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Fertilisation and pregnancy

20.1 Introduction

In the previous chapter the discussion of female reproductive physiology has assumed a situation in which cyclical ovarian activity continues uninterrupted by pregnancy, that is, with regression of the corpus luteum. If, however, an oocyte is fer­tilized, an entirely different set of events must be initiated. Loss of the uterine endometrium must be prevented, and the uterus must be maintained in a quiescent state to allow implantation of the embryo, formation of the placenta, and gestation. This chapter will be concerned with the events surrounding fertilization, pregnancy, and parturition.

20.2 The sexual reflexes

Fertilization of an ovum requires that sperm are deposited high in the vagina of a woman close to the time of ovulation. Except

for the case of fertilization by artificial insemination, this is achieved through the act of sexual intercourse. For successful intercourse to take place, the penis of the male must become erect and ejaculation of seminal fluid must occur within the vagina.

The sexual response in both males and females can be divided into four main phases: excitement, plateau, orgasm, and resolu­tion. Each phase is under the control of autonomic and somatic nerves originating in the lumbar and sacral regions of the spinal cord. The innervation of the sexual organs is derived from both the parasympathetic (S2—S4) and the sympathetic (T11-L2) divisions of autonomic nervous system, together with somatic fibers running in the pudendal nerves (see Fig. 20.1).

The main stages of the male sexual act are:

  1. the erection of the penis;

  2. the secretion of mucus by the bulbourethral gland;

Fig. 20.1 The nerve supply to the penis and accessory sex organs.


20 Fertilization and pregnancy

  1. the emission of fluid from the seminal vesicles, vas
    deferens, and prostate gland;

  2. the expulsion of the seminal fluid from the penis—

These stages have their parallel in the female, namely clitoral engorgement, mucus secretion to provide lubrication during intercourse, and orgasm. Mucus secretions are provided both by Bartholin's glands, which are adjacent to the labia minora, and by the vaginal epithelium. These secretions ensure the inter­course is associated with a pleasurable massaging sensation rather than dry frictional irritation which can inhibit both ejaculation and the female climax.

Penile erection is the result of a parasympathetic reflex

Penile erection results from either descending nerve activity originating in the higher centers of the brain {psychogenic erection) or from stimulation of the skin in the genital region {reflexogenic erection). The afferent nerve fibers from the genital region run in the pudendal nerves to the sacral region of the spinal cord. The efferent fibers are parasympathetic in origin, derived from seg­ments S2-S4. When rhey leave the spinal cord, the efferent fibers run in the pelvic nerve and synapse in the pelvic plexus. The neurons of the pelvic plexus also receive sympathetic fibers from the hypogastric nerves. The cavernous nerve provides the final limb of the efferent pathway (Fig. 20.1).

The erectile tissue of the penis consists of the two corpora cavernosa and the corpus spongiosum which surrounds the urethra (see Fig. 19-1 for more detail). The erectile tissues are essentially large venous sinusoids surrounded by a coat of strong fibrous tissue. Erection is a simple hydraulic process, mainly controlled by the parasympathetic nerves. Impulses in the cavernous nerve cause the internal pudendal artery and its main branches to dilate. Consequently, the blood flow to the penis is increased but the venous outflow remains unchanged so that blood becomes pooled in the erectile tissue, causing the penis to enlarge and extend. As the volume of blood within the erectile tissue increases, rhe pressure rises, resulting in partial occlusion of the emissory veins and the penis becomes rigid and erect. Erection is also known as tumescence.

Dilation of the internal pudendal artery and the associated arterioles is mediated chiefly by nitric oxide derived from fibers running in the parasympathetic nerves. Somatic afferent pudendal nerves provide sensory feedback to mainrain erection during intercourse. Most sensory input is derived from touch receptors in the skin of the most distal part of the penis (the glans penis). Detumescence (reversal of erection) is probably mediated by sympathetic nerves arising from the sacral ganglia.

Emission and ejaculation are the final stages of the male sexual act. When sexual stimulation becomes very intense, rhythmic contractions in the vas deferens and ampulla begin to drive sperm towards the ejaculatory duct. This is followed by

secretion of prostatic fluid and contraction of the seminal vesicles. This stage is called emission; the expulsion of this fluid from the penis is called ejaculation. During ejaculation the inter­nal urethra becomes tightly closed to prevent the seminal fluid entering the bladder or the prostate and seminal glands. Contractions of the bulbocavernous and ischiocavernous muscles drive the seminal fluid along the urethra. Together, these adapta­tions result in forward movement of the semen and its expulsion from the penis. Ejaculation is accompanied by an intense sensation known as orgasm.

In the female, orgasm is associated with rhythmic contraction of the perineal muscles, dilation of the cervical canal, and increased motility of both the uterus and possibly the fallopian tubes. These contractions may help to transport sperm towards the site of fertilization in the fallopian tube. In both males and females orgasm is followed by a feeling of warm relaxation known as resolution.


The prevention of unwanted pregnancies (conrraception) is now a major issue in most societies as human fertility leads to inex­orable population growth. The principal means of preventing conception are summarized in Box 20.1 and rely either on pre­venting contact between sperm and ovum by a physical barrier (e.g. a condom) or on preventing ovulation.

^ 20.3 Fertilization and implantation of the embryo

Around 3 ml of seminal fluid are released in each ejaculation and this will normally contain about 200 million sperm. Sperm deposited in the vagina during intercourse swim through the cervical mucus and through the uterus to the fallopian tube which is the site of fertilization. Both the male and female gametes have a limited period of viability within the female reproductive tract. Sperm are thought to retain their fertility for up to 48 hours while ovulated ova remain viable for only around 12-24 hours. There is therefore a relatively short time during each menstrual cycle in which intercourse must take place if pregnancy is to be achieved.

Before a sperm can fertilize an ovum it must undergo a process known as capacitation. This is a calcium-dependent process, also termed the acrosome reaction, involving fusion of the surface membrane of the head of the sperm with the underlying acrosomal membrane (see Fig. 19.5). It is not clear whether this process occurs spontaneously within the female tract or whether it is initiated by agents produced by the ovum as it travels along the fallopian tube. In any event, should a viable ovum meet an activated sperm in the fallopian tube, fertilization may occur. The acrosome reaction releases hyaluronidase which digests the outer granulosa cell matrix of the egg mass, allowing the sperm to pass in towards the zona pellucida. The sperm swims through this region by means of whiplash movements of its tail and then

20.3 Fertilization ond implantation of the embryo I 461

^ Box 20.1 An outline of contraceptive methods




(Estimated as percentage

of couples remaining

childless after 1 year)

Oral contraceptives


Contraceptive preparations are of two types:

(the birth control pill)

a) Combination—consisting of estrogens and progesterone in sufficiently high

concentrations to exert powerful negative feedback on gonadotropin output. They

therefore mimic the luteal phase of the menstrual cycle and prevent ovulation

b) The mini-pill—this contains only of progesterone and it acts by modifying the

secretions and environment of the reproductive tract

Side-effects include minor symptoms of early pregnancy—nausea, breast

tenderness, fluid retention, hypotension, and, in rare cases, thromboses (mainly in

smokers over 35 years of age)

Intrauterine device (IUD)


Probably works by creating an environment within the uterus that is hostile to

fertilization or implantation. May cause uterine bleeding. Increased risk of

inflammatory disease in the pelvic region

Condom (sheath)


A barrier method which is more effective if used with a spermicide. It chief

advantage is that it protects against AIDS and other sexually transmitted

diseases. It may also protect against cervical.cancer. Its disadvantage is that it

may split in use


98% when used with

An alternative barrier method. Needs to be inserted before intercourse. It fits over


the cervix and blocks the entrance to the uterus. Its use carries a small risk of


Rhythm method

Highly variable, dependent

Couple must refrain from intercourse during the fertile period of the cycle (i.e. the

on regularity of cycle and

2 or 3 days on either side of ovulation). The time of ovulation must be calculated

accuracy with which mid

on the basis of the previous cycle, assuming that ovulation occurred on day 14

cycle is calculated

before menstruation. Other indicators such as the change in body temperature at

mid-cycle and the constitution of cervical mucus may also be used


c. 100%

Requires surgery and is not always reversible. Risks are similar to those of other

(vasectomy in males

minor surgical procedures

and ligation of the

fallopian tubes in


fuses with the oocyte membtane. This fusion completes the first stage of fertilization.

The fertilized egg must now complete several important tasks in order to ensure its continued successful development:

  1. It must complete its second meiotic division to avoid
    triploidy now that it has fused with a sperm (remember
    that the secondary oocyte arrested in the second meiotic
    metaphase). This division is normally completed within
    2 or 3 hours of fertilization and the second polar body is

  2. It must avoid fusing with any further sperm (polyspermy).
    To avoid polyspermy, a special reaction takes place within
    the newly fertilized egg, the so-called 'cortical reaction'.
    Much of our information concerning this reaction has
    come from experiments using sea-urchin eggs and the
    extent to which it is applicable to humans is unclear. A
    simple diagram of the likely key events is shown in
    Fig. 20.2. After fertilization, the oocyte membrane

becomes depolarized. As a result, free calcium levels within the ooplasm rise, and this rise triggers the cortical reaction. Towards the outer region, or cortex, of the oocyte, there are granules derived from the Golgi apparatus. In response to the elevation in intracellular calcium following fertilization, these cortical granules fuse with the oocyte membrane and release their contents into the perivitelline space by exocytosis (see Section 4.5). These contents form the 'fertilization membrane' which appears to consist mainly of enzymes that act to prevent penetration of the egg by any further sperm.

Finally, the fertilized ovum must initiate changes that prevent regression of the corpus luteum and shedding of the endometrium at menstruation. The maintenance of the structure and function of the corpus luteum beyond the end of the cycle is dependent on the secretion of a glyco­protein hormone known as human chorionic gonadotropin (hCG). This hormone is a potent luteotropic agent that is

462 ] 20 Fertilization and pregnancy

Fig. 20.2 The changes that occur during fertilization. For a sperm to fuse with an egg it must first penetrate the layer of granulosa cells and the zona pellucida, as shown in (a) and (b). Fusion with the egg leads to the formation of the fertilization membrane (c) and this is followed by the second meiotic division and the extrusion of the second polar body (d). Pt and P2 are the first and second polar bodies.

secreted by the zygote. By its luteotropic action, hCG is believed to maintain the corpus luteum beyond its normal life span so that it will continue to secrete the progesterone that is required for the continuation of pregnancy.

Structurally, hCG is very similar to anterior pituitary LH but has a longer half-life. It appears in the maternal circulation within a few days of fertilization and may be detected in the urine by about 2 weeks after ovulation. Levels of hCG then con­tinue to increase steadily up until 8-10 weeks of gestation before falling rather sharply over the next few weeks. This profile of secretion is illustrated in Fig. 20.3. After 6 or 8 weeks the placenta is well established (Section 20.4) and is able to produce sufficient progesterone itself to maintain the remaining gesta­tion period. The pregnancy is then said to be autonomous, and the fall in hCG secretion seen after about 8 weeks probably reflects this diminishing requirement for the hormones of the corpus luteum.

Clinically, hCG is a very important hormone, chiefly because of its early appearance in the maternal body fluids following fer­tilization. hCG can be detected in the maternal plasma as early as 7 days after ovulation. Its presence in the urine 2 weeks or so after ovulation is used as a reliable and simple test for pregnancy, indeed so simple that it can be carried out, using a kit, by a woman herself at home. More sophisticated assay techniques can also be used by clinicians to gain information about the preg­nancy. Levels of hormone above the normal range, for example, suggest the presence of twins, a situation that may be confirmed

Fig. 20.3 The changes in the plasma concentrations of human chorionic gonadotropin (hCG) and human placental lactogen (hPL) that occur during gestation.

later by ultrasound scans. In women who have suffered habitual miscarriages because of insensitivity of the corpus luteum to hCG, it may be useful to be able to detect the presence of an embryo within a few days of fertilization so that progesterone may be administered exogenously to prevent loss of the pregnancy when the corpus luteum regresses.

^ 20.4 The formation of the placenta
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