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REPRODUCTIVE ANATOMY AND PHYSIOLOGY OF CATTLE

Understanding the anatomy and physiology of the cow’s reproductive system is fundamental to good cattle management.

ANATOMY

First, let’s look at the parts that make up the reproductive system in cattle.

There are two ovaries, two oviducts, two uterine horns, a uterine body, cervix, vagina and vulva. The bladder lies below the reproductive tract and is connected at the urethral opening located on the vaginal floor. The rectum is located above the reproductive system. The vulva is the external opening to the reproductive system.

The vulva has three main functions: the passage of urine, the opening for mating and serves as part of the birth canal. Included in this structure are the lips and clitoris. The vulva lips are located at the sides of the opening and appear wrinkled and dry when the cow is not in estrus. As the animal approaches estrus, the vulva will usually begin to swell and develop a moist red appearance. The vagina, about 6 inches in length, extends from the urethral opening to the cervix. During natural mating, semen is deposited in the anterior portion of the vagina. The vagina will also serve as part of the birth canal at the time of calving.

The cervix is a thick walled organ forming a connection between the vagina and uterus. It is composed of dense connective tissue and muscle and will be the primary landmark when inseminating cattle. The opening into the cervix protrudes back into the vagina. This forms a 360º blind-ended pocket completely around the cervical opening. This pocket is referred to as the fornix. The interior of the cervix contains three to four annular rings or folds that facilitate the main function of the cervix, which is to protect the uterus from the external environment.

The cervix opens anteriorly into the uterine body. About an inch long, the body of the uterus serves as a connection between the two uterine horns and the cervix. The uterine body is the site where semen should be deposited during artificial insemination.

The main function of the uterus is to provide a suitable environment for fetal development.

When a cow is bred, either naturally or by artificial insemination, the uterine muscles, under the influence of hormones oxytocin and estrogen, rhythmically contract to aid in sperm transport to the oviducts.

Oviducts, as their name implies, carry ova, the cow’s eggs.

The oviducts are also commonly referred to as the fallopian tubes. The oviduct has several distinct regions when examined microscopically.

The lower segment, closest to the uterus is called the isthmus. The connection between the uterus and the isthmus is called the utero-tubal junction or UTJ. The UTJ functions as a filter of abnormal sperm and the isthmus as a reservoir for healthy sperm.

The upper portion of the oviduct, closest to the ovary, is referred to as the ampulla. The interior of the ampulla is more open than the isthmus allowing for easier passage of ova.

The large funnel-like structure on the open end of the oviduct, called the infundibulum, surrounds the ovary, to recover the ova and keeps them from falling into the body cavity.

The ovaries are the primary organs in a cow’s reproductive tract. They have two functions: to produce eggs and to produce hormones, namely estrogen and progesterone, throughout the different stages of the estrus cycle.

Follicles are fluid filled, blister like structures that contain developing oocytes or eggs.

The other structure found on the ovarian surface is the corpus luteum or CL The CL is the site where ovulation occurred during the previous cycle.

THE ESTROUS CYCLE

Over a period of time, many changes take place in the reproductive system in response to changing hormone levels. These changes in normal open females repeat every 18 to 21 days. This regular repetitive cycle is called the estrous cycle.

One ovary has a large follicle approximately 15 to 20mm in diameter. This follicle has a mature egg inside ready to be released. The cells lining the follicle are producing the hormone estrogen.

Estrogen is also responsible for all signs of heat including; a red swollen vulva, allowing other cows to mount her, going off feed, bellowing considerably and holding her ears erect are but a few of the many signs.

On day 1, the follicle ruptures or “ovulates” releasing the egg to the waiting infundibulum. Several hours prior to ovulation estrogen production declines. As a result, the cow no longer displays the familiar signs of heat. After ovulation, new types of cells called, luteal cells, grow in the void on the ovary where the follicle was located.

Quite rapidly over the next five to six days these cells grow to form the corpus luteum (CL). The CL produces another hormone, progesterone. Progesterone prepares the uterus for pregnancy.

Progesterone also prevents the animal from returning to estrus by regulating the release of gonadotropins from the pituitary gland in the brain.

Days 16 through 18 of the estrous cycle are referred to as “the period of maternal recognition.” During this time, the uterus searches itself for the presence of a growing embryo. If no embryo is detected, the uterus begins to produce another hormone called prostaglandin. Prostaglandin begins to destroy the CL (Figure 15). When the CL is destroyed, no more progesterone is produced and the pituitary gland begins to increase secretion of gonadotrophins. Increased secretion of LH stimulates the dominant follicle to produce estrogen and bring the animal back into estrus.

The luteal phase begins when the corpus luteum is formed; about 5 to 6 days after the cow was in heat, and ends when the CL regresses, about day 17 to 19 of the cycle.

The other phase of the cycle; the follicular phase, begins when the CL of one cycle is regressed and ends when the new CL of the following cycle is formed. Thus, the follicular phase encompasses the period of time surrounding estrus. During this phase of the cycle estrogen levels are typically high while progesterone levels are low.