Reproductive Tract Anatomy and Physiology of the Moo-cow

Guide B-212
Reviewed past Jason Turner
Higher of Agronomical, Consumer and Ecology Sciences

Author: Professor/Extension Horse Specialist, Department of Extension Animal Sciences and Natural Resources, New Mexico Land University (Print-friendly PDF)

Introduction

Understanding the anatomy and physiology of the moo-cow's reproductive organisation is cardinal to practiced cattle management. Basic knowledge in this area volition help producers do a better chore of getting cows rebred, especially when using artificial insemination and estrus synchronization. It will too enable producers to better understand and control reproductive diseases and calving bug.

Anatomy

The ovary is the primary female reproductive organ and has 2 important functions: producing the female person reproductive cell (the egg or ovum) and producing the hormones estrogen and progesterone. The cow's 2 ovaries are oval to edible bean-shaped organs that are i—i.5 inches long and located in the abdominal cavity.

The secondary sexual practice organs are a serial of tubes that receive semen, ship sperm to the egg so it can exist fertilized, nourish the fertilized egg (embryo), and allow the calf to exist birthed. These organs include the vagina, neck, uterus, uterine horns, and oviducts (as well called Fallopian tubes), which each have a funnel-shaped opening called the infundibulum. Figure 1 presents a diagram of the complete reproductive tract anatomy.

Fig. 1: Illustration of a cow's reproductive tract.

Figure 1. Diagram of the reproductive tract of the cow. (Adapted from Rich and Turman, n.d.)

The ovary produces the egg past a procedure chosen oogenesis. In contrast to the continuous production of sperm (spermatogenesis) in the male, oogenesis is cyclic. This cycle (called the estrous bicycle) has a characteristic length and consists of a definite sequence of events, both physiological and behavioral.

The ovary contains several thousand tiny structures called primary follicles. Each follicle consists of a germ cell surrounded by a layer of cells. This germ cell has the potential to mature into an egg if the follicle completes evolution. However, most of the primary follicles never develop. Rather, they die, are absorbed by the ovary, and are replaced by newly formed principal follicles.

The relatively few primary follicles that develop completely practice and so through a series of phases. Many layers of cells are added to the single layer of cells surrounding the egg in the principal follicle, forming a key cavity. As the follicle and crenel grow larger, the egg becomes fastened to the dorsum of the follicle (opposite the ovulation site) by a stalk of cells. As the follicle continues to grow chop-chop, the side opposite the egg bulges from the surface of the ovary and becomes very thin. Once the follicle reaches this mature country information technology is chosen a Graafian follicle. At ovulation, the thin portion ruptures to release the contents of the follicle, including the egg.

Following ovulation, the cells that adult within the follicle differentiate to grade the corpus luteum, which has the of import function of producing progesterone.

The released egg is defenseless by the infundibulum and moves into the oviduct, where fertilization occurs if viable sperm are nowadays. The egg remains capable of fertilization for only a few hours. Thus, it is very of import that fertile sperm be present well-nigh the time of ovulation. The egg moves through the oviduct and into the uterus within the adjacent iii to 4 days. If it is fertilized, information technology begins embryological development in the uterus. If it is not fertilized, it degenerates.

The torso of the uterus is relatively short and poorly developed, while the uterine horns are longer and well developed. The embryo develops in the uterine horns. The fetus develops within a layer of membranes called the placenta. The mother nourishes the fetus via diffusion of nutrients across the placenta. In that location is no direct blood connection between fetus and mother.

The cervix is essentially the neck of the uterus. It has thick walls and a pocket-sized opening that is hard to penetrate because of overlapping or interlocking folds (annular rings). Information technology serves as a passageway for sperm deposited in the vagina and for the fetus at the fourth dimension of nascency. During pregnancy, it is commonly filled with a thick secretion that serves as a plug to protect the uterus from infective material entering through the vagina.

The vagina serves every bit the receptacle for the male penis during copulation. During natural mating, semen is deposited in the vagina near the cervix. The urinary bladder opens to the outside through the urethra, which opens into the vagina. This region of the vagina is restricted in size because of sphincter muscles associated with the urethral opening. The region behind the external urethral orifice is called the vestibule and is a common passageway for both the urinary and the reproductive systems. The external opening of the vagina is called the vulva.

Hormonal Regulation of the Female Reproductive Tract

Normal reproduction in the female depends on hormones, which are specific chemic substances produced past specialized glands called endocrine glands. These secretions laissez passer into the blood and lymph fluids and are transported to various parts of the torso, where they exert several specific furnishings. Reproductive hormones may originate in the hypothalamus, pituitary, gonads, uterus, or placenta.

Hypothalamic hormones are produced in the hypothalamus, a portion of the encephalon that is the neural control heart for reproductive hormones. The office of hypothalamic hormones is to regulate the release of other hormones from the pituitary gland, and many hypothalamic hormones are therefore called releasing hormones. The main releasing hormone of reproduction is gonadotropin-releasing hormone (GnRH). GnRH controls the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland, located at the base of the brain. These pituitary hormones regulate the production of estrogen and progesterone from the ovary. FSH stimulates the follicle'southward growth, evolution, and function, while LH causes the follicle to rupture and the corpus luteum to develop.

The female person hormone estrogen is produced past the Graafian follicle on the ovary. Some other hormone originating from the ovary is progesterone, which is produced by the corpus luteum. Each has an of import role in the female reproductive procedure.

Estrogen regulates several functions: the development and functioning of the secondary sexual practice organs; the onset of oestrus, or oestrus (the menstruum of sexual receptivity); the rate and type of body tissue growth, specially fat deposition; and priming or preparing of the prepubertal heifer and postpartum moo-cow for the onset of sexual activeness.

Progesterone is the hormone of pregnancy. It suppresses the further evolution of follicles and estrogen secretion. While progesterone is being produced, the female does not showroom estrus. Progesterone is necessary for preparing the uterus to receive the fertilized egg and maintains the proper uterine environment for pregnancy. Estrogen and progesterone are not completely divide in their effects because both are necessary for consummate development of some of import organs. Uterine development is initiated past estrogen and completed past progesterone. The fertilized egg will non adhere and survive in the uterus unless that tissue has been properly prepared by the action of estrogen and progesterone. Estrogen causes rhythmic contractions of the uterus. On the other hand, progesterone has a quieting result on the uterus, and so in that location are no contractions that might disturb pregnancy.

Consummate development of the mammary gland also depends on both hormones. Estrogen promotes the growth of the duct organisation, and progesterone is necessary for the development of the clusters of milk-secreting alveoli on the ducts. In general, estrogen makes things happen and progesterone calms them down.

A number of other hormones and hormone-like chemicals are known to accept of import roles in regulating the cow'due south reproductive system. Prostaglandins are secreted by many body tissues and perform a diversity of functions. The prostaglandin primarily affecting the moo-cow's estrous bicycle is prostaglandin F2α (PGF2α), which is produced by the uterus. PGF2α is the natural luteolytic amanuensis that causes the corpus luteum to backslide tardily in the estrous cycle and allows a new estrous cycle to be initiated in the nonpregnant female. In a meaning cow, a betoken is sent from the developing embryo to the uterus to forestall PGF2α release, which allows the corpus luteum to persist throughout pregnancy. Maternal recognition of pregnancy is believed to occur between days 16 and 17 after fertilization. Injecting cows or heifers with PGF2α betwixt days half-dozen and 16 of the estrous cycle will cause premature regression of the corpus luteum, with the best results achieved among females injected on days 10 to 16.

Injecting PGF2α during the outset 5 (1—v) and last five (17—21) days of the estrous cycle will generally not cause luteal regression. The luteolytic response allows the use of PGF2α in rut synchronization programs in cow herds and initiates ballgame in feedlot heifers. Estrogens, progestins, prostaglandins, and GnRH have all been used in various combinations in effective estrus synchronization programs.

The Estrous Cycle

The moo-cow'southward reproductive cycle consists of a series of events that occur in a definite order over a menstruation of days. The estrous cycle in the cow averages 21 days (range is 17—24). During this time, the reproductive tract is prepared for estrus or heat (the menstruum of sexual receptivity) and ovulation (egg release). Figures 2 and three outline the sequence of anatomical and hormonal changes that occur during a typical 21-24-hour interval cycle in which pregnancy does non occur.

Fig. 2: Diagram of a 21-day estrous cycle in which pregnancy does not occur.

Effigy 2. Diagram of a 21-day estrous bicycle in which pregnancy does not occur. (Adjusted from Deutscher, 1980)

Twenty-four hour period 0: The cow is in oestrus (standing heat) due to an increased concentration of estrogen. As estrogen levels attain a certain threshold level, a surge of LH is released past the pituitary. Near the terminate of standing rut, the mature Graafian follicle ovulates (ruptures) in response to this LH surge.

Days 1—2: The cells that formerly lined the follicle change and get the luteal cells of the corpus luteum. This modify in cell class is caused by hormonal action, primarily the activeness of LH.

Days ii—v: The corpus luteum grows speedily in both size and function. At this stage, numerous follicles may be seen on the ovary, but by solar day v they accept begun to regress.

Days 5—16: The corpus luteum continues to develop and typically reaches its maximum growth and role by day 15 or 16. It secretes the hormone progesterone, which inhibits (blocks) LH release by the pituitary gland. During this period, the ovaries are relatively inactive except for the functional corpus luteum. No follicles accomplish maturity and/or ovulate considering of high concentrations of progesterone.

Days 16—18: Increased follicular growth and accompanying estrogen secretion by the ovary stimulate PGF2αsecretion past the uterus, causing rapid regression of the corpus luteum.

Days xviii—19: The corpus luteum is nearly nonfunctional and progesterone release is suppressed, removing the blocking action of progesterone on LH and FSH. Of the several follicles that are initially recruited, one becomes dominant past a surge in rapid growth and activity. As this Graafian follicle grows, information technology secretes increasing amounts of estrogen, and the smaller follicles regress.

Days 19—20: With the increase in estrogen release past the Graafian follicle and a corresponding decrease in progesterone by the regressing corpus luteum, estrus or heat will occur (cycle has at present returned to day 0). The loftier estrogen concentration in the claret triggers a release of LH virtually the onset of heat. Post-obit this surge in LH blood concentrations, the mature follicle ruptures to release the egg, and the cellular tissue left behind becomes luteinized and forms a new corpus luteum (cycle has now returned to days 1—2). Progesterone again becomes the dominant hormone.

The timing given for these events is just judge (based on a 21-day average) and differs for dissimilar cycle lengths (range of 17—24 days).

This give-and-take of events that occur during the estrous cycle is based on a full cycle in which pregnancy does not occur. If the egg is fertilized and begins developing in the uterus, the corpus luteum does not backslide just continues to function and secrete progesterone. During pregnancy, no follicles develop to maturity and heat does not normally occur. Increased concentrations of progesterone promote uterine quiescence, providing the about favorable conditions for the developing fetus.

Fig. 3: Line graph of anatomical and hormonal changes that occur during a typical 21-day estrous cycle.

Figure 3. Graph of the anatomical and hormonal changes that occur during a typical 21-solar day estrous bike. (Adapted from Deutscher, 1980)

Any condition that prolongs the menstruation of time when blood concentrations of progesterone remain high (such as implanting, injecting, or feeding progestins for estrus synchronization) will keep the female from exhibiting heat. Occasionally, the corpus luteum does not regress normally even though the animate being does not become meaning (pseudopregnancy).

Occasionally, abnormally short estrous cycles (7—11 days) occur. This condition appears to occur because either no corpus luteum is formed or, if ane is formed, information technology is nonfunctional and progesterone concentration remains besides low to sufficiently suppress the release of gonadotropins (FSH and/or LH) from the anterior pituitary.

Cows also have periods of anestrus (noncycling). For example, an anestrous period is ordinarily observed in cows following calving. Low levels of nutrition can contribute to the duration of anestrus, specially in young cows nursing calves.

Estrus is not always accompanied by ovulation, nor ovulation by estrus. Estrus without ovulation (anovulatory rut) will not result in pregnancy, fifty-fifty if the female person is bred. Ovulation without the external signs of heat (tranquillity or silent heats) is not uncommon in cows, peculiarly the kickoff few weeks after calving. Such females will by and large not "stand" to be bred past a bull.

The discussion of the hormonal control of the estrous bicycle and pregnancy equally presented here is a greatly simplified summary of the whole process. In reality, the reproductive process is very complicated and involves a number of hormonal interactions and events.

References

Deutscher, G.H. 1980. Reproductive tract beefcake and physiology of the cow [G80-537-A]. Lincoln: University of Nebraska Cooperative Extension.

Rich, T.D., and E.J. Turman. northward.d. Reproductive tract anatomy and physiology of the cow [Beef Cattle Handbook BCH-2200]. Stillwater: Oklahoma Country University.

Original authors: Ron Parker and Clay Mathis, Extension Livestock Specialists. Adapted from Extension Beef Cattle Resources Committee Beef Cattle Handbook publication BCH-2200, by T.D. Rich and E.J. Turman, Animal Science Department, Oklahoma State University.

Photo of Jason L. Turner.

Jason L. Turner is Associate Professor and Extension Horse Specialist. Jason was agile in 4-H and FFA while growing upward in Northeastern Oklahoma. His Thousand.S. and Ph.D. studies concentrated on equine reproduction, health, and management. His Extension programs focus on proper intendance and management of the horse for youth and adults.

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Revised July 2014 Las Cruces, NM