The Reproductive System of a Cow

by mudza (student)

Reproductive System of a cow

Hormones that are issued from the ovaries and from the brain control a cow’s reproductive system. It is a very complex system, much like that of the human female, and requires a lot of time and effort to explain, making it nearly impossible to generalize without generalizing it too much.

Proestrus is when the follicles attain final growth and secrete estrogen, which causes the cow to come into heat.

Estrus is the period where the cow is the most receptive to breeding with a bull.

Metestrus occurs 12 hours after the end of standing heat, and is when ovulation occurs. This is the time where the corpus luteum forms and produces progesterone.

Diestrus, the longest time of the Estrous cycle, is when the corpus luteum continues to produce progesterone in the hope that sperm will have fertilized the ovum during this time. If not, then the CL regresses at the end of diestrus (which is at the end of the 21 days after her initial estrus period), and the cycle begins again.

Once the ovum is fertilized (or the nucleus of the sperm head merges in with the nucleolus of the ovum), the ovum becomes a zygote and it starts dividing rapidly. The Zygote becomes a blastocyst when a little pocket starts to form inside the blastocyst beside the now-called embryo. The pocket bursts, sending the embryo out further into the cow's uterus. After a little more division, cells specific to becoming a placenta attach themselves to the wall of the uterus, and holds the embryo there for the rest of the gestation period. Typically, scientists have said that when the embryo attaches itself to the uterine wall, this is when gestation officially begins. The placenta is also responsible for sending hormones to the cow to signal the halt of any other ovules from producing in the corpus luteum of the ovaries, and ultimately preventing the cow going into estrus, and instead focus on her body reserves to help this embryo grow into a fetus, and ultimately into a baby calf. The placenta is responsible for the heightened progesterone levels in the cow, which indicates on-going pregnancy.

When the gestation period is up, or the calf is getting too big for the womb, the fetal calf starts sending stress signals through the placenta to the cow's brain, which signal the uterus to start labour contractions. Estrogen levels increase rapidly, sending more signals for the onset of labour. Muscle contractions through the uterus are partly involuntary, and partly voluntary, enabling the cow to judge how hard she should push and sometimes when. The calf assumes a "diver's" position when exiting the womb through the birth canal.

Once the calf is out on the ground, oxytocin levels increase to stimulate milk production, and even mothering-up to urge the calf to get up and nurse. Throughout the next 10 to 48 hours (or even up to 10 days), uterine contractions continue to expel the placenta, and the uterus also starts to shrink in size. Estrogen levels are not at their highest during this 45 to 80 day rest period from birth to the cow's first estrus after calving, and so the cow doesn't go back into her normal cycling until after this period of time.

The Reproductive Tract

Vulva

The vulva is the external part of the reproductive tract. The thickened folds of skin of the structure are sensitive to changes in estrogen, the hormone (Fact Sheet IRM-2) responsible for estrus (heat). Swelling and redness of the vulva, due to increased blood flow, can be useful in estrous detection when coupled with other signs.

Vestibule

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The Reproductive Tract

Vulva

Vestibule

The vestibule is a part of the reproductive tract shared with the urinary system. It is approximately 4 inches long. Openings from the urinary bladder and a blind sac located below the opening of the urethra called the suburethral diverticulum are located on its floor. Dairy producers and Al technicians can prevent insertion of an inseminating rod into these openings, which could result in injury or insemination failure, by knowing their location.

Vagina

The vagina is located between the opening to the bladder and the cervix. Approximately 8 inches long, it is the site of semen deposition during natural service. The vagina also serves as an unrestrictive passageway for the calf at time of birth. One important function of the vagina is as a line of defense against invasion by bacteria. The epithelium of the vagina secretes fluids which combine with cervical fluids to inhibit growth of undesirable bacteria.

Protection from infections may not be sufficient when unsanitary housing conditions are prevalent, or dirty inseminating equipment is used. As a result, vaginal infections can be a problem. In addition, pooling of urine in the vagina adjacent to the cervix can cause infertility in some older cows.

Cervix

The cervix is a unique structure within the reproductive tract. It is 4 to 5 inches long and 1 to 2 inches in diameter and lies between the vagina and uterus. This structure is designed to restrict access to the uterus. The area around the opening of the cervix actually protrudes back into the vagina. This protrusion deflects such things as inseminating rods away from the cervical opening if care is not taken during insemination.

Also, the walls of the cervix are thick and dense in comparison to the walls of the vagina. Three or four ridges or rings within the body or the cervix, called annular folds, can be distinguished by rectal palpation. The folds must be manipulated rectally while an inseminating rod is passed through to the uterus.

The cervix has important functions. The anterior cervix may serve as a site for semen deposition during artificial insemination (Al). This occurs on services where the cycle length is not 21 days and pregnancy from a previous service is possible. Whether by deposition following Al or by migration from the vagina after natural service, the cervix acts as a reservoir for semen. The cervix provides a favorable environment for sperm survival.

Secretions of the cervix are usually thick, but these fluids thin at the time of estrus to facilitate transfer of sperm to the uterus. Some of the mucus may be seen as discharge from the vulva around the time of estrus. The cervix, or fluids of the cervix, act as a physical barrier and protect the uterus from any foreign material or bacteria during pregnancy. A thick plug forms in the canal of the cervix and blocks access to the pregnant uterus. Accidental rupture of this plug by insertion of an inseminating rod can result in abortion.

Uterus

The uterus consists of a “body” and two “horns”. It is attached to the broad ligament and suspended within the pelvic cavity and posterior portion of the body cavity. The body of the uterus is adjacent to the cervix. In a non-pregnant state it extends less than 2 inches before it divides into two separate horns.

The uterine body is the major site of semen deposition during Al. If the tip of the inseminating rod is inserted too far into the uterus, semen is deposited in only one of the uterine horns (Fact Sheet IRM-12). If the egg was released from the ovary on the other side, there is little chance that sperm and egg would unite. Remember, the body of the uterus is less than 2 inches long and caution must be used to correctly deposit semen into this region.

The uterus has many functions. Its walls are composed of several layers of muscle which aid in transport of sperm to the oviduct following insemination and in expulsion of the calf at birth. Certain glands within the walls of the uterus secrete a fluid, uterine milk, which provides nutrients to the developing embryo before and after its attachment to the uterine wall.

The uterus also develops the maternal side of the placenta to nourish and protect the developing fetus. Its surface contains many specialized areas called caruncles .Cotyledons of the fetal placenta interlock with caruncles on the uterus to provide a passageway for the exchange of nutrients and waste between fetus and cow. After calving, if the caruncles and cotyledons fail to unlock, the placenta cannot be expelled and a retained placenta (Fact Sheet IRM-21) results.

Oviduct

The oviducts are approximately 10 inches long, 1/4 inch in diameter and lie between each ovary and tip of the adjacent uterine horn. The ovarian end of the oviduct is funnel shaped and called the infundibulum. The infundibulum catches the egg as it is released from the ovary at ovulation and moves it to the enlarged upper end of the oviduct called the ampulla. Fertilization occurs here within 12 hours of ovulation. After fertilization, the fertilized ovum is transported to the uterus in a process requiring 3 to 4 days.

Ovaries

The ovaries are the primary reproductive organ of the female. In a dairy cow, each ovary is approximately 1.5 inches long and 3/4 inch in diameter. The ovaries are suspended from the broad ligament near the end of the oviduct and lie near the tips of the curved uterine horns. Their function is to produce the egg or ovum and hormones involved in regulating the estrous cycle and pregnancy.

The ovaries contain thousands of ova. These are produced by the embryo prior to birth. While the potential to collect hundreds of ova from a cow exists, only one ovum is usually released during each estrous cycle. When more than one ovum is naturally released it can lead to multiple births—an undesirable event because of freemartinism. However, superovulation, or the production of several ova following injection of hormones such as pregnant mare’s serum gonadotropin (PMSG) or follicle stimulating hormone (FSH), is an essential element in embryo transfer.All ova are surrounded by a special layer of cells in the ovary. The growth of these cells produces blister-like structures, called follicles, that are visible on the surface of the ovary.

These develop continuously throughout the life of the cow and the vast majority regress without releasing the ova. Development of ovulatory follicles begins at puberty. As the follicle enlarges, it appears as a large blister on the surface of. the ovary and can be easily detected by rectal palpation. This phase of activity is culminated by the release of ova from the follicle along with the follicular fluids.

Following ovulation, the walls of the follicle collapse and develop into the corpus Iuteum (CL) or yellow body. The CL reaches its maximum size 10-12 days after ovulation and is the dominant structure on the ovary. If a pregnancy does not result, the CL regresses 3 to 4 days prior to the next ovulation. However, the presence of an embryo in the uterus prevents this from happening.

Development of the follicle and subsequent formation of the CL are associated with the production of estrogen and progesterone, respectively. Estrogens are produced by the cells lining the wall of the follicle and are responsible for changes in behavior as well as altering the production of fluids by the vagina, uterus and cervix. In addition, estrogens also trigger the release, from the pituitary gland, of the hormone responsible for ovulation, Iuteinizing hormone (LH).

As a result of these synchronized events, the cow comes into estrus, can be mated, the fluids of the tract provide a favorable environment for survival of the sperm and ova, and ovulation occurs at the time when sperm will be available to cause fertilization.

Associated with ovulation is the transformation of the follicle wall into the CL under the influence of LH. The CL begins to produce progesterone which is required for maintenance of pregnancy. Progesterone acts upon the lining of the uterine wall to prepare it for subsequent attachment of the embryo. In addition, progesterone and low levels of estrogen prevent resumption of normal cyclic activity and allow for maintenance of pregnancy (Fact Sheet IRM-2).

Another important function of the CL is the production of a hormone called relaxin. Relaxin relaxes the cervix and suspensory ligaments in the pelvic region prior to calving, producing the “springer” look. This relaxation of the cervix is essential for the successful delivery of a new calf. Induction of parturition with oxytocin prior to relaxation of the cervix may result in damage to the uterus because the cervix may not relax sufficiently to allow for passage of the calf.

Mahdhi, Akash and Stefan