Reproductive Anatomy and Fertility

Transcription

1 Describe the functions of the parts of the male and female reproductive systems. Analyze factors that affect male and female fertility. Outline the process of estrous and estrus. UNIT Objectives 4 Discuss common hormones of livestock. Explain estrus synchronization of cattle. M SA Describe common mating practices employed in the animal science industry. Examine poultry reproductive anatomy. Reproductive Anatomy and Fertility Key Terms natural service oocytes ovary pasture mating proestrus semen spermatic cord testes testosterone PL E anestrus artificial insemination blastocyst copulation cryptorchidism diestrus ejaculation embryo transfer estrogen estrous cycle estrus estrus synchronization fertilization fetus freemartin syndrome gestation period hand mating hormones metestrus motility morphology

2 Genetics and Reproduction are among the most rapidly changing technologies in today s animal science industry. Producers utilizing advanced breeding techniques, such as artificial insemination and embryo transfer, to obtain genetically superior animals must be knowledgeable of basic reproductive anatomy before a quality breeding program can be established. Breeding is one of the most important aspects of raising a herd. Careful breeding insures that the health and quality of the herd will continue. However, before any breeding program can be devised, a thorough knowledge of the reproductive systems and cycles must be attained. Thinkstock Photos Reproductive System Parts and Functions Reproduction is the method in which organisms produce offspring. Both male and female animals have different anatomy parts that contribute to reproduction. Each livestock species has characteristics that make it unique in terms of reproduction, but all livestock have some anatomy and functions in common. Male The overall functions of the male include the capability to produce and maintain a supply of semen, detect the female in estrus, and inseminate and fertilize the female gamete (egg). The male is continually producing sperm allowing for the collection and storage of semen for the use in reproduction methods other than natural mating. The actual structure of male livestock species, such as the bull, boar, buck, ram or stallion have unique characteristics; the overall functions remain the same. 66 ANIMAL SCIENCE

3 Components of the Reproductive Tract of a Bull Prostate Cowpers Glands Vesicular Gland Ampulla Retractor Penis Muscle Bladder Penis Spermatic Cord Sheath Testicles Head of Epididymis Scrotum The testis (testes, pl.) is the male gonad. The primary function of the testis is to produce spermatozoa (sperm) and testosterone. Testosterone, the male sex hormone, will be discussed later in this unit. The testes are contained in the scrotum and attached to the body by the spermatic cord. The properties of the scrotum help to insure proper temperature for sperm production and storage are maintained. This temperature regulation (thermoregulation) of the testes is very critical. The temperature of the testes needs to be maintained approximately 3-5 degrees Celsius below body temperature for sperm production to take place. Once spermatozoa are produced by the testes, they travel out of the testes through the epididymis. As the spermatozoa enter the epididymis they are neither fertile nor motile. As spermatozoa travel through the epididymis, they obtain the capacity to be fertile, gain the ability to swim (or become motile) and are more concentrated for storage. Spermatozoa are stored in the tail, or end portion, of the epididymis. Unit 4 Reproductive Anatomy and Fertility 67

4 While the testes are the site of sperm production, the epididymis, vas deferens and several accessory glands are responsible for the production of different secretions, which collectively is referred to as seminal plasma or seminal fluid. The different secretions making up the seminal plasma provide a source of energy, buffers, antioxidants, and other compounds needed for sperm survivability, maturation, and motility. Once spermatozoa are combined with the seminal plasma, it is referred to as semen (sperm + seminal plasma = semen). Sources of seminal plasma include the ampulla, the seminal vesicles, the prostate gland and the cowpers gland (or bulbourethral gland). The accessory glands will produce different types of secretions in each of the farm species. For example, the stallion and boar each produce a gel fraction that will block the cervix to prevent the backflow of semen back into the vagina. The gel fraction is produced in the cowpers gland in the boar and in the seminal vesicles in the stallion. In the bull, the cowpers gland secretes an alkaline fluid that cleans the urethra prior to ejaculation. During copulation, or the physical act of mating, sperm is moved from the tail of the epididymis through the vas deferens and ampulla to the pelvic urethra to await ejaculation. At the same time, the retractor penis muscle relaxes allowing the penis to extend for copulation with the female. During ejaculation, the sperm is expelled through the urethra and out of the penis into the female reproductive tract. Ejaculation is the dispelling of sperm from the pelvic urethra. During ejaculation Thinkstock Photos the spermatozoa will be deposited into the vagina just outside the cervix in the cow and ewe but through the cervix and into the uterus in the sow and mare. The boar has the longest ejaculation and copulation period with a sow, sometimes lasting up to 20 minutes or more. Semen ejaculation volume and concentration are inversely related in the farm animals; for example, the boar has the highest ejaculation volume ( ml), but the lowest concentration of sperm/ml (200,000,000/ml). On the other hand, the ram has the lowest ejaculation volume (1 to 2 ml), but the highest concentration of sperm/ml (3,000,000,000/ml). Female There are numerous functions the female reproductive organs perform. For example, many hormones are produced to control the activity of the reproductive organs before, during, and after pregnancy. Sperm are also transported through the female reproductive tract to the site of fertilization in the oviduct, while the uterus of the female provides a safe environment for the embryo and fetus to grow and develop. During the birthing process, the cervix dilates and strong uterine contractions aid in the expulsion of the fetus from the uterus. Following birth, final mammary gland development allows the female to nourish the young, and the female begins the cycle over again to become pregnant. The reproductive tract of a cow and sow are somewhat different; however, the ewe and doe are similar to that of a cow. 68 ANIMAL SCIENCE

5 Components of the Reproductive Tract of a Sow Uterine Horns Broad Ligament Ovary Fallopian Tube Cervix Uterine Body Vagina Urinary Bladder Anus Clitoris Vaginal Orifice A major function of the female reproductive system is to produce oocytes (eggs) for reproduction. These viable egg cells are produced in the ovary, the female gonad, and are contained in spherical structures within the ovary called follicles. These developing follicles produce the female sex hormone, estrogen. When mature, the follicle ovulates or ruptures (similar to a pimple) releasing the egg. Around the time that the female is ready to mate, the egg is released into a funnel-like structure called the infundibulum. The infundibulum aids in transporting the egg to the opening of the oviduct (fallopian tube). Fertilization of the oocyte by the sperm occurs in the oviduct. After fertilization, the embryo continues to move from the oviduct to the uterus where it attaches to the uterine lining and develops until birth. Unit 4 Reproductive Anatomy and Fertility 69

6 Components of the Reproductive Tract of a Cow Left Horn of Uterus Right Horn of Uterus Fallopian Tube Ovary Ovary Fimbria Uterus Cervix Vagina Clitoris Vulva Several other parts of the female reproductive system also play major roles in reproduction. During the mating process, the male penis enters the female reproductive tract. First, the penis must enter the vulva which is the outer opening of the female reproductive system. Once past the vulva, the penis enters the vagina. The vagina is separated from the uterus by the cervix. In most species, the male deposits sperm into the vagina near the opening of the cervix. In swine, however, the boar s penis, consisting of a corkscrew configuration, enters the interdigitating protrusions of the cervix and deposits the sperm through the cervix into the uterus. The female and male anatomy in swine complement each other so well that it makes sows much easier to artificially inseminate than cows or ewes. 70 ANIMAL SCIENCE The vulva is the outer opening of the female reproductive system. Thinkstock Photos

7 Graphic of a 4-celled embryo that will continue to divide Thinkstock Images Fertilization is the union of the male sperm and female egg. That is, the nucleus of the sperm cell and nucleus of the egg cell fuse together (a term called syngamy) to form a zygote, or a single-cell embryo. The zygote then undergoes a series of cell divisions 2 cells, 4 cells, 8 cells, then 16, 32, 64 cells, etc. When a solid mass of cells is formed (too many to count) the embryo is called a morula. The embryo (morula) will continue to develop and differentiate until it partitions into two distinct cell populations: a group of cells that will form the body of the fetus and a group of cells that will form parts of the placenta. At this stage the embryo is referred to as a blastocyst. From this point the embryo will further develop and attach to the uterus of the female and continue to grow and develop into a fetus. After fertilization, conception begins, which is also the beginning of gestation. The gestation period is the time the female is pregnant from conception to birth. The length of the gestation within a species can vary a few days depending on several factors such as age and health. Knowledge of the gestation period will allow a producer to accurately estimate the birth date of offspring and plan breeding schedules as well as assist in other management decisions. Gestation Length by Species Cow (cattle) Ewe (sheep) Sow (swine) Mare (horse) Doe (goat) 285 days 147 days 114 days 336 days 150 days Fertility Factors Just as in humans, livestock can experience issues with fertility leaving them unable to reproduce properly. Common factors can include but are not limited to age, improper nutrition, and environmental factors such as light, temperature, relative humidity, diseases, and parasites. Producers should take all these aspects, as well as genetic issues, into account when managing his or her reproduction herd. Male Livestock A number of factors can affect the fertility of a male breeding animal. Some measures can be taken to ensure male animals are fertile and able to reproduce. For example, to determine the fertility of a bull, a breeding soundness examination (BSE) is conducted prior to the breeding season, or the artificial collection of semen is performed to assess the animal s reliability and capability for breeding. A breeding soundness exam is administered by a veterinarian and includes a physical examination of the feet, legs, eyes, teeth, and flesh cover, a scrotal circumference measurement, an internal and external examination of the reproductive tract, and a semen evaluation for sperm cell motility (ability to move independently) and morphology, which is the size and shape of the sperm. Unit 4 Reproductive Anatomy and Fertility 71

8 As an example, in order for a bull to pass a BSE, the animal must have a normal physical examination and must meet minimum requirements for scrotal circumference, sperm motility and sperm morphology. When evaluating motility, sperm are classified as non-motile, progressively motile or non-progressively motile. Progressively motile sperm swim in a forward, relatively straight motion while non-progressively motile sperm swim with an abnormal path such as a circle. A bull must have > 30% individual motility to pass. The sperm cell morphology must have > 70% normal to pass. Bulls are classified as either: 1) Satisfactory all criteria were met; 2) Unsatisfactory did not meet all criteria, has a non-resolvable problem and will need to be replaced; or 3) Deferred did not meet all criteria, but the animal should be reevaluated as the problem may be resolved with time. Sperm Morphology Normal Sperm Pear Head Double Head Abnormal Acrosome Swollen Midpiece Coiled Tail Double Tail Keeping a proper ratio of male to female animals in the herd is also important for a successful breeding season. The number of bulls required to adequately cover the breeding females is related to many factors including age, body condition, length of breeding season, and the size of the pasture. A rule-of-thumb is to place about the same number of cows or heifers with a young bull as his age is in months. For example, a bull that is 15 months in his first breeding season should be expected to breed around 15 cows; whereas, a 24-month-old (twoyear-old) bull can possibly be placed with cows. A cow-calf producer could realistically have a ratio of one mature bull per 30 to 35 head of cows and still have satisfactory results. Thinkstock Photos 72 ANIMAL SCIENCE

9 Another fertility problem with male animals is the failure of the testes to descend into the scrotum properly. The lack of testes descent into the scrotum is called cryptorchidism and is considered a heritable trait. Cryptorchidism can cause either partial or complete sterilization in the male. If both testes fail to descend into the scrotum (bilateral cryptorchid), no sperm production will take place and the animal will be sterile. If only one testis fails to descend into the scrotum (unilateral cryptorchid) the animal will be fertile but with a reduced sperm producing capacity. Female Livestock Like male animals, females occasionally experience fertility problems. These problems can be caused by genetics, disease, or other physical abnormalities. The freemartin syndrome is one of the most frequent forms of sexual abnormalities found in cattle. It is less common in other species. The freemartin condition in cattle occurs when a female is born twin to a male and occurs due to the joining of the placental membranes which exposes the female to male hormones before birth, causing the female reproductive organs to develop improperly. This syndrome typically occurs in cattle but has been recorded in other livestock species such as sheep, goats and swine. Freemartins are generally unable to reproduce. It is important for producers to recognize freemartin females to ensure resources are not spent to raise it for replacement stock. Freemartinism can be diagnosed in a number of ways ranging from a simple physical examination of the female to a DNA-based test. Infertility can cause stress to a breeding program and decrease profits for the producer. It is essential that a producer identify probable causes of infertility and take appropriate actions. The producer may need to adjust his or her heard to cull animals not contributing to the overall success of the program. The Estrous Cycle The estrous cycle, also known as the heat cycle, is the process a female animal s body undergoes to prepare itself for reproduction. Estrous varies in length between species and sometimes among individuals within the species. There are two major phases involved in the estrous cycle. One phase is under the control of estrogen, which is produced from follicles as they mature. Another phase is under the control of the hormone progesterone, which is produced from the corpus luteum (CL), a structure that forms on the ovary following ovulation of the oocyte from the follicle. If the animal does not become pregnant, the CL regresses due to the hormone prostaglandin F2α (α = alpha) that acts to kill or regress the CL, allowing the follicle to grow and eventually ovulate. Unit 4 Reproductive Anatomy and Fertility 73

10 Length of Estrous Estrus (Standing Heat) Ovulation Cow days hours hours after end of estrus Sow days hours hours after onset of estrus Ewe days hours hours after onset of estrus Mare days 2-12 days 5 days after onset of estrus Doe days 2 days hours after onset of estrus Estrous Stages Under the Control of Estrogen 1. Proestrus follicles grow rapidly to prepare for ovulation, female becomes receptive to the male 2. Estrus ovulation occurs (the release of the egg from the follicle), animal stands for breeding (standing heat) Estrous Stages Under the Control of Progesterone 3. Metestrus the formation of the corpus luteum (CL) 4. Diestrus CL produces hormones that stimulate the uterus to prepare for pregnancy With the exception of cattle, farm animals ovulate during the time the animal is in standing heat. In cattle, ovulation occurs after standing heat or in the metestrus stage of the estrous cycle. The optimum time for conception is near the time of ovulation. Anestrus is a condition where the female does not cycle due to insufficient hormonal stimuli. The failure to recognize a pregnancy is a prominent cause of anestrus and should always be considered when determining a reason for an anestrus state. Other conditions that can influence an anestrus state in animals include the postpartum period (period of time after giving birth), the nutritional state of the animal, suckling of the young, and season of the year (day length affects seasonal anestrus in sheep and horses). For instance sheep are short-day breeders whereas horses are long-day breeders. Ewes will be in an anestrus state during the summer months when the days are the longest and will begin to cycle when the days begin to shorten in the fall. Mares, on the other hand, require the long days of late spring and summer to begin the estrous cycle and will be in an anestrus state during the short days of fall and winter. Estrus in Sheep When rams are not present, ewes do not show signs of estrus. Bucks produce chemicals called pheromones produced by glands in their horns. Pheromones produced by scent glands in the buck s horns stimulate the ewe, encouraging the onset of estrous. Experiments have shown that cloths rubbed over the buck s horns and placed near the ewe s nose will, in the ram s absence, stimulate ewes causing an estral response. 74 ANIMAL SCIENCE

11 Livestock Hormones There are many hormones vital to the reproduction process. Hormones are chemical messengers carried in the blood to specific organs to change cellular activity. Estrogen, the female sex hormone, is produced from the follicles on the ovary and triggers estrus behavior. Estrogen also controls the female reproductive cycle, mammary gland development, and secondary sex characteristics in the female. Testosterone, the male sex hormone, is produced from the testes and is responsible for the male mating behavior, sperm production, maintaining the male reproductive system, and secondary sex characteristics in the male. Reproductive physiologist Tom Geary prepares to take a blood sample from a cow to measure her hormones. An ultrasound monitor on the right indicates this cow is 45 days pregnant. USDA-ARS Photo Major Hormones Affecting Growth and Reproduction FSH (follicle stimulating hormone) LH (luteinizing hormone) Inhibin Oxytocin Progesterone Prolactin Prostaglandin F2α Relaxin Stimulates follicle growth in females and sperm production in the testes of males Stimulates ovulation in females and testosterone production in males Regulates the release of FSH in males and females Stimulates muscle contraction in the female reproductive tract to aid in sperm transport, birth, and milk let down Maintains pregnancy and promotes mammary growth and secretion Stimulates milk synthesis Causes the regression of the corpus luteum and stimulates muscle contractions Stimulates the softening and dilation of the cervix for parturition Unit 4 Reproductive Anatomy and Fertility 75

12 Estrus Synchronization Producers and scientists are finding new and better techniques to improve herd quality and reproduction efficiency. One such method is estrus synchronization. Estrus synchronization is a reproductive management tool that involves the manipulation of the estrous cycle of females so they can be bred in a short amount of time either through artificial insemination (AI) or natural breeding. This allows for improved herd management by grouping females for parturition, allowing the age of the offspring to be close together. Also, this allows the producer to take advantage of superior genetics available NOTE Several drugs used for estrus synchronization are prescription drugs and require a prescription from a licensed veterinarian with whom a veterinarian-client-patient relationship (VCPR) exists. through use of AI. Estrus synchronization is never a substitute for poor herd management including health and nutritional levels. Each producer should evaluate available resources and assess the animals intended for synchronization before selecting a synchronization procedure or protocol. The farm management capabilities of the producer, time and skill available for heat detection, body condition of the animal, number of days postpartum, facilities, experience, and cost will determine which synchronization protocol is best. Protocols exist that not only synchronize estrus (standing heat) but synchronize ovulation within estrus for use with timed AI. If a synchronization protocol is being implemented to use with natural service, use only those protocols that synchronize estrus, not ovulation within estrus, and make sure the male-to-female ratio is such that the male will not be overworked. Before implementing a synchronization protocol, females should be checked for pregnancy as some of the drugs used in the synchronization protocols can cause a pregnant animal to abort. CAUTION: Females working with livestock hormones should take extra precautions by always wearing non-permeable gloves (i.e. latex). The hormones used in the synchronization protocols can interrupt the timing of the female workers own menstrual cycle. A prolonged exposure during early term pregnancy can lead to a loss of the fetus. Pregnant women should use extreme caution when working with animals in general. The following is an example protocol for fixed time AI in cows. GnRH and a CIDR (pronounced see-der) are administered on day 0, the CIDR is removed on day 7 plus an injection of Prostaglandin F2α is administered. Approximately hours following the CIDR removal and Prostaglandin F2α injection, the cow is bred by artificial insemination and administered an injection of GnRH at the time of insemination. A very similar protocol is used for timed AI in heifers except that the heifers are bred 54 ± 2 hours following CIDR removal and Prostaglandin F2α injection. 76 ANIMAL SCIENCE

13 7-day CO-Synch + CIDR Perform TAI at 60 to 66 hr after PG with GnRH at TAI. GnRH GnRH PG AI CIDR M SA hr.. treatment day 7 10 Breeding Practices PL Types of breeding practices used in livestock reproduction include natural service, artificial insemination and embryo transfer. Natural service is where the male animal (sire) is allowed to directly mate with the female (dam). Two types of natural service exist: pasture mating and hand mating. Pasture mating is simply when the male is allowed to roam freely with the females. Care should be taken to ensure the proper ratio of male to female animals. For example, one mature, good quality bull can service about 30 females. A regular BSE should be performed on males to ensure adequate semen quality. E Hand mating is another type of natural service that requires more labor and management. In this system, the female is brought to the male for service. With hand mating, it is up to the producer, not the male animal, to detect estrus. Male animals are able to service more females with a hand mating system than a pasture mating system. This mating system is quite common in the equine industry where males are often penned separately from the herd. Animal physiologist Phillip Purdy uses a water bath to thaw frozen semen for use in an insemination procedure. USDA-ARS Photo Artificial insemination (AI) and embryo transfer (ET) have gained considerable popularity with many producers. Artificial insemination refers to the technique where an insemination gun or instrument is used to deposit semen into the reproductive tract of the female. AI has had a huge impact on several species not only in the U.S. but worldwide. Success requires attention to detail in all areas of herd management and depends not only on the proper detection of estrus in the female but the proper timing of insemination following the onset of estrus. Special care must Unit 4 Reproductive Anatomy and Fertility 77

14 be taken when inserting the insemination instrument into the female reproductive tract so that the proper placement of the semen occurs. AI allows for semen to be artificially collected from a sire with superior genetics and used to breed more females than is possible by natural service. One of the most dramatic technical advances in recent years is the sexing of sperm by DNA quantification, which allows the producer to select the desired sex of the offspring before breeding. Embryo transfer is a technique where embryos from one female (the donor) are collected and then transferred into the reproductive tract of other females (the recipients). This is done so female animals with superior genetics can have more offspring than they would normally have during a lifetime (i.e. show mares can continue competing while other mares foal their offspring). Poultry Reproduction In male poultry, semen is transported from the testicles via the vas deferens to the cloaca then to the papilla. The papilla transports the sperm to the female vagina. The female does have 2 ovaries; however, only the left one is functional. The oviduct of the hen comprises five functionally distinct small segments the infundibulum, magnum, isthmus, uterus and vagina. The hen has the ability to store sperm for a prolonged period. Sperm storage tubules are located in the utero-vaginal junction and in the infundibulum of the hen s oviduct. Sperm can be stored for a few days to a few weeks after artificial insemination or natural mating. The oocyte (yolk) is transported to the magnum, where the albumen is secreted and deposited around the yolk. This process takes about 3 hours. Then the yolk and albumen are transported to the isthmus, where two shell membranes are deposited around the albumen. This process takes about an hour and 15 minutes. The egg is then transported to the uterus where it stays for about 20 hours. The egg is then ready to be laid. The total egg production process takes about 25 to 27 hours. The Male Poultry Reproductive System The Female Poultry Reproductive System Testicles Kidneys Van Deferens Oviduct Magnum Ovary Funnel of Oviduct Uterus Isthmus Vagina Cloaca Rudimentary Oviduct Papilla Cloaca 78 ANIMAL SCIENCE

15 Cloning and Reproductive Research When scientists first were able to successfully clone a mammal, it created a great deal of media interest, and Dolly become the most well known sheep in history. Dolly was cloned by Scottish scientists at the Roslin Institute in Dolly was created using reproductive cloning technology. Dolly was put down by lethal injection at six years old due to health problems. Since Dolly, scientists around the world have successfully cloned mammals including sheep, goats, cows, mice, pigs, cats, and rabbits. Although many mammals have been successfully cloned, it is not an easy process, and it is very expensive and inefficient. The majority, greater than 90%, of cloning attempts do not produce a viable offspring. But despite the low odds and high costs, many public and private organizations and institutions continue research and attempts at cloning. The future benefits of cloning range from repopulating endangered species to use in the treatment of medical disorders in humans. A lot of research is being done today around embryo transfers. This is also known as genetic engineering. Most of this research has been conducted on dairy cattle. However, what has been and will be learned can also be applied to other species. The four practical uses for genetic engineering in animals currently planned for the future include increased growth and feed efficiency, improved body composition, animals resistant to disease, and production of natural products such as valuable foreign proteins. Reproductive research will always be changing in the field of animal science and is a growing career field with many new and exciting challenges. Unit Summary Basic knowledge of livestock reproduction aids producers in many management decisions including what type of mating system to employ, when breeding should occur, and when to test animals to ensure breeding soundness, just to name a few. The main function of the male reproductive system is to produce, maintain, and store a supply of viable sperm cells. In addition to producing oocytes, the female reproductive system is responsible for housing, nurturing, and delivering new generations of offspring. Several hormones affect livestock reproduction. Male hormones are broadly known as androgens, while female hormones are known as estrogens. Female hormones control the estrous cycle; this cycle can be manipulated by producers in a process known as estrus synchronization to make females cycle at the same time. Remember, female producers should take extreme caution when working with livestock hormones. The common mating practices are natural service, artificial insemination, and embryo transfer. Producers should think through the choice of management practices to ensure it meets the goals of production. Regardless of the method chosen, knowledge of gestation periods will allow for estimation of birth dates, which will allow producers to implement proper management practices. Unit 4 Reproductive Anatomy and Fertility 79

16 Unit Review Questions 1. What are the primary functions of the testes? 2. Why is temperature regulation of the testes important? 3. What is the importance of seminal plasma? 4. Describe what a gel fraction does. 5. How are ejaculation volume and concentration related? Explain. 6. Compare and contrast the reproductive tracts of a cow and sow. 7. When does fertilization occur? 8. How is a zygote different from a blastocyst? 9. When does the gestation period begin and end? 10. What is sperm morphology? 11. Explain the three sperm-motility classifications. 12. How does cryptorchidism affect fertility? 13. What is the difference in estrous and estrus? 14. At what point during the estrous cycle is an animal bred? 15. What conditions can influence an anestrus state? 16. Name three hormones and their functions. 17. Why would a producer choose to use estrus synchronization? 18. What are the benefits of using artificial insemination? 19. Describe the process of embryo transfer. 20. How often can a hen lay an egg? 80 ANIMAL SCIENCE