Milk Production Management & Dairy Development

Module 8. Reproductive systems of dairy animals

Lesson 13
MALE AND FEMALE REPRODUCTIVE SYSTEMS OF DAIRY CATTLE

13.1 Introduction

One of the major functions of all living things is reproduction. Regular and efficient reproduction is an important factor affecting profits from a livestock enterprise. The instinct for proliferation of the species will maintain an efficient reproductive activity under natural conditions. But, under the restricted conditions of domestication, efficiency in reproduction mainly depends on the understanding of the basic facts about reproduction and the appli­cation or sound principles and techniques by the manager.

Efficient reproduction implies that the female is producing normal numbers of healthy eggs, and each of the eggs is capable of being fertilized by a normal sperm. It also implies that the female genital tract is healthy and normal, capable of nurturing the zygote during its development to produce normal healthy offsprings. As far as the male is concerned, it should be producing sufficient numbers of healthy sperms capable of fertilization on mating or insemination and in good physical condition to mate or give semen at frequent intervals.

13.2 Anatomy and Physiology of Male Reproductive System

The primary organs of reproduction in male are two testes which produce sperms. These organs are located in a bag-like out pocketing of the abdominal wall known as scrotum. Other parts of the male reproductive system consist of epididymis, vasdeferens, seminal vesicles, prostate gland, bulbo-urethral or Cowper’s glands, urethra and penis.

Testes perform the dual role of producing sperms and secreting the male sex hormone testosterone. Sperms are produced in the seminiferous tubules which are long coiled tubes, the inner walls of which are lined with ger­minal epithelium. Between the seminiferous tubules, there are blood vessels, connective tissue, nerves and the interstitial or leydig cells. These interstitial cells secrete the male sex hormone testosterone which is responsible for typical male attitude and action. It helps in the development and mainte­nance of the entire male reproduc­tive system. Increased levels of the male hormone, starting from puberty, produces libido and secondary sexual characteristics.

Epididymis is a highly coiled tubule connected to the seminiferous tubules. The sperms undergo development and become mature in the epididymis where they are stored. The vas deferens connects the epididymis to the urethra. It also possesses glands which contribute to the fluid portion of semen. The seminal vesicles are located in the pelvic cavity at the ends of the vasa deferentia as an outgrowth. They produce the largest part of the seminal plasma of the stallion, bull and ram.

The postage gland is a compound gland consisting of a large number of lobules surrounding the urethra at the neck of blad­der. The secretion from this gland contributes to the seminal fluid. The composition of the prostatic fluid varies to a great extent between species. The cowper’s glands are found on either side of the urethra. They are often called the bulbo-urethral glands and are small, firm and oval in shape. Little is known about the function of the secretion from these glands. The main function of the secretion from Cow­per's glands appears to be to cleanse the urethra in the first fraction of ejaculate of semen. The urethra is a duct connecting the bladder to the external orifice at the distal end of the penis which passes through the penis. It has two important functions:

a. Carrying urine for excretion from the bladder to the external orifice.

b. Transporting semen to the reproductive tract of the female.

The penis is the copulatory organ of the male. The urethra runs through this and, therefore, the functions of elimination of urine and transporting semen to the female genital tract can be attributed to the penis. It is made up ·of muscular and erectile tissues that become engorged with blood when the animal is sexually stimulated resulting in erection.

13.2.1 Hormonal regulation of the male reproductive tract

The primary regulation of reproduction lies with the brain which elaborates the releasing hormones. The releasing hormones in their turn stimulate the anterior pituitary to produce and release the gonadotropic hormones. Thus, the FSH-releasing factor of brain stimulates the produc­tion of Follicle Stimulating Hormone (FSH) and the LH-releasing factor, Luteinizing Hormone (LH). The brain seems to be controlled by the servo or feed-back mechanism in elaborating releasing fac­tors. If these gonadotropic hormones are not produced in sufficient quantities in the young animal, that animal will not attain sexual maturity. Failure of production of gonadotropins in the adult will result in impaired spermatogenesis, as well as impaired testosterone production. FSH brings about mainly the development of seminiferous tubules and is concerned with sperm production. LH on the other hand stimulates the development of interstitial cells and hormone production.

The role of testosterone is inducing puberty, in developing secon­dary sexual characters, in stimulating libido and controlling develop­ment and maintenance of the entire male reproductive tract may be stated that, there are a number of naturally occurring and synthesized compounds having the male sex hormone activity which are collectively called androgens or androgenic hormones. The specific androgenic hormone produced by the testes is called testosterone.

13.3 Anatomy and Physiology of Female Reproductive System

The female reproductive tract consists of two ovaries and a duct system which opens into the exterior. The duct system consists of infundibulum, fallopian tubes (oviducts), uterus, the cervix, vagina and vulva. The ovaries are analogous to the testes in the male having a simi­lar dual function that is, production of (i) gametes (eggs, ova), and (ii) female hormones. They are almond shaped or bean shaped in the cow, ewe and mare and relatively smooth. They are two in number and located on either side of the vertebral column in the area of the hip bones. From the time of birth, ovary consists of hundreds of primordial egg cells and ovary is almost completely covered with a single layer of germinal epithelium. The ova develop in structural units known as follicles which when enlarge and bulge out of the ovarian wall like pimples are known as Graafian follicles. One or more Graafian follicles develop in each reproductive cycle depending on the species. In addition to the developing ovum, the Graafian follicle produces the female hormone estrogen. The follicle eventually bursts and the ovum is released.

The fallopian tubes are oviducts which provide the connecting link between infundibulum and uterus. They are convoluted tubes with a ciliated surface inside. Ova that fall into infundibulum are slowly transported to uterus through these tubes. Fertiliza­tion normally takes place in upper part of the fallopian tube with the sperm moving through uterus and duct to meet the descending egg.

The uterus is a tubular muscular organ consisting of a body and two horns. The horns are well developed in cattle, sheep and swine. The uterus is lined with a thick layer of mucosa or endometrium which contains many glands. These glands produce the secretion which is seen at estrus and also provide nutrition to developing embryo (blasto­cyst) prior to implantation. The inner surface of the uterus of cow, ewe and other ruminants contains about 100 button-like projections known as caruncles. During pregnancy, the foetal membranes are attached to the caruncles of maternal endometrium by interlocking with corresponding area known as cotyledons. Nourishments for the foetus from the mother's blood passes mainly at the region of caruncles in ruminants.

The cervix is the thick-walled, muscular neck of the womb which lies between the uterus and the vagina. It consists of a strong mus­cular sphincter which keeps the opening closed during pregnancy and anestrous periods. This sphincter relaxes during estrus (heat) and during parturition. The musculature of cervix is thick. The mucous membrane has longitudinal folds and transverse constrictions forming a spiral canal inside. During pregnancy, thick mucous plug seals the cervix.

The vagina is the posterior part of the female reproductive tract and serves as the copulatory organ. It is a tubular canal extending from the cervix to the vulva. In farm animals it lies horizontally just below the rectum. It is a highly elastic structure. It receives the penis, and in most cases semen also, during copulation. Besides, it also acts as a birth canal during parturition. The epithelial lining of the vagina changes according to the stages of estrous cycle. During estrus, the vaigna has a highly stratified epithelial layer. During anestrous it is less active and consists of only a few layers of epithelial cells.

Vulva or labia, majora is the posterior open­ing of the female genital tract into the exterior. The clitoris is a rudimentary organ located at the ventral junction of the vulva. It is sensory and erectile to a limited extent. With the onset of heat the vulva and clitoris become turgid due to an increased flow of blood, parti­cularly so in the sow and less so in cow, ewe and mare. Clitoris is the major erotic centre in female.

13.3.1 Hormonal regulation of female reproduction. The regulatory mechanism acts in a three tier system consisting they are

i. Anterior pituitary hormone releasing factors from the mid-brain,

ii. The pituitary gonadotropins

iii. The gonadal and placental hormones.

The entire adenohypophyseal (anterior pituitary) hormones are regulated by releasing factors from hypothalamus. The two coordinating and regulating systems, i.e., nervous system and humoral system are interconnected through the releasing factors. The releasing factors are hormones produced in the hypotha­lamus which pass via the hyperphysical portal system to influence the release of all hormones from anterior pituitary including pituitary gonadotropins.

The anterior pituitary releases two separate gonadotropins. One specifically stimulates follicular growth in the ovary and is termed the Follicle Stimulating Hormone (FSH). The release of this hormone is regulated by FSH-releasing factor from hypothalamus.

The second substance does not produce any effect if FSH has not acted on ovary first. This factor is termed the luteinizing hormone (LH) and is regulated by LH-releasing factor of hypothalamus. The Graafian follicles in ovary develop under the stimulation of FSH, but LH is essential for their maturation. Estro­gen is synthesized in follicle under the influence of both FSH and LH. Ovulation and initial development of corpus luteum (in some species) are also controlled by LH.

Ovary (and placenta during pregnancy) produces three important hormones under the influence of pituitary gonadotropins. The deve­loping Graafian follicles and later the placenta produce estrogen. It is a steroid hormone responsible for the development, maintenance and cyclic changes of the female tubular genital tract. It is also concerned with the development of secondary sex characteristics, sexual behavior and development and functioning of accessory sex organs. This hormone also controls glandular and duct development in the mammary glands and uterus. As the Graafian follicle breaks to release the ovum, the remaining parts of that follicle are converted into an endocrine organ known as corpus luteum which elaborates the hormone called progesterone. The placenta also produces this hormone at the time of pregnancy. Progesterone in the presence of estrogen develops uterus for implantation of the zygote and main­tains pregnancy. Progesterone is responsible for the development of glandular tissue in the mammae and uterus.

13.4 Estrous Cycle

The combination of physiological events which take place starting with estrus (heat) and ending with the next heat is termed estrous cycle. The estrous cycle of cow range from 19 to 21 days. The whole estrous cycle may be divided into 4 phases viz., estrus, metestrus, diestrus and proestrus. Estrus or heat is the period in which female shows desire to be mated by male. During this period, ovulation and formation of corpus luteum occur in most species. Metestrus is the period just after ovulation during which the level of estrogen and LH fall and corpus luteum beings to function. The next phase is diestrus which is predominated by the influence of progesterone from corpus luteum on all accessory sex structures. If fertilization takes place, pregnancy with the accompanying high level of progesterone will stop the formation and development of new Graafian follicles in the ovaries, resulting in a condition known as anestrous. Regular rhythmic estrous cycle can start only after the termination of pregnancy.

13.5 Ovulation and Fertilization

Ovulation is the shedding of ova from the Graafian follicle. The process of ovulation is a gradual one. LH is the ovulatory hormone of mammals. In most animals, ovulation occurs at end or a little after the end of estrus. In most of the farm animals ovulation takes place at scheduled time irrespective of the fact whether coitus has taken place or not. They are known as spontaneous ovulators. Even in these animals, stimulation of the genital tract has been found to hasten ovulation. The second group (cat and rabbit) known as the induced ovulators require stimulus of coitus for ovulation to take place.

The discharged ovum enters the fimbriated end of oviduct (infundi­bulum) and begins to descend towards uterus. If copulation had taken place and sperms are available in required numbers, fertilization will take place in upper portion of the oviduct. The essential step in fertilization is the entry of a single sperm into ovum facilitating the union of nuclei of two gametes to form zygote.

Mating stimulates the estrogen-stimulated uterus to undergo strong contractions under the influence of oxytocin released by reflex action. This facilitates quick deployment of newly arrived spermatozoa in the anterior fallopian tube where it could meet the ovum. The uterine contractions are far more important than the motility of spermatozoa in transporting sperms from cervix to the anterior fallopian tube.

Even though, only one sperm cell fertilizes the ovum, it has been found that minimum concentrations of spermatozoa are required to bring about fertilization. It is hypothesized that a minimum number of sperma­tozoa are required to bring about certain biochemical changes in or around the ovum without which the penetration of ovum by a sperm cannot take place. Similarly, once a sperm penetrates into an ovum, certain changes take place in zona pellucida and vitelIine membrane of the egg, precluding further entry of sperms.

The spermatozoa of some mammalian species should remain in the female genital tract for a period of several hours to undergo capacitation to attain the capacity to fertilize the ovum.

After ovulation, the ovum is capable for undergoing fertilization for a period of five to ten hours. On the other hand, the sperms may survive in female genital tract for a day or two. Therefore, the time of mating in relation to ovulation is important for effective fertilization.

13.6 Pregnancy

Viviparity is one of the sophistications of evolution in mammals. Providing a protected environment to the young ones in the early stages of their development inside the body of the mother and supplying nutrients and removing waste products involves lot of coordination which is brought about by endocrine system. The fertilized egg descends down the oviduct and reaches the uterus by the fourth day. The prenatal life in the uterus has been subdivided into three:

1. The period of the ovum: Free living zygote which sheds the zona pellucida and becomes the blastocyst which in turn makes a loose attachment to the uterine mucous membrane. The zygote is nourished by uterine milk or secretions of the uterine glands.

2. The period of the embryo: Blastocyst undergoes further development with the formation of organs.

3. The period of the foetus: It consists of the period when developed embryo and placenta undergo rapid growth. This period is terminated by parturition.

The placental membranes consisting of amnion (forms a protective cushion against external shocks), allantois and chorion (foetal circulation ) have a complex role to play.

13.6.1 Detecting pregnancy

It is of great importance to know whether or not a female animal has become pregnant after a service. Pregnancy diagnosis assists in herd management by identifying non-pregnant animals, which can be served again or culled with minimum necessary delay. Such a pro­cedure will improve the breeding efficiency and greatly contribute towards the economy.. Pregnancy-diagnosis is also very useful in selling and buying animals and also in slaughter houses where slaughter of a pregnant animal can be prevented.

Methods for detecting pregnancy are classified as follows:

1) Detection of the physical presence of the foetus and foetal membranes by rectal or abdominal palpation.

2) Identifying the presence of the foetus in the uterus by X-ray technique (radiography) or by studying the echo of ultrasonic waves.

3) Detection of physical and behavioral changes in the mother that occur with pregnancy.

4) Detection of hormonal changes associated with pregnancy.

5) Detection of maternal changes, which are the result of endo­crinological changes.

13.5.3 Methods of pregnancy diagnosis

In cattle, the most widely used method is rectal palpation. A gloved hand is inserted through the rectum of the cow and the geni­tal organs and foetus are palpated through the rectal wall. During the first 35 days there is no visible change in the size of the uterus. However, an experienced technician can tentatively diagnose preg­nancy during this period by palpating the corpus luteum (CL) of pregnancy.

From the six weeks onwards, there will be enlargement of the uterus which can be easily studied in primeparous heifers. But, in cows which have calved already, there will be some enlargement of one horn even without con­ception. At eight weeks, the pregnant horn is approximately six times larger than the non-gra­vid horn. From eighth week to twelfth week, the gravid horn enlarges by 50 per cent more. The uterus becomes extra pelvic and starts descending in the abdominal cavity. This is the most ideal period for rectal palpation to diagnose pregnancy. From twelve weeks onward the uterus enlarges very rapidly and descends into the abdomen. But it can be easily felt up to sixteen weeks. Thereafter palpation of the uterus becomes progressively difficult as it sinks deeper and deeper into the abdomen. However, cotyledons, a double uterine membrane and an enlarged middle uterine artery with fremitus can be felt; As the time approaches, the foetus grows and extends back into the pelvic cavity and so becomes easier to palpate head and limbs.

13.7 Gestation Period

Gestation periods vary considerably between species of farm animals. Within a species variation in the length of pregnancy period can be observed between breeds and between individuals within the breeds. The gestation period of some farm animals are listed in the table below:

Table 13.1 Gestation period in farm animals

Animals	Average gestation period
Indian Dairy Cattle (Bos indicus)
Sahiwal	286
Gir	286
Red Sindhi	283
Tharparkar	287
Hariana	290
European Dairy Cattle	289
(Bos Taurus)
Ayrshire	278
Guernsey	284
Jersey	279
Brownswiss	290
Holstein-Friesien	279
Water Buffalo (Bubalus bubalis)
Murrah buffalo	309
Egyptian buffalo	318

Using the information on gestation period, the probable date of parturition can be predicted if the date of service is known. A ready reckoner to find out the probable date of parturition is given in the following table 13.1.

13.8 Parturition

In the cow, ewe and sow the corpus luteum regresses towards the end of preg­nancy with an accompanying reduction in progesterone secretion. At this stage relaxin, hormone secreted by ovary and in some species by the uterus, acts with rising levels of estrogen of late pregnancy to bring about relaxation of the pelvic structure and the cervix. In recent years, prostaglandins have been identified to play a major role in parturition. But, their activity is limited to the immediate surroundings as they are quickly metabolized in the system and are often referred to as 'local hormones'. Some of these are known to have potent electrolytic (regressing corpus luteum) and oxytocic (effect similar to oxytocin) effects. Meanwhile, it has also been observed that the foetus increases the production of adrenal cortico-steroid hormones towards the end of gestation which exert their influence on the myometrium (uterine musculature), most likely through the mediation of prostaglandin F₂ α (PGF₂ α).

Towards the end of gestation period, the foetal brain is stimulated to produce releas­ing factor which will stimulate the foetal pituitary to produce adrenocortico-tropic hormone (ACTH). This in turn will stimulate the foetal adrenal cortex to produce cortical hormones which exert their action on the foetal and maternal placenta. This will cause a lowered production of progesterone and increase in estrogen and influence maternal hypothalamus to release oxytocin which will act on the myometrium to produce labour contractions. The maternal placenta on being acted upon by foetal cortisol, produces Prostaglandin F₂ which acts on corpus luteum resulting in its regres­sion and lowering of progesterone level in the system. Prostaglandin F₂ also stimulates potent uterine contractions. This aided by voluntary effort on the part of the animal with abdominal muscles, finally results in delivering the foetus.

Knowledge of signs of approach­ing parturition is beneficial to the farmer to plan his operations and put the animal in the parturition pen. Maternal behaviour like nesting will manifest itself by the late stages of pregnancy. They will also leave the herd and seek isolation. There will be anorexia, distress and withdrawal from the environment. There will be relaxa­tion of the pelvic ligaments and muscles with rising of tail head and dropping of the quarters. The mammary glands develop fully and there will be some secretion. Milk appears to a considerable degree engorging the udder and teats one or two days before par­turition. In dairy cows, there may be dripping of milk and in the mare, formation of waxy seal by dried mammary secretion. The labour pains will give distress and the animal will be restless and pace about often trying to kick or scratch the flank region.

Fig. 13.1 Hormonal control of parturition