Gametes & HPG Axis

Question 1

Define gametes.

Answer 1

Specialised halpoid cells allowing us to pass our genes on to the next generation i.e. sperm/ova produced by sexually mature adults in the gonads (testes/ovaries) from specific diploid cells (germ cells) present from the early embryo

Question 2

What is the difference between a haploid and diploid cell?

Answer 2

Haploid: 23 individual chromosomes

Diploid: 23 chromosome PAIRS

Question 3

What is gametogenesis?

Answer 3

1. Germ cells from early embryonic life migrate to developing gonads at 6 weeks development (time/scale differs between sexes)
2. They colonise the gonads and:
   - Proliferate by mitosis
   - Reshuffle genetic material
   - Reduce to haploid by meiosis
   - Mature into sperm

Question 4

What are the steps of mitosis?

Answer 4

1. Interphase (G2): check point on duplicated DNA
2. Prophase: Centrosomes with spindle poles appear whilst nuclear envelope breaks down
3. Prometaphase: kinetochore develops from spindle poles and sister chromatids condense
4. Metaphase: sister chromatids align and attach to kinetochore
5. Anaphase: sister chromatids separate moving towards poles
6. Telophase: cleavage furrow due to cytokinesis and 2 new nucleus established
7. Interphase (G1): 2 identical daughter cells produced

Question 5

Why is meiosis different to mitosis?

Answer 5

DNA is duplicated but then there is 2 divisions/cycles instead of 1 so 4 haploid daughter cells are made with different genetic material to the original cell

Question 6

What is chiasmata?

Answer 6

Joining of genetic material between chromatids due to genetic recombination that occurs in meiosis meaning each gene can be inherited separately

Question 7

What is spermatogonia?

Answer 7

The product of mitotic proliferation of XY germ cells that colonise the medulla of the testes in the early embryo

Question 8

What happens to spermatogonia at puberty?

Answer 8

Spermatic cords hollow out to form seminiferous tubules (~250-750/testis) and the spermatogonia cluster around the edges initially. They are constantly replaced here by mitosis being available under late in life. As maturity and puberty ensue, spermatids are eventually close to the lumen ready to move down the tubule to mature into spermatozoa.

Question 9

What is the process of sperMATOgenesis?

Answer 9

A process that begins at puberty when a group of spermatogonia divide by mitosis a fixed number of times to form a clone of ~64 linked diploid cells called primary spermatocytes. Meiosis cycle 1 produces 2 haploid secondary spermatocytes from each primary one. Meiosis cycle 2 results in 4 haploid spermatids per primary. This process takes 70 days with new spermatogonia groups being recruited every 16 days.

Question 10

What is the problem with sperm production?

Answer 10

There is little quality assurance as it mainly about mass production so even if they have a problem, they will not apoptose

Question 11

What is sperMIOgenesis?

Answer 11

Maturation/remodelling of spermatids in the seminiferous into fully differentiated spermatozoa as they move down the rete testis, efferent duct and epididymis

Question 12

What are the 4 morphological features of mature spermatozoa?

Answer 12

1. Formation of head containing nucleus and a mid-piece that contains mitochondria
2. Formation of acrosome from Golgi apparatus
3. Elongation of 1 centriole to form a tail
4. Loss of excess cytoplasm

Question 13

What is the acrosome?

Answer 13

A layer surrounding the head of the spermatozoa that contains digestive enzymes that break down the zona pellucida; the glycoprotein layer surrounding the ovum

Question 14

What are spermatogenic waves?

Answer 14

Spermatogenesis occurs in sequential waves along the length of the seminiferous tubules with all stages of the process occurring at the same time in different sections of the tubule

Question 15

Why is sperm produced continuously?

Answer 15

To exploit the time limitations of female fertility

Question 16

What does semen contain?

Answer 16

1. Seminal vesicle secretions mostly
2. Prostate secretions
3. Sperm via vas deferens
4. Bulbo-urethral gland secretions

Mixed by emission into vagina by ejaculation

Question 17

How much semen does a male produce in each ejaculation?

Answer 17

1.4-4ml (>300M sperm but only ~50 get to fertilisation site)

Question 18

What is oogonia?

Answer 18

The product of mitotic proliferation of the female XX germ cells that colonise the ovaries cortex

Question 19

How does the numbers of oogonia vary throughout life?

Answer 19

Numbers peak at 7M by mid-gestation but then they start to die and are not replaced so about 2M remain at the end of gestation and they all enter meiosis before birth so females develop their entire stock of potential gametes before birth

Question 20

What is the process of oogenesis?

Answer 20

When meiosis stops before birth, a diploid primary oocyte is surrounded by a single layer of granulosa cells in a primordial follicle and from puberty, each month 1 or 2 primary oocytes complete development to form a mature ovum. Meiotic division resumes at ovulation (start of short fertility period of 24-36hrs) producing 1 haploid secondary oocyte and 1st polar body in cycle 1 and one mature haploid ovum and the 2nd polar body in cycle 2. Polar bodies are discarded.

Question 21

What is the requirement for successful fertilisation?

Answer 21

Effective transfer of viable male and female gametes to the right place at the right time

Question 22

What are the 3 requirements for successful reproduction?

Answer 22

1. Fertilisation
2. System of support for conceptus, embryo and then foetus in the female
3. Birth at the right time

Question 23

What is the hypothalamus?

Answer 23

Dominant part of endocrine system that regulates the function of several hormonal axes:

1. HPA
2. HPG
3. HPG

Question 24

What are the hormones of the Hypothalamus-Pituitary-Gonadal (HPG) axis?

Answer 24

H: GnRH

P: Ant - FSH, LH and prolactin + Post - oxytocin

G: oestrogen, progesterone, testosterone and inhibin

Question 25

What are the 6 hypothalamic releasing hormones and their relevant peptide hormones produced by the anterior pituitary gland in decreasing order of amount?

Answer 25

``` GHRH -> GH CRH -> ACTH GnRH -> LH/FSH PIH/DA -> prolactin TRH -> TSH ```

Question 26

What is the role of oxytocin?

Answer 26

Labour and lactation

Question 27

What is gonadotrophin releasing hormone (GnRH) and what does it do?

Answer 27

Decapeptide produced by cleavage of pre-pro-GnRH with a short T1/2 of < 10mins produced by the post-pubertal hypothalamus by pulsatile release every 1-2hrs transferred along pituitary stalk and acts on membrane receptors w/ IC signal transduction via 2nd messengers producing glycoprotein gonadotrophic hormones FSH and LH

Question 28

How does the release of gonadotrophin hormones change over life?

Answer 28

Low levels after birth with levels rising during puberty when pulsatile GnRH production begins

Question 29

What do gonadotrophin hormones do to the male reproductive system?

Answer 29

LH/FSH target Leydig cells and Sertoli cells lining the semiuniferous tubles of the testes. Leydig cells produce testosterone in response. Sertoli cells produce inhibin in response and support the developing sperm pushing them towards the tubule lumen ready for release. These hormones also support the 2ndary sexual characteristics of males.

Question 30

What is testosterone and what does it do?

Answer 30

Steroid hormone of the androgen family produced by Leydig cells that migrates to the seminiferous tubules, is converted to dihydrotestosterone (more active) by Sertoli cells, binds to androgen receptors in Sertoli cells to: - Stimulate sperm production - Secondary sex characteristics/libido

Question 31

What occurs in the female reproductive cycle overall?

Answer 31

1. Follicle grows, oestrogen rises and there is -ve feedback to HP 2. Dominant follicle with very high oestrogen levels and +ve feedback to HP 3. LH surge occurs disrupting the dominant follicle so ovulation occurs 4. Progesterone is high and exhibits -ve feedback

Question 32

What are the phases of the uterine cycle?

Answer 32

Days 0-7: menses Days 7-14: proliferative phase Days 14-28: secretory phase

Question 33

What are the 2 phases of the ovarian cycle?

Answer 33

1. Follicular (days 0-14) | 2. Luteal (days 14-28)

Question 34

What occurs post-ovulation?

Answer 34

Disrupted follicle forms corpus luteum and LH stimulates this to secrete progesterone and oestrogen which increase more as it expands negatively feeding back to HP - corpus luteum and therefore, oestrogen/progesterone production wane after 14 days if pregnancy does not occur

Question 35

What are the 5 roles of oestrogen?

Answer 35

1. Increases muscular contraction in uterine tubes and uterus to facilitate sperm passage 2. Proliferation of uterine lining to prepare for implantation 3. Cervical mucus thinning and alkalizing to allow sperm entry 4. Breast development at puberty and in pregnancy 5. Affects female body fat distribution, hair, genitals and supports bone growth