The Sexual Cycle

The sexual cycle averages 28 days in length, which is the basis for the timetable described in the following pages. It commonly varies from 20 to 45 days, however, so be aware that the timetable given in this discussion may differ from person to person and from month to month. As you study this cycle, bear in mind that hormones of the hypothalamus

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Development of the egg (oogenesis)

Mitosis

Mitosis

Multiplication of oogonia

2n Primary oocyte

Development of the follicle

Primordial follicle

(No change)

Before birth

Meiosis I

First polar body (dies)

If not fertilized

Adolescence to menopause n Secondary oocyte

Secondary oocyte (ovulated)

If fertilized U

Dies

Meiosis II

Second polar body (dies)

Zygote

Embryo

Granulosa cells

Antrum Oocyte

Granulosa cells

Primary follicle

Mature follicle

Ovulation

Corpus luteum n n n n

Figure 28.12 Oogenesis (left) and Corresponding Development of the Follicle (right).

and anterior pituitary gland regulate the ovaries, and hormones from the ovaries, in turn, regulate the uterus. That is, the basic hierarchy of control can be represented: hypothalamus ^ pituitary ^ ovaries ^ uterus. However, there is also feedback control from the ovaries to the hypothalamus and pituitary.

We begin with a brief preview of the sexual cycle as a whole. The cycle begins with a 2-week follicular phase. Menstruation occurs during the first 3 to 5 days, and then the uterus replaces the lost endometrial tissue by mitosis.

The ovarian follicles grow during this phase, and one of them ovulates around day 14. After ovulation, the remainder of the follicle becomes a body called the corpus luteum. Over the next 2 weeks, called the postovulatory phase, the corpus luteum stimulates endometrial secretion; and the endometrium thickens still more. If pregnancy does not occur, it breaks down again in the last 2 days. As loose tissue and blood accumulate, menstruation begins and the cycle starts over. This cycle is summarized in figure 28.13 and table 28.1.

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(a) Ovarian cycle

(a) Ovarian cycle

Developing follicle

Mature follicle

Corpus luteum

Involution Corpus albicans

New follicles tä

Days 1

Menstrual phase

Preovulatory phase

Follicular phase

HP TT1 U r2P r25 r271 r

Luteal phase

Premenstrual phase

Ovulation

Postovulatory phase

(b) Menstrual cycle

Estrogen

Menstrual fluid

Menstrual fluid ff ff

Days 1

Proliferative Phase And Secretory Phase

Proliferative phase Secretory phase

Premenstrual phase

Menstruation

Proliferative phase Secretory phase

Figure 28.13 The Female Sexual Cycle. (a) The ovarian cycle (events in the ovary); (b) the menstrual cycle (events in the uterus).

Premenstrual phase

The Ovarian Cycle

Now we can examine the ovarian cycle in more detail. We will see, step by step, what happens in the ovaries and in their relationship to the hypothalamus and pituitary gland.

The follicular phase of the cycle extends from the beginning of menstruation (day 1) to ovulation (day 14). This is the most variable part of the cycle and it is seldom possible to predict the date of ovulation reliably. The follicular phase is divided into the menstrual phase and preovulatory phase.

Menstrual Phase The menstrual phase is the period of time when a woman is discharging menstrual fluid (days 1-5 of the typical cycle). As menstruation is occurring, important developments are taking place in the ovaries:

1. Beginning around day 25 of the previous cycle

(3 days before menstruation begins), FSH secretion rises and stimulates 20 to 25 primary oocytes to begin meiosis I.

2. Meanwhile, the follicles around these oocytes develop. The primary oocyte is enclosed in a primordial follicle, which has a single layer of

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Table 28.1 Phases of the Female Sexual Cycle

Days Phase Major Events

1-14 Follicular Phase

1-5 Menstrual phase (menses) Menstruation occurring; FSH level high; primordial follicles developing into primary and then secondary follicles

6-13 Preovulatory (proliferative) phase Rapid growth of one follicle and atresia of the lagging follicles; drop in FSH level; endometrial regeneration and growth by cell proliferation; development of mature follicle; completion of meiosis I, producing secondary oocyte, which arrests at metaphase II; sharp rise in LH and estrogen levels

14 Ovulation Rupture of follicle and release of oocyte

15-28 Postovulatory Phase

15-26 Luteal (secretory) phase Formation of corpus luteum; secretion of progesterone; mucus and glycogen secretion by endometrium, causing endometrial thickening; later involution of corpus luteum and falling progesterone level

27-28 Premenstrual (ischemic) phase Endometrial ischemia and necrosis; sloughing of necrotic tissue from uterine wall, mixing with blood and forming menstrual fluid squamous follicular cells around the oocyte (see fig. 28.12). These cells enlarge and become cuboidal; the follicle is then known as a primary follicle. The cuboidal cells multiply and stratify, the follicle as a whole enlarges, and connective tissue condenses around it to form the theca29 folliculi (THEE-ca fol-IC-you-lye). Its outer layer, the theca externa, becomes a fibrous capsule. Its inner layer, the theca interna, secretes androgen, which the granulosa cells (see next paragraph) convert to estrogen.

3. In the first few days of the follicular phase (during menstruation), the follicular cells begin to secrete an estrogen-rich follicular fluid, which accumulates in little pools amid the cells. These pools soon merge and become a fluid-filled cavity, the antrum. The follicle is now called a secondary (antral) follicle (fig. 28.14) and the follicular cells lining it are called granulosa cells. A mound of granulosa cells called the cumulus oophorus30 covers the oocyte and secures it to the follicle wall. The granulosa cells secrete a clear layer of gel called the zona pellucida31 between themselves and the oocyte. The innermost layer of cells in contact with the zona pellucida is the corona radiata.32 This is the state of development when menstruation ceases around day 5.

29theca = box,case

30cumulus = little mound + oo = egg + phor = to carry

31 zona = zone + pellucid = clear, transparent

32corona = crown + radiata = radiating

Preovulatory Phase The preovulatory phase spans days 6 to 14 of the average cycle—from the end of menstruation to the day of ovulation. Its major developments are as follows:

4. The growing follicle secretes increasing amounts of estrogen, which has two seemingly contradictory effects: It reduces FSH secretion by the pituitary, and it makes the follicle more sensitive to FSH. The latter effect comes about as estrogen stimulates the granulosa cells of its own follicle to produce an increasing number of FSH receptors. FSH in turn stimulates this follicle to produce still more estrogen, completing a positive feedback loop. Overall, the most advanced follicle reduces the FSH supply to other follicles while at the same time it makes itself more sensitive to the FSH that remains.

5. The less developed, less sensitive follicles undergo atresia, while the most developed follicle attains a diameter of up to 2.5 cm. This follicle, called a mature (graafian33) follicle, protrudes from the surface of the ovary like a blister.

6. As the follicle matures, the primary oocyte completes meiosis I and becomes a secondary oocyte. This cell begins meiosis II but stops at metaphase II. It is now ready for ovulation.

7. FSH and estrogen also stimulate the maturing follicle to produce LH receptors, which are important to the next phase of the cycle.

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Before OvulationOvary Just Before Ovulation
Figure 28.14 Ovarian Follicles. (a) Note the very thin layer of squamous cells around the oocyte in a primordial follicle, and the single layer of cuboidal cells in a primary follicle. (b) A mature (graafian) follicle. Just before ovulation, this follicle will grow to as much as 2.5 cm in diameter.

Ovulation Ovulation, the release of an oocyte, typically occurs on day 14, the midpoint of the average cycle. It takes only 2 or 3 minutes. The events immediately leading up to and including ovulation are as follows:

8. In the last day or two of the preovulatory phase, the estrogen level is very high. This estrogen stimulates the anterior pituitary to secrete LH and the hypothalamus to secrete GnRH. GnRH further induces a surge in FSH and LH secretion by the pituitary (fig. 28.15). The FSH level therefore rises in the last day or two before ovulation, but the LH level rises even more markedly (see fig. 28.13).

9. The uterine tube becomes edematous, its fimbriae envelop and caress the ovary in time with the woman's heartbeat, and its cilia create a gentle current in the nearby peritoneal fluid—all in preparation for receiving the oocyte.

10. LH increases blood flow in the follicle. More serous fluid filters from the capillaries into the antrum and causes the follicle to swell rapidly. Meanwhile, LH also stimulates the theca interna to secrete collagenase, an enzyme that weakens the ovarian wall over the swelling follicle.

11. A nipplelike stigma appears on the ovarian surface over the follicle. Follicular fluid seeps from the

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GnRH

GnRH

Oogenesis Gnrh Fsh

oocyte

Figure 28.15 Control of Ovulation by Hormones of the Pituitary-Ovarian Axis. (1) The maturing follicle secretes high levels of estrogen, which stimulates the hypothalamus and anterior pituitary. (2) The hypothalamus secretes GnRH. (3) In response to estrogen and GnRH, the anterior pituitary secretes LH. (4) LH triggers ovulation.

oocyte

Figure 28.15 Control of Ovulation by Hormones of the Pituitary-Ovarian Axis. (1) The maturing follicle secretes high levels of estrogen, which stimulates the hypothalamus and anterior pituitary. (2) The hypothalamus secretes GnRH. (3) In response to estrogen and GnRH, the anterior pituitary secretes LH. (4) LH triggers ovulation.

stigma for 1 or 2 minutes, and then the follicle ruptures. The remaining follicular fluid oozes out, carrying the oocyte and the surrounding cells of the corona radiata (fig. 28.16).

12. The oocyte and its attendant cells are normally swept up by the ciliary current and taken into the uterine tube, although many oocytes fall into the pelvic cavity and die.

Insight 28.3 Clinical Application

Signs of Ovulation

If a couple is attempting to conceive a child or to avoid pregnancy, it is important to be able to tell when ovulation occurs. The signs are subtle but detectable. For one, the cervical mucus becomes thinner and more stretchy. Also, the resting body temperature (basal temperature) rises 0.2° to 0.3°C (0.4°-0.6°F). This is best measured first thing in the morning, before rising from bed; the change can be detected if basal temperatures are recorded for several days before ovulation in order to see the difference. The LH surge that occurs about 24 hours before ovulation can be detected with a home testing kit. Finally, some women experience twinges of ovarian pain known by the German name, mittelschmerz,34 which last from a few hours to a day or so at the time of ovulation. The most likely time to become pregnant is within 24 hours after the cervical mucus changes consistency and the basal temperature rises.

Uterine tube Fimbriae

Uterine tube Fimbriae

Severe Mittelschmerz
Figure 28.16 Ovulation of a Human Follicle, Viewed by Endoscopy.

_Think About It_

In chapter 17, review the concepts of up-regulation and the permissive effect in hormone interactions; explain their relevance to the ovarian cycle.

Postovulatory Phase The postovulatory phase extends from days 15 to 28, from ovulation to the beginning of menstruation. This phase of the sexual cycle is the most predictable in length. Its first 12 days are called the luteal phase and its last 2 days are the premenstrual phase. The major developments in this phase, assuming pregnancy does not occur, are as follows:

13. When the follicle expels the oocyte, it collapses and bleeds into the antrum. As the clotted blood is slowly absorbed, granulosa and theca interna cells multiply and fill the antrum, and a dense bed of blood capillaries grows amid them. The ovulated follicle has now become a structure called the corpus luteum,35 named for a yellow lipid that accumulates in the theca interna cells. These cells are now called lutein cells.

14. The anterior pituitary continues to secrete LH, which regulates the further growth and activity of the corpus luteum. For this reason, LH is also called luteotropic hormone.

15. The lutein cells produce mainly androgen, which the granulosa cells convert to progesterone and a smaller amount of estrogen. Progesterone stimulates developments in the uterus that we survey shortly.

34mittel = in the middle + schmerz = pain

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16. The lutein cells also secrete inhibin, which suppresses FSH secretion and prevents new follicles from developing.

17. For a time, the corpus luteum grows and secretes more and more progesterone. But while progesterone stimulates uterine development, it also seals the fate of the corpus luteum, because it inhibits the secretion of FSH and LH. When the LH level falls critically low, the corpus luteum involutes, or atrophies. Involution, occurring from days 24 through 26, results in declining progesterone secretion, and the lack of progesterone brings about menstruation. By day 26 or so, involution is complete and the corpus luteum has become an inactive scar, the corpus albicans.36 If pregnancy occurs, the corpus luteum remains active for about 3 months.

18. Involution of the corpus luteum also ends its negative feedback inhibition of the hypothalamus. The hypothalamus therefore begins to secrete GnRH anew, the anterior pituitary secretes FSH in response, and a new crop of follicles begins to develop.

The ovarian cycle has now come full circle to the point where we began. We now go on to see how the foregoing events are correlated with changes in the uterus—that is, how the ovarian cycle regulates the menstrual cycle.

The Menstrual Cycle

The menstrual cycle consists of a buildup of the endo-metrium through most of the sexual cycle, followed by its breakdown and vaginal discharge. The menstrual cycle is divided into a menstrual phase, proliferative phase, secretory phase, and premenstrual phase, in that order. The menstrual phase averages 5 days long, and the first day of noticeable vaginal discharge is defined as day 1 of the sexual cycle. The reason for menstruation is best understood after you become acquainted with the buildup of endome-trial tissue that precedes it, so we begin our survey of the cycle with the proliferative phase.

Proliferative Phase The proliferative phase is a time of rebuilding of endometrial tissue lost at the last menstruation. At the end of menstruation, around day 5, the endometrium is about 0.5 mm thick and consists only of the stratum basalis. The stratum functionalis is rebuilt by mitosis from day 6 to day 14. The principal processes in this phase are:

1. Estrogen from the ovaries stimulates mitosis in the stratum basalis as well as the prolific regrowth of blood vessels. By day 14, the endometrium is about 2 to 3 mm thick.

2. Estrogen also stimulates the endometrium to produce progesterone receptors, thereby preparing it for the progesterone-dominated secretory phase.

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Secretory Phase The secretory phase is a period of further endometrial thickening, but this results from secretion and fluid accumulation rather than mitosis. It extends from day 15 (after ovulation) to day 26 of a typical cycle. The principal processes in this phase are:

3. After ovulation, the corpus luteum secretes mainly progesterone. Progesterone stimulates the endometrial glands and cells of the stroma to accumulate glycogen. The glands grow wider, longer, and more coiled and secrete a glycogen-rich fluid into the lumen. The lamina propria swells with tissue fluid.

4. By the end of the secretory phase, the endometrium is about 5 to 6 mm thick—a soft, wet, nutritious bed available for embryonic development in the event of pregnancy.

Premenstrual Phase The premenstrual phase is a period of endometrial degeneration occurring in the last 2 days or so of the menstrual cycle.

5. As we have seen, in the absence of pregnancy, the corpus luteum atrophies and the progesterone level falls sharply. In the absence of progesterone, the spiral arteries of the endometrium exhibit spasmodic contractions that cause endometrial ischemia (interrupted blood flow). The premenstrual phase is therefore also called the ischemic (iss-KEE-mic) phase.

6. Ischemia leads to tissue necrosis. As the endometrial glands, stroma, and blood vessels degenerate, pools of blood accumulate in the stratum functionalis.

7. Necrotic endometrium falls away from the uterine wall, mixes with blood in the lumen, and forms the menstrual fluid.

Menstrual Phase The menstrual phase (menses) is the period in which blood, serous fluid, and degenerated endometrial tissue are discharged from the vagina. It commences when enough menstrual fluid accumulates in the uterus. The first day of external discharge marks day 1 of a new cycle. The average woman discharges about 40 mL of blood and 35 mL of serous fluid over a 5-day period. Menstrual fluid contains fibrinolysin, so it does not clot. The vaginal discharge of clotted blood may indicate uterine pathology rather than normal menstruation.

In summary, the ovaries go through a follicular phase characterized by growing follicles; then ovulation; and then a postovulatory (mostly luteal) phase dominated by the corpus luteum. The uterus, in the meantime, goes through a menstrual phase in which it discharges its stratum func-tionalis; then a proliferative phase in which it replaces that tissue by mitosis; then a secretory phase in which the endometrium thickens by the accumulation of secretions; and finally, a premenstrual (ischemic) phase in which the stratum functionalis breaks down again. The first half of the cycle is governed largely by follicle-stimulating hormone

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(FSH) from the pituitary gland and estrogen from the ovaries. Ovulation is triggered by luteinizing hormone (LH) from the pituitary, and the second half of the cycle is governed mainly by LH and progesterone, the latter secreted by the ovaries.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

8. Name the sequence of cell types in oogenesis and identify the ways oogenesis differs from spermatogenesis.

9. Distinguish between a primordial, primary, and secondary follicle. Describe the major structures of a mature follicle.

10. Describe what happens in the ovary during the follicular and postovulatory phases.

11. Describe what happens in the uterus during the menstrual, proliferative, secretory, and premenstrual phases.

12. Describe the effects of FSH and LH on the ovary.

13. Describe the effects of estrogen and progesterone on the uterus, hypothalamus, and anterior pituitary.

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