Chapter 2 The Female Reproductive System
The female reproductive system,as illustrates for the cow in Figure 2-1,consists of two ovarians and the female duct system.The duct system includes the oviducts,uterus,cervix,vagina,and vulva.The origin of the ovaries is the secondary sex cords of the genital ridge.The genital ridges are first seen in the embryo as a slight thickening near the kidneys.The duct system originates from the mullerian ducts,a pair ducts which appear during early embryonic development (Chapter 8).An overview of the organs of reproduction for the female and the major functions of these organs is shown in Table 2-1.
The ovaries are considered the primary reproductive organs in the female.They are primary because they produce the female gamete(the ovum) and the female sex hormones (estrogens and progestins).The cow,mare,and ewe are monotocous,normally giving birth to one young each gestation period.Therefore,one ovum is produced each estous cycle.The sow is polytocus,producing 10 to 25 ova each estrus cycle and giving birth to several young each gestation period.
The ovary od the cow is described as almond-shaped,but the shape is altered by growing follicles or copora lutea.The average size is about 35x25x15mm.The size will among cows,and active ovaries are larger than inactive ovaries.Therfore,one ovary is frequently larger than the other in agiven cow.The ovaries of the ewe and doe (goat) are almond-shaped and less than half the size of those of a cow.The ovaries in the mare are kidney-shaped and are two or three times larger than ovarie of a cow.The ovaries in the sow are slightly larger than those found in the ewe and appears as a "cluster of grapes".
Figure 2-1 Reproductive system and associated parts of the urinary
system of the cow as it appears in the natural state (top) and excised (bootom)
The ovary is composed of the medulla and its outer shell,the cortex.The medulla is composed primary of blood vessel,nerves,and connective tissue.The cortex contains those cell and tissue layers associated with ovum and hormone production.The outermost layer of the cortex of the ovary is the surface epithelium.The surface epithelium because it was believed to be the odigin o female germ cells (oogonia).It is now known that germ cells do not arise from this epithelial layer.They aries from embryonic gut tissue and then migrate to the cortex of the embryonic gonad.Just beneath the sureace epithelium is a thin,dense layer of the connective tissue,the tunica albuginea ovarii.Bolow the tunica albuginea ovarii is the parenchyma,known as the functional because it contains ovarian follicles and the cells which produce ovarian hormones.
It is accepted that all primary follicle are formned during the prenatal period of the female.the greastest numbe are found in the fetal gilt 50 to 90 days postconception;the fetal calf 110 to 130 days postconception.A primary follical is agerm cell surrounded by a single layer of follicle (granulosa)cells.They are located in the oarencyma and are frequently seen in groups called egg nests.It is estimated that approximately 75,000 primary follicles are found in the ovaries of a young calf.With continual follicular growth and maturation throughout her reproductive life,an old cew may have only 2500 potential ova.Some potential ova reach full maturity and are released into the duct system for possible fertilization and development of offspring.Most start development and become atretic (i.e.,they degenerate).Therdore,the potential harvest of that could produce offspring is far greater than is actually realized.
Follicles are in a constant state of growth and maturation,A histological section of the cortex of a reprodictiovely active female will reveal these maturation stages(Figure 2-2).The primary follicle has been descrubed.This stage is followed by a proliferation of granulosa cells surroinding the potential ovum.A potential ovum surrounded by two or more layer of granulosa cells is secondary follicle.Later in the development,an antrum (cavity) will form by fluid collecting between the granulosa cells and seperating them.When the antrum has formed,the follicle is classified as a tertiary follicle.The mature tertiary follicle,which appears as a fluid-filled blister on the surface of the ovary,is also called a Graafian follicle. The fluid in the antrum of a tertiary follicle is called liquor follicli.It is a viscous fluid that is rich in steroid reproductive hormones.particulary estrogens Recently a number of other reproductive hormones as well as nonhormonal factors that help regulate the function of the ovary have been identified in follicular fluid (Chapter4).
Figure 2-2 Diagram of structures that can be identified in a cross section of an ovary of a reproductively active female. Different maturation stages for follicles and the corpus luteum can be observed.(Adapted from Patten. 1964. Foundations of Embryology.(2nd ed.) McGraw-Hill.)
Several cell layers in the Graafian follicle have been identified and are of functional importance (Figure 2-3).The outer,more fibrous cell layer is the theca externa.Inside this layer is the theca interna.These two cell layers are supplied with blood by capillary network and can be differentiated microcopicaly with special histological staining techniques.Thecal cella and the capillay network are acquired as the follicle enlarges and pushes into the medulla.A basement membrane separates the theca interna from the innermost cell layer,the granulosa cells,and prevents entry of the vascular syatem into these cells(Figure 2-4 ).The granulosa cells surround the antrum.In addition,the cumulus oophorus,a hillock (mound)of granulosa cells,is located at one side of the antrum.The potential ovum rests upon the cumulus oophorus with other granulosa cells extending around the potential ovum.The granulosa cells surrounding and in immediate contact with the potential ovum are termed the corona radiata.Boththeca interna and granulosa cells are involved in production of estrogen.The theory is that the theca interna cells produce tetosterone,which diffuses through the basement membrane for conversion to estrogen by the granulosa cells. In addition,granulosa cells are the progesterone producing cells in the corpus luteum.They also secrete other compounds that have been identified infollicular fluid help regulate the function of the ovary.when ovulation occurs.the follicle ruptures expelling the liquor folliculi,some granulosa cells,and the potential ovum (oocyte)into the body cavity near the opending to the oviduct.At the time of expulsion,the oocyte is surounded by the corona radiata and a sticky mass containing other granulosa(cumulus) cells which aid the oviduct in picking up the oocyte and moving it down the oviduct.In some species the corona radiata is present at the time of fertilizition.In other species these cells are shed quickly and are not present when fertilization occurs.
Figure 2-3 Functionally important features of a Graafian follicle, (Redrawn from Hafez.1974. Reproduction in Farm Animals.(3rd ed.) Lea and Febiger.)
Figure 2-4 Structure of the wall of the Graafian follicle showing how the granulosa cells are deprived of a blood supply by the basement membrane.(Austin and Short. 1972. Reproduction in Mammals. 3. Hormones of Reproduction. Cambridge University Press.)
With rupture of the follicle,bleeding occurs and a blood clot forms at the ovulation site.The
ruptured follicle with its blood-filled cavity is called a corpus hemorrhagicum.The corpus
hemorrhagicum is replaced by the corpus luteum,which forms rapidly by proliferation of a mixture of
theca externa,theca interna,and granulosa cells.Granulosa cells,which form the main component of the
corpus luteum,enlarge and acquire a large amount of mitochondria and other intracellular structure
involved in synthesis and secretion of progesterone.The corpus luteum is a solid,yellowish body which
only ovarian source of these progestins.In a Holstein heifer between 1 and 4 days of the cycle,the
average diameter of the corpus luteum is 8mm.Between 5 and 9 days,it has grown to an average of
15mm.The average maximum size of 20.5mm is reached in 15 or 16 days in a heifer that is not
pregnant.It then regresses in size with an average diameter of 12.5 mm at 18 to 21 days.When the
corpus luteum regresses,it no longer produces progestins.It loss its yellowish color,eventually
appearing as a small white scar on the surface of the ovary,which is called a corpus albican.If the
animal is pregnant,the corpus luteum will not regress until late pregnancy for most species,but
species differences do exist(Section 8-5).
The oviducts (also called fallopian tubes) are a pair of convoluted tubes extending from near the ovaries to and becoming continuous with the tips of the uterine horns.Their functions include of ova and spermatozoa,which must be conveyed in opposite directions.In addition they are the site of fertilization and the early cell divisions of the embryo.Histologically,they contain three distinct cell layers.The outer layer,basically connective tissue,is the tunica serosa.The middle layer,composed of both circular and longitudinal smooth muscle fibers,is the tunica muscularies.The innermost layer,Which contains both ciliated and secretory epithelial cells,is the tunica mucosa.The same basic histological arrangement is found in the remainder of the female duct system with some differences in the inner two layers,which will be noted which be noted when we discuss specific organs.
An cviduct,which is from 20 to 30 cm long for most farm species,is divided into three segments(Figure
2-5).The funnel-shaped opening near the ovaries is the infundibulum.In some species(cat,rabbit,
mink,and others)the infundibulum forms a bursa around the ovary.In the cow,doe,ewe,sow,and mare the
infundibulum is separate from the ovary.There are numerous folds in the mucosa,and mucosal cells in
the infundibulum are ciliates.The ampuula,the middle segment,is from 3 to 5 mm in diameter and
accounts for about hlaf of the total length of the oviduct.The mocosal lining of the ampulla has from
20 to 40 longituidinal folds,which greatly increase the surface area of the lumen.The majority of the
cells in the mucosa of the isthmus the third segment,at the ampullary-isthmic junction.This junction
is difficult to locate anatomically and been descrubed as a physiological stricture which delay the
ovum several is similar than the ampulla,being 0.5 to 1 mm in diameter.It is further distinguished by
having a thicker smooth muscle layer than the ampulla and from 4 to 8 mucosal folds.A higher ratio of
secretory to ciliated cells is characteristic or the isthmus.The isthmus joins the tip of the uterine
horn at the uterotubal junction.In general,oviductal activity is stimulated by estrogens and
inhibited by progestins.
Figure 2-5 Anatomy of the oviduct:top, longitudinal view illustrating the macroscopic features of the oviduct; bottom, cross section of the ampulla and isthmus comparing the thickness of the musculature of the wall and the complexity of the mucosal folds.
The uterus extends from the uterotubal junctions to the cervix.For the cow,sow,and mare the overalll length may range from 35 to 60 cm.For the sow,doe,ewe,and cow the uterine horns account for 80 to 90% of the total length,wjile in the mare the uterine account for about 50% of the toal length.The uteri of the ewe and doe are less than half the size of the other mentioned soecies.The major function of the uterus is to retain and nourish the embryo,or fetus.Before the embryo becomes atatched to the uterus,the nourishment comes from yolk within the embtyo or from uterine milk which is secreted by glands in the mucosal layer of the uterus.After attachment to the uterus,nutrients and waste products are conveyed between materal and embryonic,or fetal,blood by way of the placenta(Chapter 8).
Four basic types of uteri are found in animals (Figure 2-6).Only two of these types are found in farm animals.The bicornuate uterus is found in the sow,cow,doe,and ewe.It is characterized by a small uterine body just anterior to the cervical canal and two long uterine horns.Fusion of the uterine horns of the cow and ewe near the uterine body gives the impression of a larger uterine body than actually exists and has exists and has sometimes resulted in their uteri being classified bipartite.The sow has longer uterine horns than the cow,and they may slighyly convoluted.The mare has a biparitite uterus.There is a prominent uterine body anterior to the cervical cannal and two uterine horns thet are not as long and distinct as in the bicornuate type.During pregnancy in mares,the fetal body extends into both horns,whereas the fetuses do not occupy the uterine body in the bicornate types.The duplex uterus,which consists of thwo uterine horns each with a separate cervical cannal which opens into the vagina,is found in the rat,rabbit,guinea pig,and other small animals.The simople uterus,a pear-shaped body with no uterine horns,is characteristic of humans and other primates.
Figure 2-6 Basic types of uteri found in mammals.
As in the oviduct,the tunica serosa is the outer layer of the uterus.The myometrium,the middle layer,is composed of two thin longitudinal layers of smooth muscle,with thicker cicular layer sandwiched between.estrogens increase the tone of the myometrium, causing the uterus to feelmore flaccid.The endometrium, the mucosal lining of the uterus,is more complex than the rest of the duct system and has simple glands.estrogens increase the vascularity and cause a thickening of the endometruim.in addition,estrogens stimulate growth of the endometrial glands.Progestins cause the emdometrial glands to coil and branch,and tosecrete uterine milk.The synergistic actions of estrogens and progestins on the endometrium are for preparation of the uterus of preganancy.
The endometrium provides a mechanism for attachment of the extraembryonic memberanes.This union forms the placenta,and the process is called placetation.With union forms the placeta,nutrients trom maternal blood can be transferred to embryonic or fetal blood and waste products from embryonic and fetal blood can be eliminated through the maternal systems.The nature of the placental attachment differs among soecied(figures 2-7 and 2-8).cows,does,and ewes have cotyledonary placental attachments.Chorionic villi from the extraembryonic memberanes penetrate into caruncles,which are button-like projections on the endometirum.This union,chrionic villi and caruncle,forms the placentome (also called the cotyledon).There are 70 to 120 such cotyledonary attachments in a pregnant cow in late pregnancy.There are 88 to 96 in ewes and does,and these are smaller than in cows.The sow and ,are have a diffuse (surface) placental ttachment.The extraembryonic membranes lie in folds on the endometrium,with chorionic villi ectending into the endometrium in more fragile attachment than is found in the cow,doe,or ewe.
Figure 2-7 Distribution of chorionic villi which serve as a basis of placental shape in several species.(Redrawn from Arey. 1974. Developmental Anatomy.(7th ed.) W.B. Saunders Co.)
Figure 2-8 Diffuse attachment found in the mare and cotyledonary attachment found in the cow.
The placental attachment of the mare,sow,and cow is claasified as epitheliochorial.This means thartno erosin occurs either in the tissue of the extraembryonic tissue layer of the endometrium.formation of the placenta in the human results in rather extensive erosion of the endometrium.classified as hemochorial, nutients from maternal blood must pass through only extraembryonic tissues.Erosion is not extensive enough to result in direct mixing of materal and fetal blood in any mammalian species(Figures 2-9).
Figure 2-9 Placental types showing the cellular barriers between maternal and fetal blood for several species.(Adapted from Arey. 1974. Developmental Anatomy.(7th. ed.) W.B. Saunders Co.)
While technically a part of the uterus,the cervix will be discussed as a distinct organ.It is thick-walled and inelastic,the anterior end being continuous with the body of the uterus while the posterior end protrudes into the vagina.For most farm species the length will range fron 5 to 10 cm with an outside diameter of 2 to 5 cm.It contains a canal which is the opening into the uterus.The primary function of the cervix is to prevent microbial contamination of the uterus;however,it also may serve as a sperm reservoir after mating.Semen is deposited into the cervix during natural mating in sows and mares.
The cervical cannals in the cow,doe,and ewe have transverse interlocking ridges known as annular rings(Figure 2-10) which help seal the uterus from contaminants.The cervical cannal in the sow is funnel-shaped,with ridges in the cannal having a corkscrew configuration which conforms to that of the glands penis in the boar (Chapter 3).The cervical cannal of the mare is more open than in other farm species,but mucosal folds in the canal which project into the vagina help prevent contamination.
Histologically,the outer layer of the cevix is the tunica serosa.The middle layer is connective tissue interspersed with smooth muscle fibers,whichgives the cervix its firm and inelastic properties.The inner layer,the mucosa,is composed mainly of secrectory epithelial cells,but some ciliated epithelial cells are present.High levels of estrogens cause the cervical canal to siliate during estrus (standing heat).Synergism between high levels of estrogens and relaxin causes greater dilation just before parturition.This dilation of the cannal would appear to make the uterus more vulnerable to invading organisms.However,estrogens cause the epithelial cells of the cervix to secrete mucus that antibacterial properties,thus protecting the uterus.
If cervical mucus is obtained at estrus and allowed to dry on a glass slide, it will form a fern-like
pattern.This ferning pattern does not form from cervical mucus obtained at stages of the cycle when
estrogens are low.during pregnancy the mucus thickens into a gel-like plug, which seals and protects
the uterus during the pregnancy.Removal of the mucus plug will increase the chance of abortion.
The vagina is tubular in shape,thin-walled and quite elastic.It is from 25 to 30 cm in length in the cow and mare,and 10 to 15 cm in le ngth in the sow,doe,and ewe.In the cow,doe,and ewe,semen is deposited into the anterior end of the vagina,near the opening to the cervix,during natural mating.The vagina is the female organ of copulation.
The outer layer, the tunica serosa,is followed by a smooth muscle layer containing both circular and
longitudinal fibers.In most species the mucosal layer is composed of startified squamous epithelial
cells(the cow a possible exception).These epithelial cells cornify (become cells wothout nuclei)
under the influence of estrugens.Vaginal smears can be used as an ai in detecting estrus,but are most
useful in laboratory animals. This layer of cornified cells at the time of estrus may serve as a
lubricating or protective mechaniasm which prevents abrasions during copulation. Under the influence
of progestins,the epithelial lining regenerates.
The vulva,or external genitalia,consist of the vestibule with related parts and the labia.The vestibule is that portion of the female duct system that is common to both the reproductive and urinary systems.It is from 10 to 12 cm inlength in cow and mare,half that length in the sow and one-quarter that length in the ewe and doe.The vestibule joins the vagina at the external urethral orufuce.A hymen (ridge) at that point is well defined in the ewe and mar, but less prominent in the cow and sow.A suburethral diverticulum (blindpocket) is located just posterior to the external urethral orific.The labia consist of the labia minora,inner folds or lips of the vulva,and labia majora,outer folds or lips of the vulva. The labia minora is homologous to the prepuce (sheath)in the male and is not prominent in farm animals.The labia majora,homologous to the scrotum in the male,is that portion of the female system which is visible externally.In the cow the labia majora is covered with fine hair up to the mucosa.The clitoris,homologous to the glans penis in the male,is located ventrally and about one cm inside the labia.It cintains eectile dissue and is well supplied with sensory nerves.It is erect during estrus.While not prominent enough to be used in estrus os detection in most species,the clitoris of the mare is an exception.In the mare during estrus,frequent contractions of the labia 9winking0expose the erected clitoris.vestibular glands,located in the posterior part of the vestibule,are active during estrus and secrete a lubricating mucus.The activity of thses glands accounts for moist appearance of the vulva of the cow during estrus.
2-7 Support Structures.Nerves,and Blood Supply
Even though the femlae reproductive tract may be partially resting on the floor of the pelvis,the broad ligament is considered the principal supporting structure.This ligament suspends the ovaries, oviducts,and uterus from either side of the dorsal wall of the pelvis.Blood vessels and nerves pass through the broad ligament to the female reproductive system.The female reproductive system is supplied primarily with autonomic nerves.However,spensory nerves are found the region of the vulva,especially the clitoral region.
The ovarian areries,also called utero-ovarian arteies,branch and supply blood to the ovaries,oviaucts,and a portion of the uterine horns.These arteries are larger on the side of the ovary containg an active corpus luteum in cows and species where one active corpus luteum is the rule.The middle uterine artery supplies blood to rest the uterine horns and the body of the uterus.It enlarges during middle and late pregnancy and can be palated as an aid in pregnancy diagnosis in cows and mares (Chapter 20).The hypogastric artery branches to supply the cervix,vagina,and vulva.
Interest in the circulatory patterns of the reproductive tract has increased in recent years since the discovery that the uterus.by release of prostaglandin F2メ (PGF2メ),controls the life of the corpus luteum.Prostaglandin F2メ is luteolytic(causes the regression of the corpus luteum)but is reality oxidized,and about 90% is destroyed during one passage through the pulmonary circulation.it did not seem likely that PGF2メ,if released into the systemic circulation(uterus≧veins≧heart and lungs≧arteries≧ovaries),could be responsible for luteolysis(regression of the corpus luteum).Now there is evidence of a counter current circulation pattern,whereby PGF2メ diffuses from the utero-ovarian vein into the ovarian artery,thus eaching the ovary by a local rather than a systemic route.A common utero-ovarian vein drains blood from the ovary,oviduct,and a lager portion of the uterine horn.In the sow,ewe,and cow the ovarian artery is in close apposition to the utero-ovarian vein9Figure 2-11).In the ewe and cow it is very tortuous,increasing the area of contact with utero-ovarian vein.The arterial walls are thinnest where they are in contact with this vein.Therefore,it seems likely that sufficient PGF2メ can difuse from the utero-ovarian vein into the ovarian artery to cause luteolysis in the sow,ewe,and cow.During synchronization of estrus,the effective does of PGF2メ is much smaller when infused into the uterus as composed to a systemic injection(5 verus 25 mg).
The ovarian artey of the mare is straight and caudal to the utero-ovarian vein.While a local route
for PGF2メ from the uterus to the ovary seems less likely in mares than in other species,uterine
prostaglandins have been identified as the natural luteolysin in mares.