Endocrinology 11 - The gonads II

Question 1

What is testosterone a precursor to? (2)

Answer 1

1. Dihydrotestosterone (DHT) - produced in target tissue with 5-alpha-reductase. DHT is a more potent form of testosterone that acts on same receptors.
2. Oestrogen - Aromatisation using aromatase.

Question 2

How is testosterone and DHT transported in the blood?

Answer 2

1. Sex Hormone Binding Globulin (SHBG) is the plasma protein which transports a large amount of androgens.
2. Albumin also binds to some of the androgens.
3. Only 2% free in blood.
4. Testosterone is also in seminiferous fluid bound to Androgen Binding Globulin (ABG).

Question 3

What is androgen needed for in the foetus? (5)

Answer 3

1. Development of male internal and external genitalia.
2. Stimulate general growth (male babies are larger because of effect of androgen).
3. Behavioural effect associated with androgens.
4. Androgens can be converted to oestrogen and bind to oestrogen receptors.
5. Some behavioural effects may be due to oestrogen binding to oestrogen receptors in the brain.

Question 4

What is the effect of androgens in an adult? (5)

Answer 4

1. Spermatogenesis.
2. Growth and development of male genitalia, secondary sex glands and secondary sex characteristics.
3. Stimulation of protein synthesis.
4. Behavioural (CNS) effects.
5. Pubertal growth spurt (with growth hormone).

Question 5

What are some androgenic effects mediated by?

Answer 5

Conversion to oestrogen.

Question 6

What are oestrogens? And 3 examples.

Answer 6

Any substance (natural or synthetic) that induces mitosis in the endometrium.

1. 17-beta-oestradiol - menstrual cycle horhome.
2. Oestrone - precursor.
3. Oestriol - main oestrogen in pregnancy.

Question 7

What are the effects of oestrogens (reproductive)? (6)

Answer 7

1. Stimulates proliferation of endometrium.
2. Final maturation of the follicle during follicular phase of menstruation cycle.
3. Induction of LH surge at high levels = ovulation.
4. Effects on vagina and cervix.
5. Stimulates growth ductile system of breast.
6. Decreases sebaceous gland secretion.

Question 8

What are the effects of oestrogens (other)? (6)

Answer 8

1. Increased salt and water reabsorption.
2. Increased plasma protein synthesis.
3. Metabolic actions - increased HDL’s
4. Stimulates osteoblasts.
5. Effects on other hormones (prolactin, thyrotrophin).
6. Behavioural effects.

Question 9

What are progesterones?

Answer 9

1. Any substance (natural or synthetic) which induces secretory changes in the endometrium.
2. Examples are progesterone and 17-alpha-hydroxyprogesterone.

Question 10

What are the actions of Progesterons? (5)

Answer 10

1. Stimulates secretory activity in the endometrium and cervix (thick and viscous so less easily penetrated by spermatozoa).
2. Stimulates growth of the alveolar system in the breast.
3. Decrease renal reabsorption of NaCl - competitive inhibition of aldosterone.
4. Increase body temperature.
5. Negative feedback regulation.

Question 11

What are the target cells for LH and FSH in the testis? And what does it stimulate these cells to do?

Answer 11

1. LH - Leydig cells (stimulates production of androgens).

2. FSH - Sertoli cells (stimulates production of inhibin).

Question 12

What are the effects of testosterone?

Answer 12

1. Virilisation effect (development of male physical characteristics).
2. Direct effect on pituitary (decreases LH production).
3. Indirect effect on hypothalamus (decreases amplitude of pulses of GnRH).

Question 13

What does inhibin have negative effects on?

Answer 13

1. Direct effect on pituitary (decreases FSH).

2. Indirect effect on hypothalamus (decreases amplitude of pulses of GnRH).

Question 14

Where are GnRH’s (Gonadotrophin Releasing Hormone) produced in? And what are its effects?

Answer 14

1. By neurones in the hypothalamus.
2. The activity is “pulsatile” (pulses generated).
3. They pass down to the gonadotrophs in the adenohypophysis, which produces LH and FSH.

Question 15

What are the targets for LH and FSH in the ovary?

Answer 15

1. LH - Thecal cell (increase androgen production).

2. FSH - Granulosa cells (increase aromatase activity to increase production of oestrogen).

Question 16

What is the effect of oestrogen on the granulosa cells?

Answer 16

1. Auto-positive feedback - oestrogen binds to its own oestrogen receptors.
2. Stimulates the aromatase enzyme via secondary messenger system.
3. Hence more oestrogen produced by androgens = the more oestrogen produced overall due to positive feedback.

Question 17

What happens to oestrogen and progesterone levels after fertilisation? And what causes this?

Answer 17

1. Oestrogen and progesterone levels need to rise.

2. This is caused by the placenta releasing hCG (human chronic gonadotrophin) that mimics the effects of LH.

Question 18

What is amenorrhea?

Answer 18

Absence of a menstrual cycle.

1. Primary - menstrual cycle never happened.
2. Secondary - menstrual cycle happened but then stopped (e.g. pregnancy).

Question 19

What is oligomenorrhoea?

Answer 19

1. Infrequent menstrual cycle.
2. Causes can be various.
3. Can be due to absence in LH surge.

Question 20

What is infertility?

Answer 20

1. Female -Inability to get pregnant.
2. Male - Inability to impregnate.
   Causes can be various (physical, emotional, endocrine problems). Excess prolactin (prolactinoma) can be cause.

Question 21

What happens if fertilisation does not occur?

Answer 21

Progesterone, Oestrogen and Inhibin exert negative feedback on LH and FSH production = luteolysis (breakdown of corpus luteum) and menstruation.

1. Direct to pituitary.
2. Indirect to hypothalamus.

Question 22

Does the Graafian follicle need FSH to develop and proliferate?

Answer 22

No.

Question 23

What is selective negative feedback and what does it result in, in the ovary?

Answer 23

1. Selective negative feedback occurs on FSH which is inhibited by inhibin and oestrogen.
2. The effect of this is the atresia of follicles that are still dependent on FSH (leaving the Graafian follicle).

Question 24

What does the aromatase convert androgens in the granolas cells into?

Answer 24

17-b-oestradiol

Question 25

What happens when granulosa cells become larger and multiply?

Answer 25

More oestrogen produced = Plasma oestrogen levels rise.

Question 26

What causes the negative feedback switching to positive feedback? And what kind of surge does this cause?

Answer 26

1. A rising concentration of 17-b-oestradiol in the absence of progesterone for a minimum of 36 hours and at a sufficient level.
2. LH surge.