Gametogenesis

Question 1

What happens after the germ cells have colonised the gonad?

Answer 1

• Proliferate by mitosis
• Reshuffle genetically and reduce to haploid by meiosis
• Cytodefferentiate into mature gametes.

Timing and scale varies between sexes

Question 2

What is the difference between oogenesis and spermatogenesis?

Answer 2

Ovum:

• Few gametes (around 400 in lifetime)
• Intermittent production (1 per month)
• 1/400th of reproductive potential

Sperm:

• Huge number (2million per day
• Continuous production
• Essentially “disposal” cells

Question 3

What are the two main functions of meiosis?

Answer 3

• Reduce chromosome number to 23

- Ensure every gamete is genetically unique

Question 4

What is meiosis?

Answer 4

• Used only in production of sperm and egg
• Two successive cell divisions (M1&2)
• Produces 4 daughter cells (females, only one = mature oocyte)

Question 5

How does genetic variation arise?

Answer 5

• Crossing over - exchange of regions of DNA between 2 homologous chromosomes
• Independent assortment - random orientation of each bivalent along the metaphase plate with respect to other bivalents (2^23).
• Random segregation - random distribution of alleles among the four gametes

Question 6

Where does spermatogenesis occur?

Answer 6

In the walls of the seminipherous tubules, in the spaces between the adjacent sertoli cells.

Question 7

What is the function of the rate testes?

Answer 7

Concentrate sperm as leaves seminepherous tubules.

The Ductili efferentes also helps to concentrate the sperm.

Question 8

What is the role of the blood testis barrier?

Answer 8

Mature spermatids

Stop immune system recognising sperm as foreign and triggering an immune response.

Question 9

When spermatogonia divide by mitosis, what is created?

Answer 9

Ad spermatogonium ("resting" : reserve stock)
Ap spermatogonium ("active": maintain stock and from puberty onwards produce type B spermatogonia which give rise to primary spermatocyte.

Question 10

What is the spermatogenic cycle?

Answer 10

This is the time taken for reappearance of the same stage within a given segment of tubule (around 16 days)

Question 11

What is the spermatogenic wave?

Answer 11

Distance between the same stage of the spermatogenic cycle.

This is because the different stages in spermatogenesis are ordered in space as well as time.

Question 12

What is spermiation?

Answer 12

Release of spermatids into lumen of the seminepherous tubules

Question 13

What happens during spermiation?

Answer 13

Remodel as they pass down seminepherous tubule, through rete testis and dictuli efferentes and into the epididymis to finally form spermatozoa.

They are non-motile so they are transported via Sertoli cells secretions assisted by peristaltic contractions until they reach epididymis.

Question 14

How much semen per ejaculate?

Answer 14

2ml

semen = 2-5% sperm

Question 15

What is semen made from?

Answer 15

Seminal vesicle secretions (70%)
Secretions of Prostate (25%)
Sperm (via vas deferens) (2-5%)
Bulbourethral gland secretions (Cowper glands) (less than 1%)

Question 16

What is sperm capacitation?

Answer 16

This is the removal of glycoproteins and cholesterol from sperm membrane stimulated by the conditions in female genital tract.

It Activates sperm signalling pathways (atypical soluble adenylyl cyclase and PKA are involved)
This allows the sperm to bind to zona pellucida of oocyte and initiate acrosome reaction.

Question 17

What is atresia?

Answer 17

When primary oocytes and oogonia degenerate

Question 18

What are the three stages that oocytes go through as they mature?

Answer 18

• Preantral
• Antral
• Proovulstory

Question 19

What is the corpus luteum?

Answer 19

The remaining granulosa and theca interna cells become vascularised, develop a yellowish pigment and change into lutein cells.

The corpus luteum secrets oestrogen and progesterone which stimulates the uterine mucosa to enter secretory stage in preparation for embryo implantation.

It dies after 14 days if no fertilisation occurs.

Question 20

What is spermiogenesis?

Answer 20

This is when the haploid spermatids differentiate into spermatozoa

Question 21

What is in the seminal vesicle secretions?

Answer 21

Amino acids, citrate, fructose, prostaglandins

Question 22

What is in the secretions of prostate?

Answer 22

Proteolytic enzymes,

Zinc

Question 23

What is in the cowper gland secretions?

Answer 23

Mucoproteins to help lubricate and neutralise acidic urine in distal urethra

Question 24

Before IVF can work, but must first happen to the sperm?

Answer 24

They must be incubated in a capacitation media to allow for sperm capacitation, otherwise sperm could not bind to zona pellucida and initiate the acrosome reaction.

Question 25

What happens to the oocytes before birth?

Answer 25

-Germ cells arise form the yolk sac and colonise the gonadal cortex and differentiate into oogonia which proliferate rapidly by mitosis. By end of 3rd month, oogonia are arranged in clusters surrounded by flat epithelial cells. Max number of germ cells is reached by mid gestation then cell death begins and primary oocyte degenerate. By 7th month gestation, majority of oogonia have degenerated and all surviving primary oocytes have entered meiosis 1 and are individually surrounded by follicular cells (flat epithelial cells). They are now called the primary follicle

Question 26

What happens in the prantral stage of maturation?

Answer 26

Primordial follicles begin to grow and the surrounding follicular cells change from flat to cuboidal and proliferate to produce stratified epithelium of granulosa cells.

Question 27

What happens in the antral stage of maturation?

Answer 27

- As development continues, fluid filled spaces appear between granulosa cells. These coalesce to form the antrum. - Several follicles begin to develop with each ovarian cycle. Usually only one reaches maturity (becomes atretic)

Question 28

What happens in the preovulatory stage of maturation?

Answer 28

Surge in LH induces preovulatory growth phase. This completes meiosis I meaning there are now two haploid daughter cells of unequal size (one receives most of cytoplasm and the other is a polar body). Cells then enter meiosis II but arrest in metaphase, about 3 hours before ovulation. Meiosis II is only completed if the oocyte is fertilised, otherwise the cell degenerated about 24hours after ovulation.

Question 29

What happens during ovulation?

Answer 29

-FSH and LH stimulate rapid growth of follicle several days before ovulation occurs.It is now about 2.5cm in diameter and called graafian follicle. LH surge increases collagenase activity. Prostaglandins increase response to LH and cause local muscular contractions in ovarian wall. Oocyte extruded and breaks free from ovary.

Question 30

How does the oocyte move?

Answer 30

Shortly between ovulation, fimbrae sweep over surface of ovary. The uterine tube begins to contract rhythmically Oocyte carried into tube by sweeping movements of fimbriae and by motion of cilia on epithelial lining. Oocyte then propelled by peristaltic muscular contractions of the tube and by cilia in the mucosa. If fertilised, oocyte reaches uterine lumen in 3-4 days.

Question 31

What happens if no fertilisation occurs?

Answer 31

- Corpus leteum degenerates - Forms mass of fibrotic scar tissue called the corpus albicans - Progesterome production decreases, precipitating menstrual bleeding.

Question 32

What happens if fertilisation occurs?

Answer 32

- Degeneration of corpus luteum is prevented by HCG which is secreted by the embryo - CL continues t grow and forms CL of pregnancy - Cells secrete progesterone until 4th month - Secretion of progesterone by placenta then becomes adequate.

Question 33

Describe the ovarian cycle

Answer 33

- GnRH released from hypothalamus causes anterior pituitary to release FSH and LH. - Follicles stimulated to grow by FSH and to mature by FSH and LH - Ovulation occurs on LH surge - LH also promotes development of the corpus luteum