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The physiology of the male and female reproductive systems

19.1 Introduction

Reproduction—the ability to produce a new generation of individuals of the same species—is one of the fundamental charac­teristics of living organisms. Genetic material is transmitted from the parents to the next generation to ensure that the charac­teristics of the parents and those of the species are perpetuated. The essential feature of sexual reproduction is the mixing of chromosomes from two separate individuals to produce offspring that differ genetically from their parents. At the core of the process lies the creation and fusion of the male and female gametes, the sperm and ova (eggs).

Gametes are specialized sex cells, produced by the gonads, which provide a link between one generation and the next. Spermatozoa, the male gametes, are produced by the testes, while ova, the female gametes, are produced by the ovaries. The nuclei of these cells are haploid, i.e. they contain only a single set of chromosomes. Haploid cells are created when a diploid cell divides by meiosis, a process in which the genes are parceled out afresh in single chro­mosome sets (for more details see Section 193). During meiotic division old combinations of genes are broken and new com­binations formed by chromosomal exchange, so that the genetic composition of each chromosome is modified. At fertilization, the gametes fuse to form a new cell possessing a full set of chromosomes, half of which originate from the sperm and half from the ovum. This is called a zygote. The reshuffling of genes that is permitted by sexual reproduction helps to create a genetically diverse population which is able to show greater resilience in the face of environmental challenge.

This chapter will discuss the processes that lead to the production of the male and female gametes and the hormonal mechanisms that regulate their activity. In addition, the neural and endocrine control of reproductive activity, including puberty and the menopause, will be discussed.

^ Reproductive physiology of the male 19.2 Anatomy of the male reproductive system

Figure 19-1 is a simple diagram of the adult male reproductive tract, showing the major organs within it, while Fig. 19-2 illus­trates the internal structure of the testis, the male gonad

responsible for the production of spermatozoa and the male sex hormones (the androgens). The testes lie outside the abdominal cavity, within the scrotal sac. Each testis is about 4.5 cm in diameter and weighs around 40g. The organ is made up of a large number of coiled seminiferous tubules containing Sertoli cells where the sperm are made. Between these tubules lies supportive connective tissue which contains the interstitial or Leydig cells which are responsible for the synthesis and secretion of the tes-ticular androgens, particularly testosterone. This anatomical arrangement gives the testes a lobular structure, where each lobule contains two or three tubules. At the apex of each lobule, its seminiferous tubules join and pass into the first section of the excretory ducts, the tubuli recti, these are short, straight tubes that enter the dense connective tissue of the mediastinum testis and form within it a system of irregular, epithelium-lined spaces, the rete testis. From here the tubules drain into another coiled tube, the epididymis, which in turn leads into the vas deferens, a tubular structure, 30-35 cm in length, that terminates in the ejaculatory duct close to the prostate. The seminal vesicles are located on either side of the prostate and empty their secre­tions into the ejaculatory duct, together with the sperm and prostatic secretions, to form semen. From the ejaculatory duct, the semen enters the penis, from which it is released during copulation.

The development of the testis takes place within the abdo­minal cavity of the fetus (Chapter 20). However, by the time of birth, or soon afterwards, the testes are lying within the scrotal sac outside the body cavity and experience an ambient tem­perature which is some 2 or 3 °C lower than the core tem­perature. To arrive here, the testes descend, migrating, posteriorly through the abdominal cavity and over the pelvic brim. Failure to migrate in this way results in a condition known as cryptorchidism which, if it persists until puberty, leads to an arrest of spermatogenesis and thus infertility since the testes cannot function normally at body temperature.

^ 19.3 The adult testis makes gametes and androgens

The testis performs two fundamental roles in the mature male, both of which are vital to his fertility and sexual competency.

438 19 The physiology of the male and female reproductive systems

Fig. 19-1 A posterior view of the adult human male reproductive system to show the principal structures.

Fig. 19-2 The adult testis, epididymis, and vas deferens.

These are:

1. the production of sperm, which will carry his genes and
fertilize an ovum; and

2. the secretion of testicular androgens, particularly testo­
sterone, which bring about full masculine development.

The two major products of the male gonads, the spermatozoa

and the androgenic steroid hormones are synthesized in separate

compartments within the testis. The sperm are made in the sem­iniferous tubules themselves while the androgens are synthesized and secreted by the Leydig cells that lie between the tubules. Indeed, these two compartments of the testis appear to be sepa­rated functionally as well as anatomically since there is a barrier that prevents the free exchange of water-soluble materials between them. This is known as the blood—testis barrier and arises as a result of rhe extremely tight junctional complexes that exist between the basal regions of adjacent Sertoli cells (Fig. 19-3). This barrier protects the developing sperm from any noxious bloodborne agents and so maintains a suitable environ­ment for their maturation. It also prevents antigenic materials (e.g. proteins) that arise in the course of spermatogenesis from passing into the circulation and triggering an autoimmune response to the sperm. Where this does happen, for example as a consequence of traumatic injury to the testis, it can lead to male infertility. Although the manufacture of sperm and androgens occurs in separate compartments, their production is closely related functionally as the production of mature sperm is only possible if androgen secretion is normal.

Testosterone is the major testicular androgen

The Leydig cells of the testis synthesize and secrete testosterone, the principal testicular androgen, from acetate and cholesterol, as described in Box 19.1. Adult males secrete about 4-10 mg of testosterone each day, most of which passes into the blood.

19.3 The adult testis makes gametes and androgens


Fig. 19.3 Sectional view of the wall of a seminiferous tubule to show the relationship between the Sertoli cells and the developing spermatozoa. Note the tight junctions between the basal regions of the Sertoli cells separating the basal compartment from the adluminal compartment.

However, a small amount enters the seminiferous tubule, where it binds to an androgen-binding protein secreted by the Sertoli cells, and subsequently plays a crucial role in the development of the spermatozoa (see below). Being a steroid and thus fat-soluble, testosterone is able to cross the blood-testis barrier by passive diffusion.

As the diagram shows, extensive interconversions are possible although the exact biosynthetic routes for a particular tissue will depend on the enzymes present in that tissue.

The peripheral actions of testosterone

The mode of action of steroid hormones has been described previously (Chapters 5 and 12) and the general rules are applicable to testosterone. The hormone circulates in the plasma bound either to a sex-steroid-binding globulin or to other plasma proteins. It enters cells freely, where it may be converted to dihydrotestosterone or 5a-androstenedione. All three androgens bind to specific cytoplasmic receptor proteins to form a steroid- recep-

tor complex which moves to the nucleus and interacts with chro­mosomal DNA as described in Chapter 5. In addition, they can bind to receptors on the plasma membrane or interact directly with nuclear DNA to modify gene expression. Androgen receptors are most numerous in the tissues that are specific targets for the hormone, i.e. those that depend upon androgens for their growth, maturation, and/or function. Such tissues include the accessory organs of the male reproductive tract—the prostate, seminal vesi­cles, and epididymis—as well as nonreproductive tissues such as the liver, heart, and skeletal muscle.

Dihydrotestosterone is important in the fetus for the dif­ferentiation of the external genitalia and, at puberty, for the growth of the scrotum, prostate, and sexual hair. In addition to its role in the production of sperm, testosterone stimulates the fetal development of the epididymis, vas deferens, and seminal vesicles and, at puberty, is responsible for enlargement of the penis, seminal vesicles, and larynx, together with the changes in the skeleton and musculature characteristic of the male.


19 The physiology of the male and female reproductive systems

lox 19.1 Biosynthesis of the major sex steroid hormones from cholesterol

A simple schematic diagram to show the principal pathways involved in the biosynthesis of the major sex steroid hormones from cholesterol. As the diagram shows, extensive interconversions are possible although the exact biosynthetic routes for a particular tissue will depend on the enzymes present in that tissue.

Spermatogenesis—the production of sperm by the testes

A sexually mature male produces around 200 million spermato­zoa each day. Spermatogenesis is a complex process that involves the generation of huge numbers of cells by mitosis and the halving of the chromosomal complement by meiosis. Further­more, it involves the formation of a highly specialized cell designed to carry the genetic material a considerable distance within the female reproductive tract in order to maximize the chances of fertilization. The major steps in the process of spermatogenesis are shown in Fig. 19.4.

At puberty, the male germ cells are triggered to commence mitotic division, an event that marks the beginning of spermato-

genesis and which results in the formation of a population of spermatogonia lying within the basal compartment of the semi­niferous tubules.

The first two mitotic divisions of each germ cell give rise to four cells which, in fact, remain connected to one another by a thin bridge of cytoplasm (Fig. 19-4). Three of these undergo further division to form spermatogonia while the fourth arrests at this stage and will later serve as a stem cell for a subsequent generation of sperm. The three active cells divide twice more to give rise to a population of so-called primary spermatocytes. Up until now, the cell divisions have taken place within the basal compartment of the tubule (see Fig. 19-3) but at this point the primary spermatocytes enter the adluminal tubular com­partment. They apparently achieve this by transiently disrupting

19.3 The adult testis makes gametes and androgens


Fig. 19-4 The principal stages of spermatogenesis. The primordial cells divide to form spermatogonia which undergo two more divisions to form primary spermatocytes. The primary spermarocytes undergo meiotic divisions to form secondary spermatocytes and spermatids. During meiosis the chromosome number is halved. Note the cytoplasmic bridges between the differentiating spermatids.

the tight junctions between neighboring Sertoli cells. Following a period of growth, each of the primary spermatocytes undergoes two meiotic divisions. The first of these gives rise to haploid secondary spermatocytes, which divide again immediately to form spermatids, each of which possesses 22 autosomes (that is, chromo­somes not associated with the determination of sex) and either an X or a Y sex chromosome. The progeny of a single spermatogo­nium still remain connected by cytoplasmic bridges. The genetic events of spermatogenesis are now complete. The final stages of the process involve the convetsion of the round spermatids to mature motile spermatozoa, a process known as spermiogenesis.

Spermiogenesis involves major cytoplasmic remodeling of the spermatid

Figure 19.5 shows the essential structures of a mature, motile human spermatozoon, and from this it is clear that there are con­siderable differences between this and the round spermatid. The

Fig. 19-5 (a) A mature spermatozoon in longitudinal section and (b) a transverse section through the mid-piece. Note the spiral mitochondrion and the arrangemenr of rhe microtubules.

process of spermiogenesis involves reorganization of both the nucleus and the cytoplasm of the cell, as well as the acquisition of a flagellum. The whole process takes place in close association with the Sertoli cells.

The immature sperm consists of two regions which are mor­phologically and functionally distinct, the head region and the tail. The head contains a haploid nucleus (that is, possessing half the full chromosomal complement) and a specialized secretory vesicle called the acrosomal vesicle, which contains hydrolytic enzymes that will help the sperm to penetrate the oocyte prior to fertilization.

The tail region of the sperm is motile. It is a long flagellum which has essentially the same internal structure as that seen in all cilia or flagella from green algae to humans. It consists of a central axoneme originating from a basal body situated just posterior to rhe nucleus. The axoneme consists of two central microtubules surrounded by nine evenly spaced pairs of micro­tubules. Active bending of the tail is caused by the sliding of adjacent pairs of microtubules past one another and movement is powered by the hydrolysis of ATP generated by mitochondria present in the first part of the tail, the mid-piece.

The process of differentiation of a spermatocyte to a motile sperm takes approximately 70 days in men and after this time the newly formed sperm are released from the adluminal compartment


19 The physiology of the male and female reproductive systems

of the Sertoli cells into the lumen of the seminiferous tubule and from there into the epididymis where they undergo further maturation, acquiring in particular the capacity for sustained motility. As may be seen from Fig. 19.4, connected residual bodies of cytoplasm are left behind by the released motile sperm.

The formation of seminal fluid

The epididymis can serve as a reservoir for sperm, with their passage through this coiled tube taking anything from 1 to 21 days. The sperm and other testicular secretions are then trans­ported along the vas deferens and into the ejaculatory ducts. The seminal fluid is markedly increased in volume by contributions from the seminal vesicles (about 60 per cent of total volume) and the prostate (about 20 per cent). The fluids secreted by these glands provide nutrients for the sperm. The prostatic fluid is alkaline, helping to neutralize the normally acidic fluid of the vas deferens and thus to increase the motility and fertility of the sperm, both of which are optimal at a pH of around 6.5.

19.4 The hormonal control of spermatogenesis—the pituitary—testis axis

In Chapter 12 some of the general mechanisms involved in the regulation of hormone secretion within the body were discussed, including the importance of the hypothalamic releasing hor­mones and the concept of negative feedback control. These key regulatory processes have been shown to operate in the endocrine control of male reproductive function and are summarized in Fig. 19.6.

Gonadotropin releasing hormone (GnRH) synthesized by neurons in the hypothalamus is secreted into the vessels of the hypophyseal portal tract and transported to the anterior pituitary where it stimulates the release, from pituitary basophils, of the gonadotropins FSH and LH into the systemic circulation. (LH is also called interstitial cell stimulating hormone, or ICSH, in the male). LH acts particularly on the Leydig cells to bring about secretion of testosterone, while FSH acts mainly on the Sertoli cells to cause the release of androgen-binding protein, estradiol, and a further hormone known as inhibin. In turn, testosterone inhibits the secretion of LH by exerting a negative feedback action at the level of both the anterior pituitary itself and the hypothalamus. At the same time it is thought that inhibin, and possibly also estradiol, depress the further secretion of FSH by a similar feedback mechanism. These negative feedback loops provide an important internal control system for maintaining fairly constant levels of both gonadotropic and androgenic hor­mones in the systemic circulation.

As discussed earlier (Section 19-3), testosterone exerts a variety of important effects throughout the body of the male, including the development of secondary sex characteristics at puberty, but it is also essential for normal sperm production. As it is lipid soluble, some testosterone secreted by the Leydig cells enters the intratubular compartment where it is bound by

Fig. 19.6 The relationship between the hormonal secretions of hypothalamus, pituitary gland, and testis.


  1. The adult testis produces sperm which carry the male genes. It also
    secretes steroid hormones known as androgens which bring about full
    masculine development. The principal androgen is testosterone.

  2. Spermatogenesis takes place in the Sertoli cells of the seminiferous
    tubules, while the androgens are secreted by the Leydig cells which
    lie between the seminiferous tubules.

  3. Spermatogenesis is a complex process involving the generation of
    huge numbers of cells by mitosis and the halving of the chromosomal
    complement by meiosis. It culminates in the formation of a highly
    specialized cell, the mature, motile sperm.

  4. The sperm are then mixed with a number of other secretions from the
    seminal vesicles and prostate to form seminal fluid. This is released as
    semen from the penis at ejaculation during sexual intercourse.

  5. Spermatogenesis is regulated by a variety of hormones, including
    FSH, LH, and testicular testosterone. In turn, hormone levels
    are regulated by negative feedback loops operating within the
    hypothalamic—pituitary—testicular axis.

19.6 Anatomy of the female reproductive tract


androgen-binding protein secreted by the Sertoli cells. In some way, which is as yet poorly understood, this bound testosterone helps to maintain the production of spermatozoa. While testos­terone is needed for the maintenance of spermatogenesis, pitu­itary FSH is to be required both to initiate the process and to mediate the differentiation of spermatids into spermatozoa.

^ Reproductive physiology of the female 19.5 Introduction

Like the testis in the male, the ovary produces both haploid gametes and a variety of hormones. The production of gametes by the ovary is coordinated with its endocrine activity. Unlike the testes, however, which release an enormous number of gametes in a continuous stream, the ovaries produce relatively few oocytes and their release normally occurs only once every 4 weeks or so at ovulation. This regular release of ova from the ovary is controlled by physical, neural, and, above all, endocrine mechanisms involving a complex interplay between the hypo-thalamic, pituitary, and ovarian hormones. These will be dis­cussed in some detail later but, briefly, the ovarian steroids (estrogens and progesterone) are secreted in a cyclical fashion. A period of estrogen dominance characterizes the first half of each cycle, during which one ovarian follicle (see later sections) reaches full maturity and the body is prepared for gamete trans­port and fertilization. This period culminates in ovulation roughly halfway through the cycle and is followed by a period of progesterone dominance, during which the genital tract is

Fig. 19.8 The relationship between the ovary, the fallopian tube, and the uterus. The cross-section through the ovary shows a follicle, a corpus luteum, and a corpus albicans.

maintained in a state favorable for the implantation and early development of a zygote.

^ 19.6 Anatomy of the female reproductive tract

Figure 19-7 shows a simplified diagram of rhe adult female repro­ductive organs. A more detailed diagram of the ovary, showing the important stages in follicular development is shown in Fig. 19-8.

The ovaries are about 3—4 cm long, each weighing around 15 g, and lie in the ovarian fossa of the pelvis. They are attached to the posterior wall of rhe abdomen by the mesovarium or ovarian mesentery. The adult organ is composed of stromal tissue which contains the primary oocytes housed within primordial

Fig. 19.7 Gross anatomy of the female reproductive organs and their spatial relationship to other structures.


19 The physiology of the male and female reproductive systems

follicles, and glandular interstitial cells. The physiology of the ovaries will be considered in more detail in subsequent sections.

The remainder of the reproductive tract is concerned not with gamete production itself, but with the processes of fertilization and nurture of an embryo. To achieve this, both the male and female gametes must be transported to the site of fertilization (in the fallopian tubes) and a favorable environment must be created both for implantation and for the subsequent development of the embryo. Each ovarian cycle reflects these two roles.

The fallopian tubes are thin tubes about 12 cm in length which serve to transport the ovum released at ovulation from the ovary to the uterus. The opening of the tube is expanded and split into fringes or fimbriae which move nearer to the ovary at ovulation. There are numerous cilia on the fimbriae and these create cur­rents in the peritoneal cavity so that after ovulation the egg is directed towards the mouth, or ostium, of the fallopian tube. The tube itself is muscular and covered with peritoneum. Internally it has a layer of stromal tissue which is overlaid by ciliated, high columnar secretory epithelial cells.

The non-pregnant uterus is about 7.5 cm long and 5 cm wide and is the organ that houses the fetus during its gestation period of 38 weeks. It must be adapted to receive the early embryo, and permit implantation and the formation of a placenta. While it has to contract powerfully to expel the fetus at birth, it must remain quiescent throughout gesration to allow full fetal devel­opment. The uterus consists of an outer covering or serous coat; a middle layer of smooth muscle, the myometrium, which forms the bulk of the thick wall; and the inner endometrial layer or endometrium. This is composed essentially of epithelial cells, simple tubular glands, and the spiral arterioles that supply the cells. The characteristics of the endometrium are altered considerably during each ovarian cycle.

The neck of the uterus is formed by the cervix, a ring of smooth muscle containing many mucus-secreting cells. It forms the start of the so-called birth canal which is traversed by both sperm and fetus. The mucus-secreting cells undergo important changes in activity during each monthly cycle, which are designed to optimize conditions for fertilization.

The final internal structure of the female reproductive tract is the vagina. The cells that line the vagina and the vaginal fluids also show cyclical variations. The cyclical changes in the vaginal fluid (particularly its pH) can be useful in the treatment of infertility as they can be used to determine which stage of the cycle has been reached.

The vaginal orifice, urethral orifice, and clitoris are protected by folds of tissue called the vulva, composed of the labia majora and minora. Within the walls of the vulva lie the vestibular glands which secrete mucus during sexual arousal and help to lubricate the movement of the penis within the vagina during sexual inter­course. The clitoris is a small, erectile structure which is homolo­gous with the male penis.

^ 19.7 The ovarian cycle

Like the testis in the male, the ovary plays a fundamental role in the reproductive physiology of the female. It releases mature, fer-tilizable ova at regular intervals throughout the reproductive years of a woman and it secretes a number of hormones that are involved in the regulation not only of the ovaries themselves but also of the rest of the reproductive tract. These are steroid hormones and include progesterone and a number of estrogenic hormones. In the nonpregnant woman the predominant estrogenic hormone is estradiol-17/3, while during pregnancy estrone and estriol are also produced (especially by the placenta). During the following account of ovarian function the term 'estrogen' will be used to refer to the estrogenic agents secreted by the follicular cells. In the discussion of placental function (Sections 20.5, 20.6), it will be used as a collective term for the variety of estrogenic hormones of physiological importance during pregnancy.

In the following discussion of ovarian function it will be important to bear in mind two questions:

  1. What are the mechanisms that ensure the regular release of

  2. How does the endocrine activity of the ovary prepare the
    rest of the reproductive tract for successful fertilization and
    the ensuing pregnancy?

Fig. 19.9 The internal structure of the ovary, showing the stages of follicular development, ovulation, the formation of the corpus luteum and its subsequent regression. In reality not all stages would be seen at the same time.

^ 19.7 The ovarian cycle
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