Fertilisation and the Luteal phase

Question 1

Describe the viscosity of ejaculated sperm

Answer 1

Ejaculated sperm is coagulated

• After about 20 minutes, the sperm are going to liquefy and the sperm can swim out and get into the cervical crypts in the cervix until they manage to find their way out. Some will go straight through in the endometrium.

Question 2

How long can sperm survive in the cervix?

Answer 2

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

Question 3

How does the sperm move from the cervix to the fallopian tube?

Answer 3

There are chemo-attractants that are released from the oocyte that may attract the sperm.

Once the sperm become close to the egg, they become hyperactivated. They start forcefully beating their tails with increased frequency and amplitude. This is mediated by Ca2+ influx via CatSper channels.

Question 4

Why does the acrosome reaction not happen immediately?

Answer 4

The acrosome reaction cannot happen immediately as the sperm need to be outside the seminal fluid, and in the female, from between 4-18 hours before they can undergo that. It gives them time to swim to the egg.

Question 5

What is capacitation?

Answer 5

It is the biochemical arrangement of the surface glycoproteins and changes in membrane composition which must occur before the acrosome reaction can occur.

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

Question 6

What is the acrosome reaction?

Answer 6

Acrosin bound to the inner acrosomal membrane digests the zona pellucida so the sperm can enter the egg.

It occurs when the sperm comes into contact with the zona-cumulus complex. The acrosomal membrane on the sperm head fuses, releasing enzymes that cut through the complex

Question 7

Describe ovulation of the egg as it is released from the ovary

Answer 7

The LH spike causes the resumption of meiosis and ovulation, converting the primary oocyte to the secondary oocyte

The basement membrane breaks, so blood pours into the middle

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

Theca and granulosa become mixed and the empty follicle is known as the corpus luteum, which produces progesterone in the luteal phase

Question 8

Describe the role of progesterone in the luteal phase?

Answer 8

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

Question 9

Describe the role of oestrogen in the luteal phase?

Answer 9

• Helps to maintain endometrium in luteal phase

- Causes proliferation in follicular phase

Question 10

What does the menstrual cycle achieve?

Answer 10

• 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 corpus luteum progesterone
  Initiating a new cycle if fertilisation does not occur

Question 11

Describe the breakdown of the corpus luteum

Answer 11

If fertilisation does not occur, the corpus luteum has an inbuilt finite 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 12

What is the cumulus oophorus?

Answer 12

A protective layer derived from granulosa cells that protects the egg

Question 13

What is the corona radiata?

Answer 13

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 14

What is the zona pellucida?

Answer 14

The thick transparent membrane secreted by the egg. It becomes impenetrable after the cortical reaction

Question 15

Describe sperm binding and penetration

Answer 15

1) The acrosome reaction occurs in contact with the zona-cumulus complex. The sperm penetrates the cumulus and binds to the zona pellucida via adhesion molecules and receptors, ZP3.
2) The sperm enzymes cut through the zona pellucida and the sperm fuses with the plasma membrane.
3) The sperm is taken in by phagocytosis. Phospholipase Zeta (PZeta) is activated by the basal Ca2+ inside the egg, and causes further release of intracellular Ca2+, leading to a large Ca2+ spike.
4) The cortical reaction occurs as a wave of Ca2+ sweeps around the egg. This reaction involves the release of proteases, peroxides and hyaline, which prevents polyspermy.

Question 16

Describe the syngamy post-fertilisation

Answer 16

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 decondenses 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.