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activity of the ovaries, it is known that a variety of other factors, both neural and hormonal, may affect the control of fertility.

The reproductive behavior of many farm, domestic, and labo­ratory animals illustrates how environmental stimuli mediated by the CNS, such as olfactory, tactile, coital, and visual cues, play a role in regulating gonadal function. For example, the fertile periods of certain animals, such as sheep, appear to be controlled by the relative lengths of day and night. Others, such as cats and rabbits, are reflex ovulators, i.e. they ovulate in response to coitus. Such regulatory mechanisms must be medi­ated via the CNS, the afferent information being integrated in the hypothalamus in order to control the output of GnRH and thus of the gonadotropins.

While human gonadal function is clearly not subjected to external control mechanisms in such a rigid fashion, the poss­ibility of neurally mediated influences on gonadotropin release cannot be ruled out. Indeed, the wealth of neural inputs to the GnRH-secreting neurons of the hypothalamus would argue strongly in favor of such mechanisms. It is certainly well estab­lished that factors such as anxiety and emotional stress can upset cyclical ovarian activity and fertility.

Physiologically, the role of prolactin in the normal menstrual cycle is unclear, but clinically it is well known that oversecretion of prolactin (hyperprolactinemia), is a common cause of female infertility. The condition may be physiological, as in lactating women (see Chapter 22), or pathological, arising from a pitu­itary tumor. It is often associated with anovular cycles (cycles in which ovulation fails to occur) or a complete loss of cyclical ovarian activity. High levels of prolactin seem to impair the response of the anterior pituitary to GnRH so that LH surges are lost and ovulation fails to take place.

Summary

  1. Cyclical variations in the levels of steroid and gonadotropic hormones
    act together to ensure the regular release of mature ova and to prepare
    the body for fertilization and pregnancy.

  2. While the gonadotropins control ovarian function, the estrogens and
    progesterone secreted by the ovaries regulate the secretion of
    pituitary FSH and LH by both negative and positive feedback.

  3. Very high levels of estrogens stimulate the anterior pituitary to
    initiate an LH surge, which is crucial to the events of the pre-
    ovulatory stage and to ovulation itself. For the remainder of the cycle,
    negative feedback prevails, and gonadotropin output is relatively low.

  4. The menstrual cycle appears to be sensitive to both neural and hor­
    monal influences in addition to the fundamental regulation exerted
    by the interactions between the pituitary gonadotropins and the
    ovarian steroids.

  5. A variety of emotional and physical factors, mediated via neural
    inputs to the hypothalamus, can influence cyclicity.

  6. Prolactin, another anterior pituitary hormone, appears to inhibit
    ovulation.

19.10 Activation and regression of the gonads— puberty and the menopause

Menarche and the menopause

In the female, the fertile years are defined by two events. These are the onset of menstruation at puberty (menarche) and the cessa­tion of cyclical ovarian activity which occurs at around the age of 50 (the menopause or climacteric).

Puberty is a collective term which includes the variety of changes taking place within the body of an adolescent girl as her ovaries mature. Menarche, the onset of menstruation, is the outward sign that these changes have taken place and that cyclical secretion of ovarian steroids has begun. The other changes that take place during the 2 or 3 years preceding menarche include the adolescent growth spurt, the development of secondary sexual characteristics (pubic hair and mammary development), and changes in body composition—adult females have about twice as much body fat as males and a smaller mass of skeletal muscle.

Circulating levels of the pituitary gonadotropins, FSH and LH, rise gradually up to the age of around 10 years. After this, with the approach of menarche, pulsatile release is established, with spurts of secretion during sleep. With this increase in gonado­tropin rhere is also a rise in the output of ovarian estrogens and, under the influence of these steroids, budding of the breasts occurs. This is usually the first outward physical sign of puberty.

From 2 to 3 years prior to the onset of menstruation, andro-gen secretion from the adrenal cortex is increased (adrenarche) and these hormones are important in the stimulation of pubic hair growth. It has also been suggested that androgen secretion plays a part in the control of menarche, although no clear link has been established. What is clear, however, is that the increas­ing synthesis and secretion of FSH and LH eventually results in the onset of menstruation, although the first few cycles are usually anovular and progesterone is not produced in large amounts. Bleeding therefore tends to be light and to occur irregularly at first.

The exact trigger for menstruation is still not fully under­stood, but various suggestions have been made. The ovaries might become more sensitive to gonadotropins, or the anterior pituitary may become more sensitive to the positive feedback effects of estrogens.

The average age for menarche in the United Kingdom is around 12 years, with the first ovulatory cycles occurring 6-9 months later. The range of normality, however, extends from 10 to 16 years of age (Table 19-1). There has been a trend towards earlier menarche in the past 150 years, possibly as a result of improved health care and nutrition. The latter may be particularly imporrant since it is thought that menarche requires the attainment of either a critical body mass (around 47 kg) or possibly a critical ratio of fat to lean mass. It is certainly true that regular menstrual cycles are disrupted in girls who lose

454

19 The physiology of the male and female reproductive systems


Table 19.1 Summary of principal changes during puberty



Characteristic

Age range of

Principal hormones




first appearance

responsible




(years)

for development

Girls







Breast bud

8-13

Estrogens, progesterone,







GH

Pubic hair

8-14

Adrenal androgens

Menarche

10-16

Estrogens, progesterone

Growth spurt

10-14

Estrogens, GH

Boys







Growth of testis

10-14

Testosterone, FSH, GH

Growth of penis

11-15

Testosterone

Pubic hair

10-15

Testosterone

Facial and axillary

12-17

Testosterone

hair







Enlargement of

11-16

Testosterone

larynx







Growth spurt and

12-16

Testosterone, GH

male pattern of







skeletomuscular







development







large amounts of weight either through anorexia, excessive exercise, or starvation.

The cessation of menstrual cycles (the climacteric)

The menopause or climacteric marks the end of a woman's fertile years. It is the progressive failure of the reproductive system and usually occurs between the ages of 45 and 55. The numbers of oocytes in the ovaries have been depleted by atresia and the ovarian responsiveness to gonadotropins declines. Cycles often become anovular and irregular before ceasing altogether. Pituitary FSH and LH levels are high in postmenopausal women because of the loss of negative feedback inhibition by estrogen, though LH surges are no longer seen.

Many somatic and emotional changes accompany the loss of ovarian steroids. The uterine muscle becomes fibrous, the vagina may become dry, and there is a loss of breast tissue. Depression, night sweats, hot flushes, and an increased susceptibility to myocardial infarction also accompany the menopause and there is often an increase in bone weakness due to increased bone resorption. Most of these changes are attributable to the loss of ovarian estrogen and can be treated successfully by hormone replacement therapy (HRT) should they be sufficiently serious to warrant medical intervention.

Puberty in the male

Testosterone is the key to reproductive function in the male. It plays a crucial role in sexual differentiation during embryonic life (Chapter 20) and its concentration in the plasma rises at the onset of puberty to reach adult levels by about 17 years of age.

Between early infancy and the start of puberty, testosterone secretion by the testes is low. Pituitary gonadotropin levels are also low. During the years between 10 and 16 (on average), boys develop their full reproductive capability. At the same time they acquire secondary sexual characteristics and adult musculature and undergo a linear growth spurt which is halted by closure of the epiphyses when adult height is reached (see also Table 19-1). The first endocrinological event of puberty is an increase in the secretion of pituitary LH. As a result there is maturation of the Leydig cells and the initiation of spermatogenesis. Testosterone production is also enhanced and this hormone is responsible for the anatomic changes that are characteristic of puberty. These include enlargement of the testis, growth of pubic hair, starting at the base of the penis, reddening and wrin­kling of the scrotal sac, and, later on, an increase in size of the penis. Facial hair begins to appear, the scalp hair takes on the masculine pattern, and there is deepening of the voice due to thickening of the vocal cords and enlargement of the larynx. These maturational changes take place over a period of several years and Table 19.1 shows the average timing of the major events that occur during male puberty.

Is there a male menopause?

While there is no obvious event marking the end of reproductive capacity in the male comparable to the female menopause, sperm production does decline between the ages of 50 and 80. There is also a reduction in plasma testosterone levels in men over 70 and a parallel increase in plasma levels of FSH and LH, although this is much less marked than in the female. Nevertheless, these changes are relatively small and many elderly men maintain active sex lives and retain their reproductive capacity.

Summary

  1. In the female, the fertile years are denned by menarche and the
    menopause, the commencement and cessation of cyclical ovarian
    activity, occurring at around 12 and 50 years respectively.

  2. Many changes take place within the body of an adolescent girl in
    addition to the onset of menstrual cycles. These include a growth
    spurt and the developmenr of secondary sexual characteristics.

  3. FSH and LH are secreted in increasing amounts prior to menarche
    but the exact trigger for the onset of cyclical ovarian activity is
    unclear.

4. The menopause marks the progressive failure of the reproductive
system and is due to depletion of the oocyte pool by atresia and a
reduced ovarian responsiveness to pituitary gonadotropins. Many
somatic and emotional changes accompany the loss of ovarian
steroids.

  1. Between 10 and 16 years of age, boys show a growth spurr and
    develop their full reproductive capacity.

  2. Leydig cells mature under the influence of pituitary LH and start
    to produce sperm. Testosterone output rises and this hormone is
    responsible for the development of secondary sexual characteristics.

Answers

455


Recommended reading

Anatomy

MacKinnon, P. С. В. and Morris, J. F. (1988). Oxford textbook of functional anatomy, Vol. 2, pp. 99—109. Oxford University Press, Oxford.

Cell biology of germ cells

Alberts, В., Bray, D., Lewis, J., Raff, M., Roberts, K., and Watson, J. D. (1994). Molecular biology of the cell, (3rd edn), Chapter 20. Garland, New York.

Histology

Junquieira, L. C, Carneiro, J., and Kelley, R. O. (1995). Basic histology, (8th edn), Chapters 22 and 23. Prentice-Hall, London.

Physiology

Case, R. M. and Waterhouse, J. M. (ed.) (1994). Human physiology: age,

stress and the environment. (2nd edn) Chapters 3 and 4. Oxford Science

Publications, Oxford. Ferin, M., Jewckwicz, R., and Warres, M. (1993). The menstrual cycle.

Oxford University Press, Oxford. Griffin, N. E. and Ojeda, S. R. (1992). Textbook of endocrine physiology,

(2nd edn), Oxford University Press, Oxford. Johnson, M. H. and Everitt, B. J. (1995). Essential reproduction, (4th

edn), Chapters 3—7. Blackwell Scientific, Oxford.

^ Self-test questions

Each statement is either true or false. The answers are given below.

1. The following statements apply to sexual reproduction in
humans:

a. The male gametes are known as sperm;

b. A mature sperm contains a full complement of chromo­
somes;

с At birth an ovary contains all the oocytes it will ever have;

d. Primary and secondary spermatocytes divide by mitosis to give rise to spermatids.

2. In the testis:

a. Leydig cells secrete testosterone;

b. Testosterone synthesis requires both FSH and LH;

с Sertoli cells prevent free diffusion of water-soluble substances between the seminiferous tubules and the blood;

d. Sertoli cells respond to testosterone by synthesizing
androgen-binding protein;

e. Sertoli cells line the seminiferous tubules and directly
give rise to the developing sperm.

3. During the ovarian cycle:

a. The initiation of the preantral phase of follicular development is under the control of LH;

b. The development of the antral follicle depends on the expression of receptors for FSH and LH;

с Ovulation occurs after about 14 days;

d. Ovulation occurs in response to a sudden increase in
plasma LH;

e. The period before ovulation is known as the luteal
phase;

f. The luteal phase is associated with a large increase in
plasma progesterone;

g. The myometrium proliferates under the influence of
estrogens.

4. The following questions relate to the hormonal control of the menstrual cycle:

a. Estrogens are synthesized mainly by the cells of the
theca interna;

b. In the absence of receptors for FSH and LH, preantral
follicles undergo atresia;

с After ovulation, the ruptured follicle is converted into a corpus albicans which secretes progesterone;

d. Progesterone promotes full development of the
endometrium;

e. If no egg is fertilized, progesterone levels fall and this is
the trigger for menstruation.

Answers

1. During spermatogenesis, the number of chromosomes
is halved by meiosis. The primary spermatocytes undergo
two meiotic divisions to give rise to spermatids. As a result,
mature sperm are haploid. This is also true of a mature ovum
which undergoes meiosis prior to ovulation. Following fer­
tilization the full complement of chromosomes is restored.

a. True;

b. False;
с True;
d. False.

2. Sertoli cells synthesize androgen-binding protein in
response to FSH. Testosterone binds to the androgen-
binding protein and the hormone-protein complex
enables the Sertoli cells to maintain the production of
spermatozoa. While the spermatozoa are nurtured by the
Sertoli cells until they develop into mature sperm, they are
derived from germ cells known as spermatogonia which lie
within the basal compartment of the seminiferous tubules.

a. True;

b. True;
с True;

d. False;

e. False.

456 19 The physiology of the male and female reproductive systems

The initial recruitment of primordial follicles into the preantral phase is hormone independent. During the pre-antral phase the follicular cells acquire receptors for various hormones. The granulosa cells develop receptors for FSH and estrogen while the thecal cells acquire receptors for LH. The period before ovulation is known as the follicular phase, while that after ovulation is called the luteal phase.

a. False;

b. True;
с True;

d. True;

e. False;

f. True;

g. True.

The cells of the theca interna secrete mainly androgens which are converted to estrogens by the granulosa cells in response to FSH. After ovulation the follicle collapses around a blood clot to form a corpus luteum. If no preg­nancy occurs, the corpus luteum regresses to form a corpus albicans which has no secretory role.

a. False;

b. True;
с False;

d. True;

e. True.
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