Fertilization and the luteal phase

Question 1

Describe the composition of ejaculated semen.

Answer 1

first coagulated

then liquefies (10-15 mins) via action of prostatic and seminal vesicle secretions

Question 2

Describe the composition of the Cervical mucous mid-cycle.

Answer 2

less viscous/more watery (low progesterone) allowing sperm to pass

Question 3

When does fertilisation occur after ejaculation?

Answer 3

typically 24-48 hours

Question 4

Give some Factors which contribute to the sperm’s journey to egg.

Answer 4

Uterine tubes set up currents to aid in passage of sperm through uterus

Oocyte cumulus complex releases chemoattractants to attract sperm

Sperm tail is hyperactivated and beats more when close to the egg, mediated by Ca2+ influx via CatSper channels

Question 5

What is Sperm capacitation?

Answer 5

rearrangement of surface glycoproteins and changes in membrane composition of sperm before the acrosome reaction

Question 6

Describe the Acrosome reaction.

Answer 6

Acrosome membrane fuses with egg membrane, releasing digestive enzymes which digest the zona-cumulus complex

Question 7

What enzyme digests the zona pellucida?

Answer 7

Acrosin

Question 8

What triggers the acrosome reaction?

Answer 8

Contact between sperm and zona-cumulus complex of the egg

Question 9

Outline the Steps of the Acrosome Reaction.

Answer 9

1) Sperm contact zona-cumulus complex, penetrate cumulus cells and bind zona pellucida.
2) Digestive enzymes released (e.g. Acrosin) which bind to adhesion molecules on zona pellucida and digest zona pellucida. Sperm lipid membrane then fuses with egg lipid membrane.
3) Sperm taken in by phagocytosis. Phopholipase Zeta enzyme activated by basal Ca2+ inside egg, converting PIP2 to DAG +IP3, releasing Ca2+ from intracellular stores resulting in large Ca2+ spike. This spike triggers resumption of meiosis II.
4) Increase in Ca2+ triggers cortical reaction, where cortical granules release their content (peroxidases, proteases, hyaline) which cause cross-linking and degradation of adhesion molecules on zona pellucida preventing further sperm entering.

Question 10

Describe the Formation of the zygote.

Answer 10

nucleus of sperm transforms to male pronucleus after entering oocyte and triggering Ca2+ spike

female pronucleus forms after Ca2+ spike induces completion of meiosis II

both pronuclei come together, membranes break down, alignment of chromosomes on spindle, then mitosis

Question 11

What is the Effect of progesterone in luteal/secretory phase?

Answer 11

• makes endometrium secretory and receptive to implantation
• suppresses cilia in uterine tubes once oocyte has already passed
• makes cervical mucus viscous again to prevent further implantation

Question 12

What is the Effect of fertilisation on corpus luteum?

Answer 12

hCG (released from blastocyst) binds LH receptors and rescues corpus luteum

corpus luteum continues to produce progesterone, therefore maintaining the endometrium

Question 13

What happens if there is no fertilisation?

Answer 13

In the corpus luteum:

• cell death occurs
• vasculature breaks down
• corpus luteum shrinks, becoming the corpus albicans over time

Question 14

What happens to the primary oocyte at ovulation?

Answer 14

LH surge induces completion of meisois I and ovulation:

• egg divides asymmetrically into a secondary oocyte and polar body
• zona pellucida secreted by egg to prevent further sperm penetrating once egg is fertilised
• corona radiata- innermost of cumulus granulosa cells adhere to the zona pellucida