Human Physiology, 14th edition (2016)

734 Chapter 20

20.6 FERTILIZATION, PREGNANCY,

AND PARTURITION

Once fertilization has occurred, the secondary oocyte com-

pletes meiotic division. It then undergoes mitosis and forms

an early embryonic structure called a blastocyst. Cells of

the blastocyst secrete human chorionic gonadotropin, a

hormone that maintains the mother’s corpus luteum and

prevents menstruation. Birth is dependent upon strong con-

tractions of the uterus, which are stimulated by oxytocin.

Menopause

The term menopause means literally “pause in the menses”
and refers to the cessation of ovarian activity and menstruation
that occurs at about the age of 50. During the postmenopausal
years, which account for about a third of a woman’s life span,
the ovaries are depleted of follicles and stop secreting estra-
diol and inhibin. The fall in estradiol is due to changes in the
ovaries, not in the pituitary; indeed, FSH and LH secretion by
the pituitary is elevated because of a lack of negative feedback
from estradiol and inhibin.
The only estrogen found in the blood of postmenopausal
women is the weak estrogen estrone, formed by the mesenchymal
cells in adipose tissue. Estrone is formed from weak androgens,
such as androstenedione and dehydroepiandrosterone (DHEA),
secreted from the adrenal cortex. Because adipose tissue is the
only source of estrogen, postmenopausal women who have more
adipose tissue have higher levels of estrogen and less propensity
toward osteoporosis.
It is the withdrawal of estradiol secretion from the ovaries
that is most responsible for the many symptoms of menopause.
These include vasomotor disturbances and urogenital atrophy.
Vasomotor disturbances produce the “hot flashes” of meno-
pause, where a fall in core body temperature is followed by feel-
ings of heat and profuse perspiration. Atrophy of the urethra,
vaginal wall, and vaginal glands occurs, with loss of lubrication.
There is also increased risk of atherosclerotic cardiovascular dis-
ease and increased progression of osteoporosis.

Figure 20.36 Changes in basal body temperature during the menstrual cycle. Such changes can be used in the
rhythm method of birth control.

• 14
  0

37.0

36.6

36.2

Oral

temperature (

°C)

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  • 10
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14

+ 2

16

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18

+ 6

20

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28

Days from LH peak

Cycle day

| CHECKPOINT

10a. Describe the changes that occur in the ovary and

endometrium during the follicular phase and explain

how these changes are regulated by hormones.

10b. Describe the formation, function, and fate of the

corpus luteum. Also, describe the changes that

occur in the endometrium during the luteal phase.

11a. Describe the hormonal regulation of ovulation.

11b. Explain the significance of negative feedback control

during the luteal phase and describe the hormonal

control of menstruation.

LEARNING OUTCOMES

After studying this section, you should be able to:

12. Describe the process of fertilization and blastocyst
    formation, and the significance of hCG.


13. Describe the formation and functions of the
    placenta.


14. Explain the mechanisms that promote parturition.


15. Explain the hormonal control of lactation.

Sperm are stored in the epididymis, where they are fully devel-

oped yet incapable of fertilization. This is largely because they are

kept in a slightly acidic state, with a cytoplasmic pH below 6.5.

During the act of sexual intercourse, the male ejaculates an average

of 300 million sperm into the vagina of the female. This tremen-

dous number is needed because of the high sperm fatality—only

about 100 survive to enter each fallopian tube. During their passage

through the female reproductive tract, about 10% of the sperm gain

the ability to fertilize an ovum; this ability is called capacitation.

Capacitation requires several hours in the female reproduc-

tive tract and involves a number of chemical changes in the sperm,

including increases in pH, Ca^2 1 concentration, and cAMP. The

swimming ability of sperm is suppressed by low cytoplasmic pH

until the sperm enter the alkaline female reproductive tract (for

example, the cervical mucus can have a pH of 9). This alkalin-

ity, together with the extrusion of H^1 (in exchange for Na^1 ) from

the sperm, raises the pH of sperm cytoplasm, and the increased

pH activates dynein in the flagellum. Dynein proteins are ATPase

molecular motors (chapter 3, section 3.2) in the flagellum that pro-

duce beating movements, allowing the sperm to swim. The rise