Gametes + HPG axis

Question 1

What are gametes?

Answer 1

Sperm/ ova
Haploid cells containing 23 individual chromosomes

[Fusion of sperm and ovum produces a diploid cell which then divides to form an embryo]

Question 2

How are gametes made?

Answer 2

Gametogenesis begins in early embryonic life (germ cells) which migrate to developing gonads at approx. 6 weeks development - there they proliferate (by mitosis), reshuffle genetic material, reduce to haploid (by meiosis) and then mature into sperm or ova

[more maturation occurs for sperm and quantity of sperm production is larger than female gametogenesis]

Question 3

How does spermatogenesis occur?

Answer 3

1. XY germ cells colonise medulla of testes
2. Form spermatogonia by mitosis (proliferation)
3. Spermatogonia begin to cluster around the edges of the seminiferous tubules at puberty
4. At puberty, spermatogonia divide by mitosis to form a clone of about 64 linked diploid cells (primary spermatocytes)
5. Meiosis I produces two haploid secondary spermatocytes from each primary spermatocyte and meiosis II results in four haploid spermatids per primary spermatocyte

[Spermatogenia constantly replaced at the edges of the seminiferous tubule whilst spermatids are close to the lumen]

Question 4

What is spermiogenesis?

Answer 4

Maturation of spermatids into fully differentiated spermatozoa as they are released into tubule and pass down the rete testis, efferent duct and epididymis

Question 5

What morphological changes occur as spermatids mature into spermatozoa?

Answer 5

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

Question 6

What is the acrosome of a spermatozoa and it’s function?

Answer 6

Surrounds head of spermatozoa and contains digestive enzymes that break down the zona pellucida of an ovum

[Zona Pellucida = glycoprotein layer surrounding ovum]

Question 7

What are spermatogenic waves?

Answer 7

Sequential waves of spermatogenesis along the length of the seminiferous tubule (process takes approx. 70 days with new spermatogonia recruited every 16 days)

Question 8

What is the contents of semen?

Answer 8

Seminal vesicle secretions
Prostate secretions
Spermatozoa (via Vas Deferens)
Bulbo-urethral gland secretions

Question 9

What are the main hormones of reproduction released from the hypothalamus?

Answer 9

Gonadotrophin Releasing Hormone (GnRH)

Question 10

What are the main hormones of reproduction released from the anterior pituitary?

Answer 10

Follicle Stimulating Hormone (FSH)
Luteinising Hormone (LH)
Prolactin
Oxytocin (Posterior Pituitary)

Question 11

What are the main hormones of reproduction released from the gonads?

Answer 11

Oestrogen
Testosterone
Progesterone
Inhibin

Question 12

What is the role of Gonadotrophin Releasing Hormone (GnRH)?

Answer 12

Pulsatile release from hypothalamus, causes release of gonadotrophic hormones (FSH + LH) from the anterior pituitary

Question 13

What are the phases of mitosis?

Answer 13

Interphase (gap 2) - nucleus with decondensed chromosomes and 2 centrosomes
Prophase (centrosome begin to migrate to opposite poles, breakdown of nuclear envelope, condensing of chromosomes and development of microtubules/ spindles from centrosomes)
Prometaphase (chromosomes condense further into distinctive X shaped structure)
Metaphase (chromosomes aligned on metaphase plate joined to spindles at the kinetochore)
Anaphase (sister chromatids separate and begin to move apart so each daughter cell has identical copy of parent cell DNA, cytoplasm begins to elongate and narrow)
Telophase (cleavage furrow/ cytokinesis, re-establishment of nuclei)
Interphase (gap 1) - two separate daughter cells - enter S phase where DNA is synthesised before next mitotic division

Question 14

What is the difference between mitosis and meiosis?

Answer 14

Mitosis results in formation of 2 genetically identical daughter cells whilst meiosis results in formation of 4 identical daughter cells (haploids), each with half the number of chromosomes than the parent cell

Meiosis occurs in two phases (meiosis I and II)

Question 15

What are oogonia?

Answer 15

Immature female reproductive cell formed by mitosis of XX germ cells

[females develop all potential gametes prior to birth with approx. 2 million oogonia at birth]

Question 16

What is oogenesis?

Answer 16

Meiosis of oogonia results in a diploid primary oocyte surrounded by a single layer of granulosa cells in a primordial follicle - this stops at very early stage and then resumes again at ovulation where meiosis I produces one haploid secondary oocyte and the first polar body, and meiosis II produces one mature haploid ovum and the second polar body

Question 17

What does successful fertilisation and successful reproduction require?

Answer 17

Successful fertilisation requires effective transfer of viable male and female gametes at the right place at the right time.

Successful reproduction requires fertilisation, a system of support for the conceptus, embryo and foetus (coordinated by HPG axis)

Question 18

What is the difference between a conceptus, embryo and foetus?

Answer 18

Conceptus = very early embryo 
Embryo = up to approx. 8-10 weeks 
Foetus = from 8-10 weeks to birth

Question 19

What is Oxytocin important for?

Answer 19

Labour and lactation

Question 20

Where is testosterone produced and what is its function?

Answer 20

Leydig cells lining the seminiferous tubules in the testicles and converted to dihydrotestosterone (its more active form) by Sertoli cells - binds to androgen receptors in Sertoli cells to stimulate sperm production and leads to secondary sex characteristics/ libido

Question 21

What are the actions of oestrogen?

Answer 21

1. Increases muscular contractions in uterine tubes and uterus to facilitate sperm passage
2. Causes proliferation of the uterine lining to prepare for implantation
3. Makes the cervical mucus thin and alkaline to allow sperm entry
4. Supports breast development in puberty and in pregnancy
5. Affects female body fat distribution, hair, genitals and supports bone growth