ANIMAL REPRODUCTION Agriscience 332 Animal Science #8405 TEKS: (c)(4)(A)
Introduction Reproduction is the process by which animals produce offspring for the purpose of continuing the species. Photo by Ron Nichols courtesy of USDA Natural Resources Conservation Service.
The process of reproduction begins with copulation, which is the mating of a male and female of the species. Sperm cells from the male are deposited in the female reproductive tract and try to unite with an egg cell. When fertilization (a sperm cell and egg cell unite) occurs, an embryo begins to develop.
The embryo attaches to the wall of the uterus where it is protected, receives nourishment, and develops. When the new offspring reaches the end of the gestation period, it is delivered from the female reproductive tract in a process called parturition.
To completely understand the process of reproduction, a basic knowledge of the reproductive tract structures and functions is required.
Structure and Function of the Male Reproductive System Photo by Scott Bauer courtesy of USDA Agricultural Research Service.
The major functions of the male reproductive system include: • Production, storage, and deposition of sperm cells, • Production of male sex hormones, • Serve as passageway for expelling urine from the urinary bladder.
The male reproductive system is made up of several organs, glands, and muscles; each will be discussed.
Male Macrostructure and Function Testes and Related Structures: Testes – paired, ovoid shaped organs that produce sperm cells and testosterone. Testosterone – the male sex hormone that is responsible for the development of secondary male characteristics and sex behavior (libido).
Spermatic cord - a protective fibrous sheath consisting of smooth muscle, blood vessels, and nerves. Cremaster muscle – primary muscle supporting testes and coursing the length of the spermatic cord. Scrotum – sac outside the body cavity that protects and supports the testes.
The spermatic cord extends from the body through the inguinal ring to suspend the testes within the scrotum. The cremaster muscle, spermatic cord, and tunica dartos muscle raise and lower the testes to maintain a constant temperature (4 – 6 degrees below body temperature) for sperm to develop.
Epididymis and Related Structures: Epididymis – a coiled tube connected to each testis that is responsible for the maturation, storage, and transport of sperm cells. Deferent Duct (Vas Deferens) – part of the spermatic cord that is the passageway for sperm from the epididymis to the urethra.
Ampullae – an enlargement of the deferent duct that opens into the urethra and may serve as a temporary storage depot for sperm. Urethra – a passageway for both semen and urine that extends from the ampullae and bladder to the end of the penis.
Accessory Glands: The accessory glands are responsible for the production of secretions that contribute to the the liquid non-cellular portion of semen known as seminal plasma. Semen and ejaculate are terms given to the sperm plus the added accessory fluids.
Vesicular glands (seminal vesicles) – paired accessory glands that secrete seminal fluid that nourishes the sperm and provides protection and transportation medium for sperm upon ejaculation. Prostate gland – secretes thick, milky fluid that mixes with seminal fluid to provide nutrition and substance to the semen.
Bulbourethral glands (Cowper’s glands) – secretes fluid that cleanses and neutralizes the urine residue that can kill sperm cells in the urethra.
Penis and Related Structures: Penis – the organ that allows for the deposition of semen into the female reproductive tract. The penis of the stallion is vascular, which means that it depends on the engorgement of blood within certain tissues for erections to occur and it forms no sigmoid flexure when relaxed.
The penises of the bull, ram, and boar are fibroelastic, which means that they are primarily composed of connective tissue and depend little on blood for erections. The rear portion of the fibroelastic type penis forms an S-shaped curve or sigmoid flexure when relaxed.
Retractor penis muscle – muscle that contracts to retract the penis and form the sigmoid flexure and relaxes to extend the penis upon sexual excitement. Sheath – external portion of the male reproductive tract that serves to protect the penis from injury and infection.
Male Microstructure and Function Microscopic cellular parts within the testes produce sperm cells and testosterone. Spermatogenesis refers to the development of sperm cells (spermatozoa) through a process of cell division and maturation.
Seminiferous Tubules – tubular structures that coil throughout the testes and are the site of spermatogenesis.
Spermatozoa (sperm cells) – haploid gametes of the male that are motile and tadpole-like. Photo by Keith Weller courtesy of the USDA Agricultural Research Service.
Once matured, the sperm cells proceed to the epididymis where they are stored until ejaculation or absorbed by the body. Unusual climatic conditions (extremely high temperatures) or stress on the male can temporarily halt sperm cell production causing reproductive failure upon breeding.
Interstitial Cells (Cells of Leydig) – group of cells between the seminiferous tubules that produce testosterone. Testosterone – an androgen hormone that directs the development of secondary male characteristics and influences libido.
Secondary male characteristics include coarse hair, horns that are long and large at the base, a deep voice, and pronounced muscularity. Photo from IMS.
Structure and Function of the Female Reproductive System Photo from IMS.
The functions of the female reproductive system include: • Produce egg cells (ova), • Serve as receptacle for the penis during copulation, and • House and nourish the fetus until parturition.
The female reproductive system is made up of several organs with specific functions; each will be discussed. Photo from IMS.
Female Macrostructure and Function Ovaries – paired structures that produce eggs (ova) and the female hormones, estrogen and progesterone. Broad ligaments – ligaments that support the female reproductive tract and arteries, veins, and nerves of the ovaries in the abdominal cavity.
Oviducts (Fallopian tubes) – paired tubes that transport the eggs from the ovaries to the uterus and serve as the site where sperm and ova meet and fertilization occurs. Infundibulums – two funnel-like openings of the oviducts that pick up the eggs at ovulation and direct them to the body of the oviducts.
Uterus – a major reproductive organ that consists of the uterine body and two uterine horns. The embryo attaches to uterine body or uterine horn, depending on the species. The uterus varies in shape between livestock species from long uterine horns of the sow to relatively short uterine horns in the mare.
Functions of the uterus include: • Passageway for sperm during copulation, • Incubation and nourishment of the embryo during pregnancy, and • Expulsion of the fetus during parturition by contractions.
Vagina – reproductive structure that serves as the receptacle for the penis during copulation and the birth canal at parturition; it also serves as a passageway for expelling liquid wastes, as the urethra joins the bladder to the vagina prior to the opening at the vulva.
Cervix – a thick-walled mass of connective tissue with a small tube-like opening that joins the uterus to the vagina; it serves as a passageway for semen during copulation. It also contains glands that secrete a waxy-like substance that seals off the uterus during pregnancy and between heat periods to protect against infection, disease, or foreign matter.
Vulva – the external portion of the female reproductive tract that serves to protect the internal system from infection, to initially receive the penis at copulation, and to act as a passageway for urine.
Female Microstructure and Function Oogenesis is the process of producing ova (eggs) in the follicles of the ovaries. Oogonia cells develop in the ovaries of the fetus and mature into oocytes by birth. Only a small proportion of oocytes develop into ova or reach ovulation.
Follicle – a blister-like mass on the surface of the ovary that contains a developing ovum and produces and stores estrogen. The follicle secretes estrogen as a signal to the rest of the reproductive tract to prepare for ovulation (release of the ovum from a mature follicle).
Corpus hemorrhagicum – a small hemorrhage or blood-clotted area that develops at the site of a ruptured follicle and lasts 2 – 3 days. Corpus luteum – a yellow body of cells that develops in place of the corpus hemorrhagicum and produces progesterone.
Progesterone – the female sex hormone that functions to prepare the female reproductive system for pregnancy; it is produced by the corpus luteum and lasts about twelve days, unless the ovum is fertilized. Corpus albicans – a white body of connective tissue that is the result of the degeneration and re-absorption of luteal tissue.
Oogenesis is part of the estrous cycle. The number of ova produced per cycle varies with each species. A cow or mare normally produces one ovum per cycle. A ewe produces two ova per cycle. A sow produces eight to fifteen ova per cycle.
Structure and Function of the Reproductive System in Poultry The reproductive anatomy of poultry differ when compared to that of other animal species.
Male Reproductive System in Poultry The male poultry anatomy consists of two testes (each with an epididymis and vas deferens) that lead to papillae and a rudimentary copulatory organ.
Unlike other livestock species, the testes of poultry are located within the abdominal cavity along the backbone. The epididymis, which still functions in sperm storage, is relatively small in relation to the testes. The vas deferens extend from the epididymis to the cloaca and are located on each side of the vertebral column.