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Chapter 5: Functional anatomy of reproductive systems: genital organs.

CHAPTER OBJECTIVES

* Describe the general anatomic organization of male and female reproductive systems.

* Describe the gross and microscopic anatomy of the male genital organs.

* Describe the gross and microscopic anatomy of the female genital organs.

* Relate structures of the genital organs to their functions.

* Compare and contrast structures of male and female genital organs.

OVERVIEW: REPRODUCTIVE TRACTS

The reproductive system consists of several organs that communicate with one another via neuronal and humoral pathways to regulate reproductive activity. Before we study how these organs interact, it is necessary to learn the structures of these tissues. For convenience, we will divide our analysis into two major components: the genital organs and neuroendocrine systems. This chapter will focus on the anatomy of the genital organs. The following chapter will focus on the anatomy of the nervous system and pituitary gland.

The genital organs include the paired gonads, genital duct system, and external genitalia. Collectively these organs are referred to as the reproductive tract. A tract is a longitudinal arrangement of organs that have related functions. Our discussion begins with general descriptions of the male and female reproductive tracts, followed by more detailed descriptions of the gross and microscopic anatomy of each reproductive organ.

The Male Reproductive Tract

It is convenient to organize the male reproductive organs into three major groups: the testes and their adnexa (attached structures), the pelvic reproductive organs, and the external genitalia. The adnexa of the testes include the excurrent ducts (efferent ducts, epididymis, and ductus deferens), and scrotum. The pelvic reproductive organs of the male are the urethra and accessory sex glands. The penis and prepuce make up the external genitalia. The gross anatomy of the male reproductive organs is fairly consistent among four-legged mammals (Figure 5-1). Our discussion of the gross anatomy of the male reproductive system will focus on the dog (Figure 5-2), a species with which most students are familiar. Remembering this anatomy is facilitated by relating structure to function. The major functions of the male genital organs include the production, maturation, packaging, and transportation of spermatozoa. Spermatozoa are produced by the testes, which are suspended between the hind legs in a sac of skin called the scrotum (Figure 5-3a). Closely associated with the testis is the system of excurrent ducts (Figure 5-4). The efferent ducts (not shown) provide a means for spermatozoa to leave the testes. These ducts drain into a larger duct called the epididymis. The epididymis concentrates and nourishes the sperm cells and allows them to mature. This duct also serves as a storage depot. During ejaculation, spermatozoa and epididymal fluid enter the ductus deferens (vas deferens), which arise from the epididymal ducts and empty into the pelvic urethra. At its point of entry into the urethra, the deferent duct widens to form the ampulla. The pelvic urethra is continuous with the external or spongy urethra that runs through the penis emptying into an external orifice located at the apex of the penis. The accessory sex glands (prostate gland, vesicular glands, and bulbourethral glands) are aligned along the urethra distal to the point where the deferent ducts empty into the urethra. These exocrine glands produce fluids that are released to the urethra during ejaculation. Accessory gland fluids, together with the spermatozoa and epididymal fluids make up the semen (seminal plasma).

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The external genitalia of the male includes the penis and scrotum. As noted previously the scrotum encases the testes, which are suspended from the abdominal cavity via the spermatic cord, which is connective tissue that supports the ductus deferens, lymphatics, nerves, and blood vessels that carry blood to and from the testes. The spermatic cords extend through the abdominal muscles on each side of the body via the inguinal rings (Figure 5-3b). The penis is protected by a layer of skin called the prepuce. In primates this is called the foreskin. In most other mammals, the penis retracts into the abdominal cavity during the non-erect state. In these cases, the prepuce is the fold of skin surrounding the opening through which the penis protrudes. Four skeletal muscles are associated with the male reproductive organs. The locations of these muscles are depicted in Figures 5-1, 5-2 and 5-3. The ischiocavernosus (see Figure 5-2) and bulbospongiosus muscles lie lateral and dorsal to the base of the penis, respectively. These muscles help empty the urethra after urination and play roles in erection. The cremaster muscles are fascia-like muscles that are enmeshed in the spermatic cords, and when contracted they lift the testes within the scrotum. The paired retractor penis muscles arise from the caudal vertebrae and extend to and connect with each side of the penis. In ruminants, the retractor penis muscles attach at the second bend of the so-called sigmoid flexure, an S-shaped curve in the body of the retracted penis. These muscles are less developed in mammals such as the dog and stallion. In this latter case, they originate on the rectum and pass along the ventral surface of the penis, eventually becoming enmeshed with the bulbospongiosus muscle, and ending at the tip (glans) of the penis.

The Female Reproductive Tract

The genital organs of the female include the ovary, uterus (including the cervix), uterine tubes (oviducts), vagina, and external genitalia. Like the male reproductive tract, a major function of this system is the production and transport of gametes. However, unlike the male, this system is involved with transporting both male and female gametes, as well as the developing embryo. Oocytes produced by the ovaries enter the oviducts and move toward the uterus. In addition, spermatozoa deposited into the vagina by the male during copulation are transported to the oviducts, the site of fertilization. The embryo eventually enters the uterus and attaches to the uterine wall, where the placenta develops. This highlights another difference in function between male and female reproductive tracts; the female tract is responsible for maintaining pregnancy and plays an important role in the birthing process.

Although the positions and shapes of the reproductive organs vary among species, the general organization of these tissues is similar among mammals (Figures 5-5, 5-6, 5-7, 5-8, and 5-9). In all cases, the female reproductive tract lies ventral to the rectum. In large species such as cattle and horses, this arrangement permits per rectum examination of the female reproductive tract. Unlike the testes of the male, the female gonads are located internally. Each ovary lies in close proximity to one of the oviducts, which consist of an infundibulum, which is closely apposed to the ovary; the ampulla, the middle section characterized by a narrowing diameter; and the isthmus, the narrow section that connects with the uterus. The shape of the uterus varies among species. In all cases the caudal portion consists of a cervix, a thick-walled segment with a narrow lumen that acts as a sphincter regulating the flow of material to and from the uterus. The cranial portion of the uterus is often referred to as the womb, and is the region in which embryos implant and pregnancy is established and maintained. The shape of cranial uterus varies considerably among mammals, ranging from a structure that consists of two separate uterine horns and no uterine body to one that consists of one large uterine body with no horns. The true vagina extends from the caudal limit of the cervix to the entrance of the urethra (urethral orifice). The vagina is strictly a reproductive passage serving as the female copulatory organ as well as part of the birth canal. The external genitalia or vulva (wrapping or covering) lies caudal to the vagina. This region consists of the labia (lip-shaped folds of skin covering the entry to the female reproductive tract), clitoris, and vestibule. The vestibule extends from the urethral orifice to the inner labia. This portion of the tract serves both urinary and reproductive functions. The clitoris lies in the ventral portion of the external genitalia at the point where the folds of the labia join. In primates the labia is divided into an inner labia minora and an outer labia majora.

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The female reproductive tract is suspended from the body wall by the broad ligament, a peritoneal fold that attaches the ovaries, oviducts, and uterus to the walls of the pelvis (Figure 5-10).

This concludes the general descriptions of the male and female reproductive tracts. The next sections include detailed accounts of the gross and microscopic anatomy of these organs.

For convenience, the reproductive tracts will be divided into the following major subdivisions, each of which is the focus of a separate discussion:

* gonads,

* genital ducts,

* suspension and vasculature,

* and external genitalia.

GONADS

The ovary and testis have homologous anatomic and physiologic features. With respect to function, both of these gonads have gametogenic and endocrine functions. Moreover, the organization of tissues within each of these organs is similar. Each is encapsulated by a white, fibrous tissue known as the tunica albuginea. The parenchyma of both organs can be divided into two major compartments; one concerned with the development of gametes and another consisting of richly vascularized endocrine cells. A major anatomic difference between the ovary and testis lies in the structure of these compartments. In the testis, spermatozoa are produced within a network of small tubules (seminiferous tubules) and exit the testes via an excurrent duct system. The ovary does not contain a duct system. Oogenesis occurs within pouchlike depressions (follicles). The exiting of an oocyte from the ovary involves rupture of the follicular wall. Another major anatomic difference between the testis and ovary is that the testis is located outside of the abdominal wall and is intimately associated with a portion of the external genitalia; that is, the scrotum. In contrast the ovary remains in the abdomino-pelvic cavity and is not located near the external genitalia.

Testis and Scrotum

The testes are paired, ellipsoidal organs located in the scrotum. The position of the testes varies among species, ranging between a pendulous scrotum located in the caudal portion of the abdomen (e.g., ruminants) to more restrained scrotum nestled beneath the anus (e.g., pigs and cats). The orientation of the testes tends to vary with location. In ruminants, the long axis is positioned vertically, whereas the long axis lies more in the horizontal plane in the dog, cat, pig, and horse.

Gross Anatomy of the Testis

The testis is intimately associated with the epididymis (Figures 5-11 and 5-12), which provides reference points for identifying opposing ends of the testis. The end associated with the point of origin (or head) of the epididymis is known as the extremitas capitata, whereas the part of the testis associated with the tail of the epididymis is referred to as the extremitas caudate.

The testis and epididymis are suspended in the scrotum by the spermatic cord (Figures 5-12 and 5-13). The foundation for this structure is a double layer of peritoneum known as the tunica vaginalis (see Figures 5-7 and 5-8). As noted earlier, this connective tissue supports the ductus deferens, as well as lymphatics, nerves, and blood vessels. The tunica vaginalis also covers the epididymis and testis. This sheath consists of inner (visceral) and outer (parietal) layers separated by a thin microscopic (vaginal) space (Figure 5-14).

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Beneath the inner layer of the vaginal tunic lies the tunica albuginea, a thick capsule that supports branches of the testicular arteries and veins (Figure 5-14). This testicular capsule is not an elastic tissue, and therefore exerts pressure on the underlying parenchymal tissue. This becomes evident when one cuts into this tissue. A small cut through the tunica albuginea causes the underlying tissue to evert through the opening. Projections of the tunica albuginea extend to the interior of the testis. These septa divide the organ into numerous lobules and provide pathways for blood vessels and nerves to enter and leave the lobules. The septa meet at the center of the testis to form the mediastinum, a mass of connective tissue that houses fine tubules. Each lobule contains a small number of seminiferous ("semen-carrying") tubules, which are tortuous (tubulus contortus) in the peripheral portion of the lobule. The tubules unite and straighten (tubulus rectus) near the mediastinum, and then drain into rete tubules, which are embedded in the mediastinum. The rete tubules flow to the extremitas capitata where they penetrate the outer tunica albuginea and connect with the efferent ducts. These small-diameter ducts drain directly into the head of the epididymis. Collectively the system of ducts that transport sperm cells and seminal fluid out of the testes is referred to as the excurrent duct system.

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Microscopic Anatomy of the Testis

As noted in the previous section, the lobules of the testis are filled with tightly compacted bundles of seminiferous tubules (Figure 5-15a). The tubules are highly convoluted and densely packed within the lobules of the testis. A thin basement membrane forms the wall of the tubule and divides the testicular tissue into tubular and interstitial compartments. The large interstitial cells are called Leydig cells and produce testosterone. Several different types of cells reside in the tubular compartment. Large Sertoli cells extend from the basement membrane to the lumen of the tubules. These cells surround numerous layers of smaller cells, which are the developing germ cells; that is, cells that develop into sperm cells.

Closer examination of a section of seminiferous tubule reveals a more elaborate structure (Figure 5-15b). The walls of these microscopic ducts consists of connective tissue; that is, a thin basement membrane supported by reticular fibers. A thick layer of cells lies along the inner surface of the basement membrane. These cells include spermatogenic germ cells, which are embedded in much larger Sertoli cells. The Sertoli cells are tightly aligned around the tubule, forming a barrier between the outside and inside of the tubule, and creating a narrow lumen. The spermatogenic cells exist in stratified layers engulfed by the Sertoli cells. Each layer contains cells in a particular stage of stage spermatogenesis. Spermatogonia occupy the outermost layers, whereas spermatids occupy the innermost layers. Spermatocytes reside in intermediate layers. The main function of Sertoli cells is to provide support for the spermatogenic cells. The connective tissue between the seminiferous tubules contains blood vessels, lymphatic vessels, and Leydig cells. The Leydig cells produce androgens that either enter adjacent capillaries or diffuse across the basement membrane into the seminiferous tubules.

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Scrotum

As noted earlier, the scrotum is intimately associated with the testes. In addition to housing the male gonads, it plays a role in regulating testicular function. The scrotum consists of several tissue layers. An outer layer of thin skin contains both sweat and sebaceous glands and, depending on the species, may be bare or covered with hair of varying densities. The inner layer of the scrotal skin is adhered to the tunica dartos, a fibrous layer of smooth muscle. The tunica dartos envelops each testis and extends between them to form a septum that isolates the testes in separate compartments. Within each testicular compartment, the inner surface of the tunica dartos borders on the tunica vaginalis which, as noted earlier, consists of the outer parietal layer and the inner visceral layer.

The two major functions of the scrotum are protecting and cooling the testes. The temperature in the scrotum is a few degrees cooler than that of the body cavity. This lower temperature appears to be required for adequate production of spermatozoa. In most mammals, production of sperm is markedly reduced at normal body temperatures. The scrotum provides two mechanisms for cooling the testes. First, the presence of sweat glands in the scrotal skin facilitates evaporative cooling. Second, the tunica dartos muscle relaxes during high ambient temperatures, moving the testes away from the warm body cavity. The most important mechanism regulating temperature of the testes involves the vasculature of the spermatic cord.

Ovary

The ovaries are paired organs with an ellipsoid shape and nodular surface (Figures 5-16, 5-17, and 5-18). Unlike the testes, they are located in the sub-lumbar region of the abdominal cavity caudal to the kidneys and their morphology changes depending on the reproductive state of the individual. Each ovary is suspended by the cranial part of the broad ligament. Blood vessels, lymphatics, and nerves run along this tissue and enter the ovary at the hilus.

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Cutting a longitudinal section through the ovary reveals the organization of its tissue layers (Figure 5-19). Like the testis, the ovary is encapsulated by the tunica albuginea. However, directly beneath this capsule is the germinal epithelium, a single layer of cuboidal cells that is absent from the testis. The underlying tissue of the ovary can be divided into two clearly distinguishable layers. The outer parenchymatous zone, commonly referred to as the cortex, is a dense layer of tissue. The central medulla is a more loosely organized and vascularized region that is continuous with the hilus.

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The ovarian cortex contains important structures that play important roles in the reproductive physiology of females. Ovarian follicles are one type of structure found in this region. At the microscopic level, five types of follicles can be distinguished (Figures 5-20 and 5-21): primordial, primary, secondary, tertiary, and atretic. Only the tertiary follicles can be seen with the naked eye; the others are microscopic structures. Primordial follicles are the least developed type. They consist of an oocyte surrounded by a single layer of squamous cells. A primordial follicle can develop into a primary follicle, which can be distinguished by a surrounding layer of cuboidal (follicular) cells. If a primary follicle continues to develop, it forms a secondary follicle; that is, an oocyte surrounded by two or more layers of follicular cells. The oocyte of a secondary follicle develops an outer, translucent envelope called the zona pellucida. Very few follicles develop into tertiary follicles. This type of follicle is also called a "vesicular follicle" due to the presence of a fluid-filled cavity (antrum). These follicles appear as fluid-filled blisters and contribute to the nodular appearance of the ovarian cortex (Figure 5-16). Tertiary follicles are surrounded by a capsule of stromal tissue called the theca. This capsule consists of two concentric layers of cells: an outer theca externa and an inner theca interna. The inner layer is supplied with capillaries and contains endocrine cells. The theca interna cells rest upon a thin basement membrane. Beneath this membrane is the granulosa layer; that is, several layers of follicle cells, also known as granulosa cells. The rapidly expanding population of granulosa cells produces follicular fluid, which accumulates and pushes the granulosa cell to the outer limits of the follicular cavity. The oocyte remains centrally located and is supported by a mound of granulosa cells called the cumulus oophorus. The granulosa cells that surround the oocyte are typically referred to as the corona radiata. These cells penetrate the space between the oocyte and zona pellucida forming intimate contacts with the germ cells. Like the Sertoli cells of the testis, these cells are thought to provide support to the gametogenic cell.

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Tertiary follicles have one of two fates. Most of these follicles undergo degeneration and are resorbed by the ovaries. This process is known as atresia (Figure 5-21). Atretic follicles are essentially dying follicles and can be identified microscopically by examining the appearance of follicular cells. A select few tertiary follicles go on to ovulate; that is, rupture and release an oocyte.

The other structures located in the ovarian cortex are associated with ovulation (Figures 5-17 and 5-18). Ovulation of a tertiary follicle involves rupture of the follicle wall and release of follicular contents. This leads to the collapse of follicular tissue and hemorrhaging of small blood vessels located within the follicular capsule. The remaining tissue resembles a small blood clot and is called a corpus hemorrhagicum ("bloody body"). The remaining granulosa and theca interna cells undergo transformations and re-organize to form a corpus luteum, or "yellow body." In most species, this structure is much larger than the tertiary follicle, and protrudes markedly from the surface of the ovary. This structure is highly vascularized and therefore has a red color on its surface. However, the core of the corpus luteum is yellow. As the corpus luteum degenerates, it forms fibrous scar tissue called the corpus albicans, or "white body."

It is important to point out that the combination of ovarian structures varies considerably with the physiologic state of the female. At any given time, the ovary contains follicles and/or corpora lutera at different stages of growth and degeneration. It is rare to see all of the aforementioned structures on a single ovary at one time. The pattern of growth of a particular follicle is best described as a follicular wave; that is, a period of enlargement followed by a rapid decrease in size due to atresia or ovulation.

GENITAL DUCTS

The basic characteristics of the genital ducts are similar to other tubular organs; that is, the parenchyma consists of concentric layers of tissue: tunica serosa (or tunica adventitia), tunica muscularis, tela submucosa, and mucosa. It is common to refer to these tissue layers simply as the serosa, muscularis, and mucosa. The degree to which these tissue layers are developed varies considerably within the reproductive tracts of males and females.

Genital Ducts of the Male

A system of ducts provides the means for transport of spermatozoa and seminal plasma within the male reproductive system. This duct system consists of the efferent ducts, epididymis, and ductus deferens. The urethra is also involved, but this duct is also part of the urinary system and is therefore considered separately.

Efferent Ducts

The efferent ducts connect the rete testis with the epididymis. There are between six and 20 of these ducts within each testis. The mucosal epithelium consists of columnar cells, some of which are ciliated. These cells exhibit both secretory and absorptive abilities. Beneath these cells is the lamina propria mucosae, which blends in with surrounding connective tissue. Isolated smooth muscle cells are also present.

Ductus Epididymidis

The ductus epididymidis (epididymis) consists of three major regions: head (caput), body (corpus), and tail (cauda). The head and tail of the epididymis are firmly attached to the testis. The attachment to the body may be loose, creating a space between the duct and the testis. The head is attached to the testicular capsule, whereas the tail is fixed to the proper ligament of the testis as well as to the parietal layer of peritoneum that covers it.

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The histology of the epididymis varies from head to tail (Figure 5-22). Throughout the organ, tissue layers include a mucosa consisting of pseudost-ratified columnar cells, a thin and vascularized lamina propria mucosa, a lamina muscularis mucosa comprised of circular smooth muscle cells, and a submucosa which is continuous with the tunica albuginea. Distinguishing features of the head include a small lumen, very tall mucosal cells with non-motile cilia (stereo cilia), and a thin layer of smooth muscle cells. In contrast, the tail is characterized by a large lumen, more compact epithelial cells with motile cilia, and well-developed layers of smooth muscle cells. The body of the epididymis has characteristics that intermediate between those of the head and tail. The anatomic differences among the various regions of this duct reflect their different functions. The head of the epididymis is involved primarily with absorption of fluid from the testes, whereas the tail of the epididymis serves as a storage depot and is important in the transport of sperm during ejaculation. Spermatozoa emerge from the rete testis suspended in rete fluid. As they make their way through the efferent ducts and head of the epididymis, the fluid is absorbed by the mucosal epithelium, causing the concentration of spermatozoa to increase. Movement of spermatozoa through the head and body of the epididymis is facilitated by rhythmic contractions of the lamina muscularis mucosae. In contrast, the tail remains quiescent unless the male becomes sexually excited. The amount of time required for spermatozoa to move from the proximal head to the distal tail varies among species (4-19 days). Removal of spermatozoa from the tail of the epididymis occurs in two ways. The most familiar (and noticeable) involves ejaculation. During sexual stimulation, smooth muscle cells in the tail contract and push the epididymal contents into the ductus deferens. A second mechanism involves periodic contractions of the epididymal tail and ductus deferens, causing more sustained release.

In addition to transporting and storing spermatozoa, the epididymis plays an important role in the maturation of spermatozoa. When spermatozoa emerge from the testis and enter the head of the epididymis, they are not completely developed; they lack motility and cannot bind to an oocyte. As the cell moves through the body of the epididymis, it takes on a mature form, becomes motile, and gains the ability to bind to the female gamete.

Ductus Deferens

The ductus deferens is coiled where it emerges from the epididymis. It then straightens and runs medial to the epididymis before entering the spermatic cord near the head of the epididymis. After the duct deviates from the epididymis, it runs within the spermatic cord through the inguinal canal and finally joins the urethra distal to the bladder. Throughout most of its length, the ductus deferens has a uniform diameter with a narrow lumen and thick muscular wall (Figure 5-23). In most mammalian species, the terminal end of the duct is enlarged and spindle-shaped. This is due to the presence of mucosal glands, not widening of the lumen. This region is called the ampulla or ampullary gland.

The lamina epithelialis mucosa of the deferent duct is a pseudost-ratified columnar epithelium. The most striking feature of this duct is the thin submucosal layer and thick, well-organized tunica muscularis varying from two distinct inner and outer layers to intermingled fibers. Unlike the epididymis and efferent ducts, the ductus deferens has a well-defined tunica serosa.

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Urethra and Accessory Sex Glands

In the male, the urethra originates in the neck of the bladder and terminates in an orifice at the end of the penis. Major subdivisions include the internal (pelvic) and external (spongy) urethras. In this section we will consider only the internal urethra and its relationship with the accessory sex glands. It is more convenient to discuss the spongy urethra in relation to the anatomy of the penis.

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The pelvic urethra is a tube lined with transitional epithelium that overlays a lamina propria-submucosa that contains glands and some erectile tissue. Erectile tissue in the pelvic urethra is inconspicuous; that is, a minute system of interconnected vascular spaces. This is continuous with the more extensive erectile tissue of the spongy urethra. The structure of the tunica muscularis varies along the length of the duct, ranging between three layers near the bladder and a striated urethral muscle in more distal sections, where it becomes continuous with the bulbospongiosus muscle. The outer covering of the urethra is the tunica adventitia.

The vesicular glands exist in pairs. Each one is associated with the distal portion of its ipsilateral deferent duct. In the horse and human, the glands are smooth and have an appearance similar to the bladder. In contrast, these glands are knobby and have thick walls in most other species. Each seminal vesicle is divided into lobules, each containing numerous pockets that drain into a central duct. The lumens of the pockets are lined with a glandular epithelium arranged in a simple columnar fashion. The central duct is lined with pseudostratified epithelium. In each case a lamina propria-submucosa is present along with a muscularis and serosa.

The prostate can exist in several forms that reflect the relative development of the two main parts of the gland: corpus prostate and disseminate prostate. In small ruminants such as sheep, the gland is disseminate; that is, glandular tissue is distributed along the dorsal and lateral surfaces of the pelvic urethra. In carnivores and horses the compact corpus is the dominant portion. The boar has a well-developed disseminate prostate, but the corpus is clearly present. Both portions of the prostate are present in the bull. In all cases, the corpus prostate is located peripheral to the urethralis muscle of the pelvic urethra. Both parts of the prostate consist of many small ducts that drain into the urethra.

A pair of bulbourethral glands lies dorsolateral to the urethra near the base of the penis. These glands are of a compound, tubuloalveolar type, meaning that they consist of small sacs (alveoli) each of which is drained by a small duct. The entire gland drains into the urethra via one or more main ducts. Both structures are lined by epithelial cells of varying arrangements.

Genital Ducts of the Female

The genital ducts of the female include the vagina, uterus, and oviducts. As noted earlier, the uterus consists of two main parts: a caudal section that serves as a valve (cervix) and a cranial portion that is the site of pregnancy. In humans, the oviducts are frequently referred to as the fallopian tubes. It is important to emphasize that the vagina is distinct from the vestibule, which is part of the vulva, or external genitalia.

Uterus

The uterus plays a central role in mammalian reproduction by serving several essential functions. First, it is the site of semen deposition in some mammals (horses, swine). Second, it is important in transporting spermatozoa following insemination. Third, the uterus (and oviducts) capacitate spermatozoa; that is, induce changes that allow spermatozoa to accomplish fertilization of oocytes. Fourth, the uterus is the site of pregnancy; that is, development of the embryo and fetus. Fourth, uterine contractions are essential for the birthing process.

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The gross structure of the uterus varies among species (Figure 5-25). Most domestic animals have a bicornuate uterus, consisting of two horns, a body, and a single cervix. However, the relative sizes of the horns and body can vary considerably. Some anatomists differentiate between a bicornuate uterus, that has two long horns and no body, and a bipartite uterus, that has two horns and a distinct body. In addition, there are differences in the degree of fusion of the paramesonephric ducts during development. Primates have a simple uterus, consisting of a large body, two very small horns, and a single cervix. Lagomorphs (e.g., rabbits), monotremes, and marsupials have a duplex uterus that is characterized by two separate cervices and uterine compartments.

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The uterus is a highly organized organ consisting of all the major tissue layers characteristic of tubular organs (Figures 5-26). In the uterus, these tissues are given special names.

* Tunica mucosa = endometrium

* Tunica muscularis = myometrium

* Tunica serosa = perimetrium

The endometrium contains a simple columnar epithelium, but the height of these cells changes depending on the reproductive state of the individual. Sections of this mucosal epithelium invaginate deep into the lamina propria-submucosa to form uterine glands, which secrete mucus, lipids, proteins, and glycogen into the lumen of the uterus. The distal ends of these glands may be highly coiled in some species. In ruminants, certain highly vascularized regions of the submucosa are devoid of uterine glands. These are referred to as caruncles, and are points of contact between the maternal and fetal tissue that make up the placenta (Figure 5-27)

The myometrium consists of a thick inner layer of circular smooth muscle, and a thin outer layer of longitudinal smooth muscle. A layer of blood vessels lies between the smooth muscle layers.

The structure of the cervix is considerably different from that of the uterine body and horns. This region serves as a valve between the vagina and body of the uterus. The lumen (cervical canal) is narrow and constricted (Figure 5-28). The shape of the cervical canal varies among species. In all cases, the mucosal epithelium is highly convoluted, creating folds that run longitudinally along the length of the cervix. In addition, circumferential folds form several annular rings that extend into the lumen of the cervix, which create a tortuous canal. In the sow the pattern is complementary to the corkscrew shape of the boar's penis, which becomes embedded in the cervix during coitus.

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Microscopically, the cervix resembles the other portions of the genital ducts, but with some notable exceptions (Figure 5-29). The mucosal epithelium of the cervix ranges from stratified squamous in the bitch to columnar cells in the cow and ewe. In both cases, the mucosa contains goblet cells that secrete mucus. The secretory activity of these cells varies with physiologic status. During estrus, the cells secrete thin, clear mucus. In contrast, the cervix produces thick mucus that forms a cervical seal during pregnancy. We will consider the physiologic significance of the cervical mucus in a later chapter. The submucosa contains both loose and dense collagenous tissue and the tunica muscularis is well-developed with extensive fibrous fibers. These characteristics make this region of the uterus feel like a rope or a chicken neck.

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Oviducts

The oviducts are narrow extensions of the uterine horns and can be divided into three segments: infundibulum, ampulla, and isthmus (Figures 5-30, 5-31, and 5-32). The cranial end of the oviduct has a wide opening near the ovary, but then tapers to a smaller diameter distal to the ovary. This region of the oviduct is the infundibulum ("funnel"). The edge of the infundibulum is fringed with irregular processes (fimbriae), some of which attach to the ovary. A small orifice, located at the center of the infundibulum, opens to the tubular portion of the oviduct, which can be divided into two segments of similar lengths. In many species these regions have different diameters. The proximal ampulla is wider than the distal isthmus. The isthmus connects with the apex of the uterine horn forming the uterotubal junction. Although this region varies in appearance among species (from gradual to abrupt transitions), in all cases it acts as a physical barrier to impede movement of ascending spermatozoa and descending oocytes or embryos.

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The lumen of the oviduct is tortuous due to numerous folds in the mucosa. Major folds run longitudinally along the length of the tube. Smaller secondary and tertiary folds create small clefts that branch from the main canal. The degree of convolution is highest in the ampulla and lowest in the isthmus. The mucosal epithelium of the oviduct consists of columnar cells, some of which have motile cilia. Ciliated cells are most prominent in the cranial region and facilitate movement of the oocyte. Nonciliated cells secrete various substances that nourish the oocyte and capacitate spermatozoa. Unlike the uterus, submucosa does not contain glands. The tunica muscularis is more developed in the isthmus than in the ampulla, and consists of the familiar longitudinal and circular layers. A tunica serosa is present and contains a distinct vascular layer.

Vagina

As noted earlier, the vagina consists of two major parts. The cranial portion (between the uterine cervix and urethral orifice) is primarily a reproductive passage, whereas the caudal portion serves both reproductive and urinary functions. Overall, the vagina is a thin-walled tubular organ that is situated in the medial pelvic cavity ventral to the rectum and dorsal to the bladder and urethra. With the exception of the cranial portion, the organ lies in a retroperitoneal position.

[FIGURE 5-33 OMITTED]

The mucosa of the vagina is lined with an epithelial layer that varies in structure. Columnar epithelial cells are characteristic of the cranial vagina, whereas the caudal vagina is lined with stratified squamous epithelial cells (Figure 5-33). The thick layer of epithelial tissue in the mucosa of the caudal vagina helps this portion of the vagina withstand trauma associated with coitus. Mucosal glands are confined to the cranial region. The cervix protrudes into the cranial vagina reducing its lumen and forming a ring-like space; that is, the fornix. The muscularis of the vagina is similar to that of the uterus. A transverse fold of mucosal tissue at the junction of the vagina and vestibule (hymen) may exist in virgin females (especially in the gilt and filly). This usually breaks down, but is sometimes intact until first coitus.

SUSPENSION AND VASCULATURE OF THE INTERNAL GENITAL ORGANS

The reproductive organs located in the abdomino-pelvic cavity are suspended by the genital fold, a ligament formed by the peritoneum. In females the genital fold is called the broad ligament (Figures 5-34). The cranial portion supports the ovary and is called the mesovarium. The oviducts are supported by the middle section known as the mesosalpinx (referring to the "trumpet-shaped" oviduct). The mesometrium is the caudal end of the broad ligament and it supports the uterus. The mesovarium and mesosalpinx form a pouch around the ovary; that is, the ovarian bursa (Figure 5-35). The relationship between the ovary and its bursa ranges from a shallow sac that does not contain the ovary (e.g., the mare) to a deep enclosure that completely surrounds the ovary (e.g., the bitch; Figure 5-36). In rodents, the ovary is completely trapped by the bursa leaving no contact between the cavity and the surrounding peritoneum.

[FIGURE 5-34 OMITTED]

[FIGURE 5-35 OMITTED]

[FIGURE 5-36 OMITTED]

The blood vessels that supply and drain the female reproductive tract (Figure 5-37) are supported by the broad ligament. A branch of the abdominal aorta known as the ovarian artery supplies the ovary, oviducts, and the cranial uterine horn. This blood vessel is highly convoluted and intertwines with the ovarian vein. Blood flows to the body and horns of the uterus via the uterine artery, which branches from the internal iliac artery. Blood is supplied to caudal portion of the tract via the vaginal artery. Branches of the uterine artery run cranially and caudally and form anastomoses with the uterine branch of the ovarian artery as well as with the vaginal artery. Smaller arteries branch from these main branches forming an arterial arcade that supplies blood along the entire length of the reproductive tract. Blood drains from the ovary, uterine horn, and uterine body via the ovarian vein (also called the utero-ovarian vein). The accessory vaginal vein and the vaginal vein drain the caudal uterine body and vagina. The ovarian vein is plexiform (made up of a web of interconnected vessels) near the ovary, oviducts, and uterine horn. These smaller vessels converge on a larger vessel that runs laterally to the uterine body.

[FIGURE 5-37 OMITTED]

The ovarian artery is intimately associated with the utero-ovarian vein (Figure 5-37). The convoluted artery wraps around and makes numerous contacts with the utero-ovarian vein. This anatomic relationship has tremendous physiologic significance. The close proximity of the uterine artery to the utero-ovarian vein permits the transfer of an important uterine hormone from the utero-ovarian vein to the ovary artery via a counter-current exchange mechanism. Prostaglandin F2_ ([PGF.sub.2[alpha]]), present in high concentrations in the blood of the utero-ovarian vein, is transported via diffusion to the ovarian artery, which has little or no prostaglandin. In this way, high concentrations of this hormone are shunted directly from the uterus to the ovary.

As with the female, ligaments support the reproductive tract in males. The structure that is analogous to the broad ligament in females is much smaller in males and supports only the ampullae of the ductus deferens and the vesicular glands. The testis, epididymis, and most of the ductus deferens are enveloped by the vaginal tunic, an evagination of abdominal peritoneum that extends through the inguinal canal into the scrotum. The proximal portion is narrow and surrounds blood vessels, lymphatics, nerves, and the ductus deferens. As noted earlier, these tissues are known collectively as the spermatic cord.

The dominant feature of vasculature of the testis is the spermatic cord, which supports the testicular artery surrounded by a network of veins (Figure 5-38). As noted earlier, these structures are encased in an inward fold of the vaginal tunic. The outer layer is the parietal layer of the vaginal tunic, whereas the inner layer is the visceral layer. The vaginal cavity lies between these layers. This arrangement extends from the vaginal ring to the tail of the epididymis.

[FIGURE 5-38 OMITTED]

The arrangement of blood vessels in the spermatic cord is remarkable and provides the primary means for thermoregulation of the testis. Each testicular artery branches from the abdominal aorta, courses directly to the vaginal ring, and then follows the spermatic cord to the testis. In the spermatic cord, the artery is highly convoluted (7 m of artery compacted into 10 cm of spermatic cord) in a way that is similar to that of the ovarian artery within the broad ligament in the female. The contortions of the artery are embedded in a mesh of veins called the pampiniform plexus (Figures 5-38), the structure of which is homologous to the ovarian artery/utero-ovarian vein complex in the female. The testicular veins eventually converge and flow into the vena cava. Arteriovenous anastomoses exist between the testicular artery and surrounding veins permitting the direct shunting of blood between vessels. This intimate association facilitates heat exchange between the blood vessels. Heat from the artery is lost to the veins, which are cooled in the suspended testis. Thus blood flowing to the testis falls below body temperature creating a temperature gradient along the testis; that is, the temperature of the dorsal portion (near the spermatic cord) can be as much as 6[degrees]C warmer than the temperature of the ventral portion. In addition to cooling the testis, the pampiniform plexus allows testosterone, secreted into the arterial circulation by the testes, to be shunted back to the testes via the testicular veins. This contributes to the high concentrations of testosterone normally found in the testes. The physiologic significance of this mechanism lies in the fact that testosterone is necessary for spermatogenesis.

EXTERNAL GENITALIA

The external genitalia of males and females develop from the same embryonic tissues. Therefore, it should come as no surprise that the external sex organs of adult males and females consist of homologous structures. The penis and clitoris arise from the genital tubercle, whereas the scrotum and vulva arise from the genital folds (see Chapter 4). Although the external genitalia of males and females are anatomically related, their functions are less comparable. For example, the penis is the copulatory organ in males, whereas the clitoris is not necessary for the female to copulate. Likewise, the scrotum and vulva have different functions. The scrotum is intimately associated with the testis and influences its activity. In contrast, the vulva has nothing to do with ovarian functions.

Vestibule and Vulva

The external genitalia of the female include the vestibule and vulva. The vestibule lies caudal to the ischial arch (the ventral and caudal portion of the hip bone) and slopes downward to the vulva. The urethra opens at the floor of the vestibule in most species. However, in some cases (e.g., the cow) the urethra opens into a small sac called the suburethral diverticulum. Familiarity with this anatomy is particularly useful when performing procedures such as artificial insemination. For example, it is common for novice inseminators to place the tip of an inseminating gun into the suburethral diverticulum, instead of the vagina. A pair of vestibular glands empties into the vestibule lateral and ventral to the urethral opening. These glands produce mucus that helps lubricate the passage during coitus and parturition. In addition, these secretions contain aromatic compounds (pheromones) that arouse the sexual interest of males during mating. The vestibule is a highly vascularized tissue. In particular, a lateral vestibular bulb, consisting of a network of veins, forms a patch of erectile tissue that is homologous to the bulb of the penis (see subsequent discussion).

The walls of the vestibule are continuous with the vulva, a vertical opening below the anus (Figure 5-39). The labia, lateral folds of tissue on either side of the vulva, meet at the dorsal and ventral commissures. In humans, the labia can be divided into the inner labia minora and the outer labia majora. The labia minora is not distinct in domestic mammals. The clitoris lies within the ventral commissure and consists of a body and glans. It is homologous to the male's penis and contains spongy erectile tissue analogous to that found in the male phallus.

[FIGURE 5-39 OMITTED]

Penis and Prepuce

The scrotum, penis, and prepuce make up the external genitalia of the male. The anatomy of the scrotum was considered in an earlier section dealing with the gross anatomy of the testis. The penis is located between the thighs, and is suspended below the trunk of the body. In the larger species (bull, boar, ram, and stallion) the penis is anchored to the floor of the pelvis by a ligament. In mammals other than primates, the penis is concealed by an invagination of abdominal skin called the prepuce. The penis is made up of three independent columns of erectile tissue (Figure 5-40). The cura of the penis (crus penis) consists of a pair of columns situated dorsally. At their site of origin, near the ischial arch, they are separated. A short distance distal to this location, the columns converge, bend cranially, and then run along the floor of the pelvis, where they unite. Each of these columns contains a core of cavernous tissue (corpus cavernosum) covered by the tunica albuginea (Figure 5-41). A separate erectile compartment (corpus spongiosum) lies beneath the corpus cavernosum and surrounds the urethra. A groove of connective tissue separates these two vascular compartments. The origin of the corpus spongiosum lies at the base of the penis and is characterized by a bi-lobed enlargement called the bulb of the penis. It continues along the shaft of the penis and expands over the distal end, where it is called the glans penis. The corpus cavernosum does not extend to the tip of the penis. During sexual arousal, each of these cavernous structures fills with blood to cause erection of the penis.

[FIGURE 5-40 OMITTED]

[FIGURE 5-41 OMITTED]

There are several variations in penis structure among mammals. In the dog and cat (and some other species of mammals), the distal corpus cavernosum becomes ossified and exists as a boney tissue called the os penis (bacculum). The shape of the glans also varies considerably. Perhaps the most extraordinary structure is the filiform appendage of the ram's and buck's penis. Another major anatomic difference deals with the type of tissue found in the corpus cavernosum. In swine and ruminants, this tissue contains strands of fibro-elastic tissues that divide the space into numerous small compartments (sinusoids). This so-called fibroelastic penis exists in a semi-erect state and requires little blood to achieve full erection. In contrast, the corpus cavernosum of the stallion, dog, and primate contains much larger sinusoidal compartments separated by septa that are more muscular and less elastic than those of the fibroelastic penis; that is, a musculocavernous penis. The shapes of these two types of penis differ considerably. The fibroelastic penis has a sigmoid flexure; that is, the shape is bent in the shape of an S. This is maintained by a pair of retractor penis muscles that run bilaterally from the coccygeal vertebrae to the ventrolateral sides of the penis. These are made up of smooth muscle fibers that keep the penis retracted most of the time. However, upon sexual arousal, they relax and allow the penis to protrude from the prepuce.

In both types of penis, erection results from the blood flowing into the sinusoids of the corpora cavernosum and spongiosum (see Figure 5-41). The artery of the penis supplies all of the blood flowing into these tissues. Before entering the penis, the artery splits into three branches: one enters the bulb of the penis to supply the sinusoids of the corpus spongiosum; another other passes through the tunica albuginea to supply the corpus cavernosum; a third runs dorsally along the shaft to supply the apex of the penis.

SUMMARY OF MAJOR CONCEPTS

* The genital organs of the male and female include the gonad, genital ducts, and external genitalia.

* The testis and ovary are similar in that they both consist of gametogenic and endocrine tissues. They differ with respect to location, structure, and how the developing gametes are housed and released from them.

* The genital ducts of the male and female are tubular organs consisting of concentric layers of tissue. The number of tissue layers and the organization of cells within each layer vary among the organs of the reproductive tracts.

* The genital ducts of the male are primarily concerned with the transport of spermatozoa, whereas the female genital ducts are involved with the transport of male and female gametes, as well as transport and nourishment of the conceptus.

* The reproductive tracts of the male and female are suspended by evaginations of peritoneum, which also support afferent and efferent blood vessels and nerves. In each sex, the main artery providing blood to the gonad is intimately associated with a venous plexus, which permits a counter-current exchange of heat and/or hormones.

* The external genitalia of males are homologous to that of females, but there are marked anatomic and physiologic differences between the external genitalia of the two sexes.

DISCUSSION

1. Discuss the major similarities and differences between the reproductive tracts of male and female mammals. Restrict your discussion to the major components and the arrangement of these components.

2. Compare and contrast the histologies of a section of semini-ferous tubule and a tertiary ovarian follicle. Pay particular attention to the basement membrane and cell types.

3. Compare and contrast the arrangement of tissue layers in the ductus deferens and the oviduct.

4. Describe the counter-current vascular systems found in the male and female reproductive tracts. Explain the significance of each one.

REFERENCES

Banks, W.J. 1993. Applied Veterinary Histology (Third Edition). St. Louis, MO: Mosby Year Book.

Dyce, K.M., W.O. Sack, and C.J.G. Wensing. 2002. Textbook of Veterinary Anatomy (3rd Edition). Philadelphia: W.B. Saunders Company.

Evans, H.E. and G.C. Christensen. 1979. Miller's Anatomy of the Dog (2nd Edition). Philadelphia: W.B. Saunders Company.

Mullins, K. J., and R.G. Saacke. 2003. Illustrated Anatomy of the Bovine Male and Female Reproductive Tracts, From Gross to Microscopic. Blacksburg, VA: Germinal Dimensions, Inc.

Keith K. Schillo, PhD

Department of Animal and Food Sciences

University of Kentucky

Lexington, Kentucky
TABLE 5-1 Accessory sex glands present in several species of
domestic mammals

Species   Ampullary   Vesicular   Prostate   Bulbourethral

Dogs      Yes         No          Yes        No
Cats      Yes         No          Yes        Yes (vestigial)
Horses    Yes         Yes         Yes        Yes
Cattle    Yes         Yes         Yes        Yes
Sheep     Yes         Yes         Yes        Yes
Swine     Yes         Yes         Yes        Yes
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Author:Schillo, Keith K.
Publication:Reproductive Physiology of Mammals, From Farm to Field and Beyond
Geographic Code:1USA
Date:Jan 1, 2009
Words:8465
Previous Article:Chapter 4: Sexual differentiation.
Next Article:Chapter 6: Functional anatomy of reproductive systems: neuroendocrine systems.
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