4.6 Reproductive System

Overview

Theriogenology is a branch of veterinary medicine that specializes in animal reproduction. Animal reproduction is necessary for the survival of a species, and the reproductive system is responsible for producing offspring. There are unique differences in the reproductive systems of domestic animals, but many have similarities, too. In most cases, males are the producers of sperm, and the females are the producers of eggs. The production of offspring requires both male and female organs where the male gamete (sperm) fertilizes the egg for development inside the female’s body. In some species, the eggs may be released into the environment for external fertilization (Clark et al., 2018).

Male Reproductive System

The primary structures involved in the male reproductive system are the testes, accessory sex glands, and external genitalia.

The pathway of sperm is as follows:

1. Sperm is produced in the testes, then is stored and matures in the epididymis.
2. Sperm travels via the vas deferens towards the urethra. On the way, it passes the accessory sex glands, which add secretions to form semen.
3. Semen is excreted from the body through the urethra via the penis.

Structures

Epididymis: A coiled, tubular structure in which sperm start to mature and are stored until ejaculation

Penis: The male organ of copulation

Scrotum: The external pouch of skin and muscle that houses the testes; scrotum is Latin for “bag”

Sperm: The male gamete or sex cell. The process of producing sperm is called spermatogenesis.

Semen: Ejaculatory fluid composed of sperm and secretions from the accessory sex glands

Testes or testicles: The male gonads (sing. testis) or male sex organs that produce sperm

Vas deferens: Also called the ductus deferens; the duct that transports sperm from the epididymis to the penis

Accessory sex glands: Glands present in males that produce components of semen. Note that not all glands are found in all species, and which ones are present varies among species.

• Prostate gland: A gland at the base of the bladder that surrounds the urethra and contributes fluid to the semen during ejaculation
• Seminal vesicles: Glands that produce seminal fluid, which contributes to semen
• Bulbourethral glands: Glands that secrete a lubricating mucus that cleans and lubricates the urethra prior to and during ejaculation

Female Reproductive System

The primary structures of the female reproductive system are the ovaries, oviducts, uterus, cervix, vagina, and external genitalia.

The pathway of the egg:

1. The ova (sing. ovum) or eggs are formed in the ovaries.
2. Ovulation occurs when the ovaries release an ovum into the oviducts.
3. Fertilization takes place in the oviduct if sperm are present.
4. The fertilized egg travels down the oviduct to the uterus and implants in the uterus wall for fetal development.
5. After gestation, the neonate passes through the cervix and vagina on its way out of the body.

Ovaries: The female gonads; contain thousands of follicles, each of which contains an egg or ovum

Oviducts: Also called uterine tubes; the tubes that transport the ovum from the ovary to the uterus

Uterus: A hollow, muscular organ that nourishes and supports the growing fetus.

Cervix: The narrow portion of the uterus that projects into the vagina; prevents debris from entering the uterus from the outside world.

Vagina: A muscular canal that is the entrance to the female reproductive tract.

Vulva: The external female reproductive structure.

Mammary glands: Glands in females that produce milk; referred to as the udder in large animals.

The Estrous Cycle

The estrous cycle is similar to the menstrual cycle in humans and is where the female body experiences hormonal and developmental changes as it prepares the body to reproduce. In the canine estrous cycle, there are four stages: proestrus, estrus, diestrus, and anestrus.

1. Proestrus: Characterized by the presence of bloody discharge; males begin to be attracted to the bitch.
2. Estrus: Ovulation occurs and the ova will mature in the oviducts until potential fertilization.
3. Diestrus: Characterized by a drop in the bitch’s progesterone.
4. Anestrus: The period of the least reproductive activity as the uterus regenerates.

Breeding

Breeding occurs during estrus. This can happen naturally or by artificial insemination (AI).

Gestation length in canines and felines averages 63 days.

Common Reproductive Pathological Conditions

Abortion: A miscarriage or the elective ending of a pregnancy

Artificial insemination (AI): Introducing semen into the female reproductive tract by mechanical or artificial means

Cesarean section: Also called a c-section; the delivery of a baby via abdominopelvic surgery

Dystocia: A difficult birth process

Mastitis: Inflammation of the mammary tissue

Metritis: inflammation of the uterus

Stillbirth: When a neonate is born dead

Pyometra: An infection of the uterus; pus in the uterus

Uterine prolapse: The eversion of the uterus; that is, the uterus is displaced outside the body

Example

A uterine prolapse (shown in Figure 4.24) is a medical emergency and a potentially life-threatening condition. After a calf is born, the uterus protrudes into and pushes outside the vagina. Some predisposing factors increase the risk of a prolapse, including prolonged labour or dystocia. Treatment could include returning the uterus to its original position in the body or complete removal of the uterus.

Common Procedures

Induction: A medical process that causes labour to begin

Mastectomy: The excision of mammary tissue

Orchiectomy: The removal of part or all of a testis; known as a neuter in companion animals or castration in large animals

Ovariohysterectomy: The surgical removal of the ovaries and uterus; also known as a spay

Pregnancy radiography: An imaging technique used to determine whether an animal is pregnant; can also be used to estimate the number of fetuses

Reproductive ultrasound: An imaging technique used to determine whether an animal is pregnant; can also be used to estimate the number of fetuses

Vaginal cytology: Microscopic examination of the vagina cells

Additional Reproductive Terms

Castrated: Refers to livestock; removal of the testes

Cryptorchidism: Also known as undescended testicles; a condition in which one or both testes have not descended into the scrotum and often remain within the abdomen

 

 

Electroejaculation: When electric stimuli is used to stimulate ejaculation in order to obtain semen

Fertilization: Occurs when sperm and an ovum combine

Gestation: Pregnancy

Infertility: The inability to achieve pregnancy

Lactation: Milk production

Mastitis: Inflammation of the mammary glands

Neonate: A newborn infant (see Figure 4.27); pertaining to a newborn infant

 

 

Ovarian: Pertaining to one or both ovaries

Parturition: The act of giving birth

Preg check: An examination to determine whether an animal is pregnant

Speculum: An instrument for opening a body cavity to allow visual inspection

Standing heat: When the female stands to be mounted by the male for copulation

Testicular: Pertaining to the testicle or testicles

Uterine: Pertaining to the uterus   

References

Clark, M. A., Douglas, M., & Choi, J. (2018). Biology 2e. OpenStax. https://openstax.org/details/books/biology-2e, licensed under CC BY 4.0

Image Credits
(images are listed in order of appearance)

Male repro system labelled by Rlawson at English Wikibooks, CC BY-SA 3.0

Female repro system labelled by Rlawson at English Wikibooks, CC BY-SA 3.0

Spay 1 by Joel Mills, CC BY-SA 3.0

Kittens by Milchdrink, Pixabay licence

Inguinal cryptorchid 1 by Joel Mills, CC BY-SA 3.0