Bio 3201 Unit 1B part 2

Regulation of the Reproductive system.

The chromosomal sex, or genetic sex, of an individual is determined at fertilization. In humans

and other mammals, an embryo that carries the sex chromosome combination XY is genetically

male. An embryo that carries the sex chromosome combination XX is genetically female.

The chromosomal sex remains constant throughout the life of an individual the presence or

absence of specific hormones determines which sex organs develop, when the sex organs

mature, and how they function.

The ovaries and testes are ultimately endocrine glands that regulate reproduction and are

controlled by the anterior pituitary of the brain along with the hypothalamus.

Sex Hormones and the Male Reproductive System

The male sex hormones are also known as androgens. The presence of androgens initiates the

development of male sex organs and ducts in the fetus.

Puberty begins when the hypothalamus increases its production of gonadotropin releasing

hormone (GnRH). GnRH acts on the anterior pituitary gland, causing it to release two different

sex hormones: follicle stimulating hormone (FSH) and luteinizing hormone (LH).

In males, these hormones cause the testes to begin producing sperm and to release

testosterone. Testosterone acts on various tissues to complete the development of the sex

organs and sexual characteristics.

1. Gonadotropin releasing hormone (GnRH): Stimulates the release of FSH and LH from

the anterior pituitary

2. Inhibin: hormone that acts on the hypothalamus to slow down the production of

release factors that trigger FSH secretion. Inhibin acts on the anterior pituitary to inhibit

the production of FSH. The result is a negative feedback loop. As the level of FSH drops,

the testes release less inhibin. A decrease in the level of inhibin causes the anterior

pituitary to release more FSH. This feedback loop keeps the level of sperm production

relatively constant over time.

3. Follicle-stimulating hormone (FSH) – in males, FSH increases sperm formation and

Stimulates the development of the sex organs (released by the anterior pituitary)

4. Luteinizing hormone (LH) – produced by the anterior pituitary; LH stimulates interstitial

cells to produce testosterone

5. Testosterone – steroid hormone produced in the testes that is responsible for

secondary sex characteristics in males, such as:

- Enlargement of primary sex characteristics

- Enlargement of larynx (Adam’s apple)

- inhibiting fat storage and increasing muscle development

- Formation of hair on chest, arms, face, genitals

High levels of testosterone in the blood inhibits the production of LH. This feedback loop

keeps the testosterone level relatively constant in the body.

GnRH from the hypothalamus stimulates the anterior

pituitary to release LH and FSH. LH and FSH act on

the testes to trigger sperm production and the

release of testosterone and inhibin.

Testosterone and inhibin exert regulatory control

over the anterior pituitary and hypothalamus.

Aging and the male reproductive system.

A man in good health can remain fertile for his entire life. Even so, most men experience a

gradual decline in their testosterone level beginning around age 40.

This condition is called andropause. In some men, the hormonal change may be linked to

symptoms such as fatigue, depression, loss of muscle and bone mass, and a drop in sperm

production.

Other hormonal changes associated with aging can also affect the male reproductive system.

For example, the prostate gland often begins to grow gradually in men over age 40. This can

lead to discomfort and urinary difficulties, because the prostate squeezes on the urethra as it

grows. Older men have an increased risk of cancer of the prostate gland, as well.

Note: Substances that interfere with the hormonal feedback system can cause changes in the

reproductive system. For example, anabolic steroids mimic the action of testosterone in

promoting muscle development. For this reason, some athletes illegally use steroids to increase

their speed or strength. Steroids, however, also disrupt the reproductive hormone feedback

systems. The side effects of steroid use in men may include shrinking testicles, low sperm count,

loss of body hair, and the development of breasts.

Sex Hormones and the Female Reproductive System

North American girls usually begin puberty between 9 and 13 years of age. The basic hormones

and hormonal processes of female puberty are similar to those of male puberty.

A girl begins puberty when the hypothalamus increases its production of GnRH. This hormone

acts on the anterior pituitary to trigger the release of LH and FSH.

In girls, FSH and LH act on the ovaries to produce the female sex hormones estrogen and

progesterone. These hormones stimulate the development of the female secondary sex

characteristics and launch a reproductive cycle that will continue until about middle age.

1. Gonadotropin releasing hormone (GnRH): Stimulates the release of FSH and LH from

the anterior pituitary

2. Follicle stimulating hormone (FSH) – in females, FSH is responsible for the follicles

developing into eggs and secreting estrogen

3. Luteinizing hormone (LH) – in females, LH triggers the release of an ovum from the

follicle

4. Estrogen – produced by the follicles; responsible for the thickening of the endometrium

and keeps FSH secretions down (to prevent further follicles from developing)

5. Progesterone – sex hormone that is produced in females, first by the corpus luteum and

the ovary to prepare the uterus for fertilization, and later by the placenta to maintain

pregnancy

The Menstrual cycle

In humans, female reproductive function follows a cyclical pattern known as the menstrual

cycle.

During each month, the endometrium thickens as it prepares to receive a zygote. The zygote

implants itself in the endometrium, and development of the embryo begins. If the egg is not

fertilized, it does not implant in the endometrium. The endometrium disintegrates, and its

tissues and blood flow out the vagina in a process known as menstruation.

The menstrual cycle is usually about 28 days long, although it may vary considerably from one

woman to the next, and even from one cycle to the next in the same woman.

The menstrual cycle is actually two separate but interconnected cycles of events. One cycle

takes place in the ovaries and is known as the ovarian cycle.

The other cycle takes place in the uterus and is known as the uterine cycle.

Both cycles are controlled by the female sex hormones estrogen and progesterone, which are

produced by the ovaries.

Ovarian cycle. - The ovarian cycle can be roughly divided into four stages.

Follicular stage. (10-14 days) Begins with an increase in the level of FSH released by the

anterior pituitary gland.

• FSH stimulates one follicle to mature. As the follicle matures, it releases estrogen and

some progesterone.

• Increased estrogen levels stimulate the endometrium to thicken with blood, and stimulate LH

secretion and inhibit further FSH secretion

• The rising level of estrogen in the blood acts on the anterior pituitary to inhibit the

release of FSH and triggers a sudden release of GnRH from the hypothalamus. This leads

to a sharp increase in LH production by the anterior pituitary triggering ovulation

Ovulation (1day) marks the end of the follicular stage and the beginning of the second stage.

• Egg is released by a follicle due to the release of LH.

• Follicle changes to become corpus luteum

Corpus luteum: body that develops from a follicle that has been emptied of its egg; produces

progesterone

Luteal stage. (Days 14-28) LH causes the follicle to develop into a corpus luteum.

• The corpus luteum secretes progesterone and some estrogen. (As the levels of these

hormones rise in the blood, they act on the anterior pituitary to inhibit FSH and LH

production.)

• As the corpus luteum degenerates, this leads to a decrease in the levels of estrogen and

progesterone. The low levels of these sex hormones in the blood cause the anterior

pituitary to increase its secretion of FSH, and the cycle begins again.

Menstruation (3-5 days)

• As LH levels drop, corpus luteum degenerates causing progesterone levels to drop

• The uterine lining is not maintained and the blood and unfertilized egg are passed through the

vagina

The hypothalamus produces GnRH, which stimulates the

anterior pituitary to produce FSH and LH. FSH stimulates

the follicle to produce estrogen. LH stimulates the corpus

luteum to produce progesterone.

As the levels of these hormones rise in the blood, they act

on the anterior pituitary to inhibit FSH and LH production.

The corpus luteum degenerates, leading to a decrease in

the levels of estrogen and progesterone. The low levels of

these sex hormones in the blood cause the anterior

pituitary to increase its secretion of FSH, and the cycle

begins again.

If the ovum is fertilized and implants in the endometrium, blood hormone levels of progesterone

and estrogen remain high under stimulus of a hormone called human chorionic gonadotropin

(hCG), which is released by embryo-supporting membranes. The continued presence of

progesterone maintains the endometrium to support the developing fetus. The continued

presence of estrogen stops the ovarian cycle so no additional follicles mature.

The Uterine Cycle

The timing of the uterine cycle ensures that the uterus is prepared to receive and nurture a new

life. The events of the uterine cycle cause a buildup of blood vessels and tissues in the

endometrium.

The uterine cycle begins on the first day of menstruation (which is also the first day of the

ovarian cycle). On this day, the corpus luteum has degenerated and the levels of the sex

hormones in the blood are low.

Beginning around the sixth day of the uterine cycle, the estrogen level is high enough to cause

the endometrium to begin thickening. After ovulation, the release of progesterone by the

corpus luteum causes a more rapid thickening of the endometrium. Between days 15 and 23 of

the cycle, the thickness of the endometrium may double or even triple.

If fertilization does not occur, the corpus luteum degenerates. The levels of the sex hormones

drop, the endometrium breaks down, and menstruation begins again.

Investigation 13.B Page 502.

Aging and the Menstrual Cycle

The number of functioning follicles in the female reproductive system decreases with age.

This, in turn, leads to a gradual overall decline in the amount of estrogen and progesterone in

the blood. As hormone levels drop, a woman’s menstrual cycle becomes irregular. Within a few

years, it stops altogether. The end of the menstrual cycle is known as menopause. Among

North American women, the average age of menopause is approximately 50, but menopause

can begin earlier or later.