Accessory Glands of the Animal Reproductive System

Accessory glands of the reproductive system in animals, also often known as accessory sex glands are organs that are distinct from the main reproductive tract but aid in the process of reproduction. In most cases, the male accessory glands are different from the female accessory glands. The accessory reproductive glands facilitate reproduction in a number of ways. 

On this page, we will attempt to learn about the accessory sex glands (male and female), details about the male accessory sex glands, the function of accessory glands in males and females and more. 

Accessory Glands of Male Reproductive System

Accessory genital glands are discernible outgrowths of the genital tract in the case of mammals.  The male accessory sex glands majorly include the bulbourethral (seminal fluid), the seminal vesicles, the prostate and the ampullary glands. The accessory glands of male reproductive systems are outgrowths of the urethra or the spermatic duct. All four types of male accessory sex glands are seen in the case of horses, elephants and most moles, rodents, bats, cattle, rabbits and primates. Ampullary glands and/or seminal vesicles are, however, lacking in a few members of these groups. In certain other cases such as whales, porpoises (Cetaceans), certain carnivores including weasels, dogs, ferrets and bears only have prostate glands. Let us now study the male accessory glands in detail: 

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Prostate

It is a structure that is firm and dense and located just below the urinary bladder. The size of the prostate gland roughly resembles that of a walnut. The prostate surrounds the urethra as it exits the urinary bladder. A number of small ducts arising from the prostate gland carrying the substance which is emptied into the prostatic urethra. The prostate secretes milk coloured, thin, alkaline secretions. These secretions are meant for sperm motility enhancement. 

Among the males, the prostate is the most widely distributed male accessory sex gland Echidna is the only marsupial where it is not present along with a few carnivores. Numerous insectivores, rodents and lagomorphs possess three distinct prostatic lobes. In some mammals (including some primates and carnivores) the prostate is present as a single mass with lobules where at the base of the urinary bladder, it encircles the urethra. A few mammals such as opossums do not have a compact mass for the prostate but a gland that is partially diffused. 

Seminal Vesicles

The seminal vesicles are typically seen in pairs. They are elongated with fibromuscular sacs that are coiled in nature. These are saccular glands present posteriorly to the urinary bladder. Each gland of the seminal vesicles possesses a short duct that combines with the ampulla of the ductus deferens and forms an ejaculatory duct. This duct is then emptied into the urethra or the spermatic duct. The seminal vesicles produce a fructose-rich fluid and are viscous in nature. This fluid acts as a source of energy for the sperm. The fluid also powers the prostaglandins that contribute to the sperm’s motility and viability. The proteins present in the fluid cause the semen to slightly coagulate after ejaculation. They, however, are not sperm reservoirs.

Monotremes, marsupials, cetaceans, carnivores as well as some primates, insectivores and chiropterans lack seminal vesicles. They are also not seen in domesticated rabbits. These vesicles are exceptionally large in the rhesus monkeys, large in armadillos while they are small in man and rudimentary in cottontails. In sloths, they are of variable size. 

Bulbourethral Glands

These are also known as Cowper's. They are present in pairs and their size is small, comparable to that of a pea. These glands arising from the urethra are located in proximity to the penis’ base. The muscle of the penis or the urethra surrounds them. A short duct arising from each of the glands enters the penile urethra through the proximal end. The bulbourethral glands secrete a mucus-like, alkaline fluid as a response to sexual stimulation. The urethra contains urine residue (acidic in nature) which is neutralised by this fluid. The vagina which is typically acidic is also neutralised by this fluid and it also acts as a lubricating agent for the penis’ tip during intercourse.

While only a single pair of bulbourethral glands are usually seen, in marsupials up to three pairs may be found. The size of these glands is small in man; they are large in elephants, rodents, and ungulates such as camels, pigs and horses. In mustelids (mink, weasel, etc.), pholidotans (pangolins), cetaceans, sirenians (manatees, dugongs), certain edentates, and in carnivores including sea lion, walrus, bear and dog, the Cowper’s glands are absent.

Seminal Fluid

Seminal fluid, also known as semen is a mixture consisting of male accessory glands’ secretions and sperm cells. The seminal fluid is alkaline in nature. About 60 percent of the semen’s volume is made up of seminal vesicle secretions. The remainder of the volume is made up of prostate gland secretions. The bulbourethral gland secretions and sperm make up only a small percentage of volume.

In a single ejaculation, semen volume may vary anywhere between 1.5 and 6.0 ml. Typically, 50 to 150 million sperms are present per millilitre of semen. Sperm counts dropping below 10 to 20 million per millilitre can cause fertility problems. Although the ovum is fertilised by only a single sperm, several million sperms in ejaculation are needed to ensure the happenstance of fertilisation.

Accessory Glands of Female Reproductive System

In comparison to the male sex accessory ducts, female accessory glands in the reproductive system are fewer in number. The accessory glands of the female reproductive system include:

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Bartholin’s Glands 

These are also known as bulbo vestibular glands. These are homologous to the bulbourethral glands of males. These are present as a single pair that empties into the urinogenital sinus in mammals. In the case of primates, the pair opens into a shallow vestibule present at the vagina’s opening. 

Prostates 

In several female embryos, they are developed as buds from the urethra. However, they mostly remain partially developed. Their complete development is seen in the case of some rodents, chiropterans and lagomorphs. The function of the prostate in these animals is obscure. 

Labial, Urethral, Preputial Glands

These glands are present in the mucosa also called the mucous membrane. The uterine mucosal glands provide the embryos with nourishment before implantation. Mucus secreted by the cervical uterine glands helps in the lubrication of the vagina.