REPRO: Fertilisation and the Luteal Phase

Question 1

How does the sperm get coagulated, and how is that reversed when they enter the cervix?

Answer 1

Prostatic and seminal vesicle secretions comprise of the seminal fluid, which coagulates the sperm - prevents loss, later than liquifies (10-15 mins)

Movement through the cervical mucus removes seminal fluid, abnormally morphological sperm and cellular debris. This allows the sperm to swim free and fertilise the egg.

Question 2

Describe the sperm’s passing through the cervix.

Answer 2

Cervical mucus is less viscous in the absence of progesterone, allowing sperm to pass.

Sperm can inhabit the cervical crypts, which may form a reservoir of sperm. There is some evidence of thermotaxis, but the mechanism is not yet elucidated.

Question 3

When does fertilisation occur after ejaculation?

Answer 3

Fertilisation typically occurs within 24-48 hours, but sperm has been recovered alive after 5 days.

Question 4

Factors that contribute to the sperm’s journey to the egg

Answer 4

Uterine tubes set up currents to aid in the passage of sperm through the 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 capacitation?

Answer 5

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

This is partly achieved by removing the sperm from the seminal fluid; also, the uterine and tubal fluid may contain factors that promote capacitation.

Question 6

What is the acrosome reaction?

What triggers the acrosome reaction?

Answer 6

Acrosome membrane fuses with egg membrane, releasing digestive enzymes (acrosin bound to the inner acrosomal membrane) which digest the zona-cumulus complex so the sperm can enter the egg.

Contact between sperm and zona-cumulus complex of the egg

Question 7

Briefly, describe the ovulation of the egg as it is released from the ovary.

Answer 7

The LH spike causes the resumption of meiosis and ovulation. It converts primary oocytes to secondary oocytes, plus the first polar body

The basement membrane breaks so that blood pours into the middle.

The oocyte cumulus complex is extruded out and caught by the fimbriae of the uterine tube.

The theca and granulosa cells become mixed and the empty follicle is known as the corpus luteum. It produces progesterone in the luteal phase.

Question 8

Describe the role of progesterone in the menstrual cycle (luteal phase).

Answer 8

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

Question 9

Describe the role of oestrogen in the menstrual cycle (luteal phase).

Answer 9

• helps to maintain the endometrium in the luteal phase (causes proliferation in the follicular phase)

Question 10

Steps of the Acrosome Reaction

Answer 10

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 the 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- preventing polyspermy

Question 11

Formation of the zygote

Answer 11

the 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 the spindle, then mitosis

Question 12

Describe the demise of the corpus luteum.

Answer 12

If fertilisation does not occur, the corpus luteum has a lifespan of 14 days. The regression of the corpus luteum is essential to initiate a new cycle.

The fall in corpus luteum-derived steroids causes an inter-cycle rise in FSH. Cell death occurs, the vasculature is broken down and the corpus luteum shrinks. Over time, it becomes a corpus albicans.

The corpus luteum would be rescued in pregnancy by hCG (from the embryo) binding to its LH receptors. The corpus luteum then continues to produce progesterone and maintains the endometrium.

Question 13

What does the menstrual cycle achieve?

Answer 13

• selection of a single follicle and oocyte
• regular spontaneous ovulation
• correct haploid number of chromosomes in the oocyte
• cyclical changes in the cervix and uterine tubes, to enable egg transport and sperm access
• preparation of the endometrium of the uterus to receive the fertilised egg
• support of the implanting embryo and endometrium by the corpus luteum progesterone
• initiating a new cycle if fertilisation does not occur

Question 14

What is the cumulus oophorus?

Answer 14

It is derived from granulosa cells, and it protects the egg.

Question 15

What is the corona radiata?

Answer 15

It is the innermost layer of cumulus cells in contact with the zona pellucida.
It is formed by granulosa cells adhering to the oocyte before it leaves the ovarian follicle.

Question 16

What is the zona pellucida?

Answer 16

It is secreted by the egg.

It becomes impenetrable after the cortical reaction.

Question 17

Describe the syngamy post-fertilisation.

Answer 17

After meiosis 1, the oocyte has 23x chromosomes, but 2 chromosomes are arranged as sister chromatids.
Entry of the sperm causes an increase in Ca2+, probably because of PZeta from the sperm. The Ca2+ causes the completion of meiosis 2, expelling the second polar body, and the cortical reaction.

The sperm nuclear membrane breaks down, the chromatin decondensed and the chromosomes separate.

4-7 hours after the fusion, the two sets of haploid chromosomes become surrounded by distinct membranes, forming pronuclei. These haploid structures synthesis DNA in preparation for the first mitotic division.

The pronuclei fuse and the mitotic metaphase spindle forms with the chromosomes, assuming their position at its equator. Mitosis is completed and the one-cell zygote becomes a two-cell embryo.