory sex glands. These glands, excluding the prostate, are paired. There is considerable variation among species in the shape and size of the various accessory sex glands, but their relative location is consistent.Accessory sex glands of the bull

Ampullae:

Ampullae are glandular enlargements connected to the terminal portions of the ductus deferentia. They are developed in stallions, bulls, and rams, but absent in boars. Glands of the ampullae drain into the ductus deferens and contribute volume to the semen.

 

Vesicular Glands:

Previously known as seminal vesicles, the vesicular glands are a pair of glands located in the genital fold. In most domesticated species, each vesicular gland fuses with the ipsilateral ductus deferens to form the short ejaculatory duct, which empties into the pelvic urethra. The vesicular glands of the boar open into the urethra independently of the ductus deferens. The vesicular glands of a stallion are hollow, pear-shaped sacs, whereas those of a bull, ram, or boar are large, lobulated glands.

 

Prostate Gland:

The prostate gland is a single gland that surrounds the urethra of the pelvis. Under the protection of the urethral muscle, the prostate gland in farm animals consists of various combinations of diffuse and compact parts extending along the pelvic urethra. The prostate gland’s multiple ducts open in two parallel rows, one on each side of the urethral lumen. The prostate produces an alkaline secretion that imparts a distinctive odour to sperm. In mature intact males, the prostate may swell and impede urination if it becomes enlarged.

Bulbourethral Glands:

The bulbourethral (formerly Cowper’s) glands are paired glands on either side of the pelvic urethra just cranial to the ischial arch but caudal to the other accessory glands. Bulbourethral glands are especially large in the boar.

Functions of the Accessory Sex Glands:

The accessory sex glands depend on testosterone for their full development and maintenance of their structure and function. These glands are responsible for producing secretions which contribute to the seminal plasma (the non-cellular portion of semen). Seminal plasma is not required for fertility but is important in natural insemination for the transport of sperm. Seminal plasma consists of fructose (an energy source for spermatozoa), citric acid, inositol, ergothioneine, and prostaglandins.

 

Penis

The penis, the male organ of copulation, can be divided into three major regions:

1. The glans or free extremity is the specialised distal end.
2. The body or shaft – the main portion.
3. Two crura, or roots, attach to the ischial arch of the pelvis.

Most of the penile body’s internal structure is composed of paired columns of erectile tissue, the corpora cavernosa (sing. corpus cavernosum). Each corpus cavernosum is supplied with blood sinusoids that are separated by sheets of connective tissue called trabeculae. These are derived from the heavy, fibroelastic capsule surrounding the penis, the tunica albuginea. In species with a fibroelastic penis (ruminants and swine), the trabeculae make up most of the penis; therefore, in these species, the penis is firm when not erect. The stallion has a musculocavernous penis, with the blood sinusoids outweighing the connective tissue. Therefore, the equine penis is flaccid when not erect.

The ventral portion of the penis immediately surrounding the penile urethra is another area of spongy erectile tissue called the corpus spongiosum.

The penis is responsible for depositing semen into the female reproductive tract during copulation and serves as the passage through which urine is excreted out of the animal body from the urinary bladder.

Erection is defined as a stiffening of the penis without a change in diameter and is necessary to achieve vaginal penetration.