Endocrinology 10 - The Gonads I

Question 1

What are the function of the gonads?

Answer 1

1. Gametogenesis (production of gametes)
   - Males = Spermatogenesis.
   - Females = Oogenesis.
2. Steroidogenesis (production of steroid hormones)
   - Males = Androgens (small amounts of oestrogen + progesterone).
   - Females = Oestrogen + progesterone (small amounts of androgen).

Question 2

When are germ cells activated? Where do male and female germ cells derive from?

Answer 2

They are activated during embryogenesis where germ cells proliferate reaching around 6-7 million. Male germ cells come from spermatogonia and female germ cells come from oogonia.

Question 3

Describe spermatogonia levels in males throughout life.

Answer 3

During childhood, spermatogonia are dormant and are only activated during puberty. Spermatogonia levels remain high throughout life in males.

Question 4

Describe oogonia levels in females throughout life.

Answer 4

Oogonia levels reach a maximum at 24 weeks, after this no more oogonia are produced so there are only a finite number of eggs. The number of oogonia decrease (atresia) throughout life, rapidly at first. At menopause the ovary is depleted of oogonia.

Question 5

What is Atresia?

Answer 5

When the cells start degenerating and dying off.

Question 6

Explain the process of spermatogenesis.

Answer 6

1. Germ cells in embryogenesis multiply and differentiate into spermatogonia (DIPLOID).
2. During puberty = high levels of FSH + testosterone cause spermatogonia to divide by mitosis - either more spermatogonia or primary spermatocytes (DIPLOID).
3. Primary spermatocytes enter 1st meiotic division = Secondary spermatocytes (HAPLOID).
4. Secondary spermatocytes enter 2nd meiotic division = spermatids.
5. Spermatids mature and differentiate into spermatozoa.

Question 7

Explain the process of oogenesis.

Answer 7

1. Germ cells multiply to produce oogonia (DIPLOID).
2. Oogonia multiply to produce primary oocytes (DIPLOID).
3. Primary oocytes enter 1st meiotic division, however halted at prophase = form primordial follicles.
4. Primary oocytes are dormant for 12-50 years - atresia takes place as well.
5. Some primary oocytes will be rescued by increased levels of FSH (ovulation) = secondary oocytes + polar body.
6. Secondary oocyte enters 2nd meiotic division and will only complete it during fertilization = ovum + polar body.

Question 8

What is a polar body?

Answer 8

In female meiosis, there is an unequal distribution of cytoplasm. One cell retains all the cytoplasm and the cell resources and the other daughter cell only contains the chromosomes and will die off - this is the polar body.

Question 9

Describe the journey of spermatozoa.

Answer 9

1. Spermatogenesis takes place in the seminiferous tubules.
2. Semiferous tubules are lined with layers of spermatogonia and sertoli cells.
3. Spermatozoa travel down to Rete testis and they are concentrated and drained by the Vasa efferentia into the epididymis (highly coiled) where they are stored.
4. In the epididymis, nutrients are secreted for them and they mature here.
5. They are propelled by vas deferens via the urethra.

Question 10

Where do the testes develop?

Answer 10

They develop in the abdomen and descend into the scrotum just before birth. The scrotum is 2-3 degrees cooler than core temperature = critical for spermatogenesis.

Question 11

Describe the function of sertoli cells. (4)

Answer 11

1. Sertoli cells line the seminiferous tubule and form tight junctions that prevent large proteins getting inside to the spermatozoa. This forms a blood-testis barrier.
2. Sertoli cells also provide structural and metabolic support.
3. Synthesize FSH and androgen receptors.
4. Produce inhibin in response to FSH (negative feedback for FSH).

Question 12

How do spermatogonia get through the sertoli cell layer?

Answer 12

1. They are enclosed in the cytoplasm of the sertoli cells.
2. Here they undergo spermatogenesis.
3. As they develop, they move towards the lumen and are released into the lumen.

Question 13

What are Leydig cells? (3)

Answer 13

1. Leydig cells are located close to the seminiferous tubule.
2. Produce LH receptors.
3. They contain enzymes that allow them to synthesize and release testosterone in response to LH.

Question 14

Where does oogenesis take place?

Answer 14

In the ovaries. The ovaries contain follicles at all stages of development (e.g. some will be undergoing atresia).

Question 15

What is a graffin follicle?

Answer 15

A follicle which has reached its maximum size and is ready for ovulation.

Question 16

Where are all the follicles embedded in?

Answer 16

In the ovarian stroma.

Question 17

What is corpus luteum?

Answer 17

What the follicle transforms into after ovulation.

Question 18

What are the steroids produced in the gonads? (3)

Answer 18

1. Progesterone (C21).
2. Androgens (C19).
3. Oestrogens (C18).

Question 19

What are the similarities and differences between the production of steroids in the gonads and the adrenals?

Answer 19

Similarities - the first 4 reactions in steroidogenesis are common in both adrenals and gonads.
Differences - Only adrenals have 21-hydroxylase to produce deoxycorticosertone or 11-deoxycortisol. Gonads have 17-hydroxysteroid dehydrogenase that converts androsteronedione to testosterone.

Question 20

What is the process of dihydrotestosterone production?

Answer 20

Cholesterol => prognenolone => progesterone => 17-OH-progesterone => androstenedione => testosterone => dihydrotestosterone.

Question 21

What is the process of 17-beta-oestradiol production?

Answer 21

Cholesterol => prognenolone +> progesterone => 17-OH-progesterone => androstenedione => oestrone => 17-beta-oestradiol. (or testosterone => 17-beta-oestradiol).

Question 22

What are the 2 related events in the menstrual cycle?

Answer 22

1. Ovarian cycle - follicular phase => ovulation => Luteal phase.
2. Endometrial cycle - proliferative => secretory.

Question 23

What does oestrogen do to the endometrium?

Answer 23

It causes the proliferation of the endometrium. There is an increase in mitosis and increase in progesterone and oestrogen receptors. The endometrium gets thicker, blood vessels longer and glands get bigger.

Question 24

What is the main hormone of the luteal phase? And what are its effects? (3)

Answer 24

Progesterone is the main hormone of this stage although oestrogen is still produced as well.

1. Decreases the proliferation that oestrogen caused.
2. Increases secretory activity of the cells in the myometrium to make it more suitable for implantation.
3. Reduces oestrogen receptors.

Question 25

Describe the variation hormones throughout the menstrual cycle.

Answer 25

1. FSH slightly raised at beginning in order to rescue dormant follicles.
2. Follicles grow and produce 17-beta-oestradiol.
3. 17-beta-oestradiol has a negative feedback on gonadotrophins.
4. One of the follicles are selected to be the dominant follicle = produces large amounts of oestrogen.
5. High oestrogen levels (36hours) stimulates release of gonadotrophins (LH + FSH) = ovulation.
6. After ovulation, empty follicle converts into corpus luteum = continues oestrogen and high progesterone production (Luteul phase)
7. Oestrogen + progesterone give negative feedback for gonadotrophins.
8. If fertilization does not occur, progesterone and oestrogen levels decrease = endometrium changes (vasospasm, necrosis and loss of tissue + blood = menstruation).
9. Negative feedback on FSH reduced and hence FSH levels rise to the start of the cycle.

Question 26

What happens to the body temperature after ovulation?

Answer 26

Body temperature increases.

Question 27

Describe the development of the follicles in the ovaries.

Answer 27

1. Follicle with oocyte stuck in early stage of meiosis.
2. Follicle will develop over embryonic life into pre-antral stage (gonadotrophins not needed).
3. FSH required for next step - develop into early-antral follicle.
4. Under influence of FSH - selected follicle will grow into maximum size = graffian follicle (ready for ovulation).
5. LH surge causes rupture of follicle and release of egg.
6. After ovulation, follicle converted to corpus luteum.
7. Surge of LH stimulates egg to complete first meiosis.

Question 28

How does the follicle produce oestrogen and progesterone?

Answer 28

1. Outer follicle cells called Thecal cells have LH receptors = LH binds to this.
2. LH stimulates the synthesis of androgens by the thecal cells.
3. Androgens diffuse into the inner cells called the granulosa cells.
4. Granulosa cells have FSH receptors that FSH binds to and stimulate the aromatase enzyme = converts androgen to oestradiol.
5. After ovulation, thecal and granulosa cells will continue to be stimulated by LH and FSH to produce oestrogen and high progesterone.