Fertilisation

Question 1

How long can the sperm survive for in the female apparatus?

Answer 1

Sperm cells can survive in the female apparatus for up to 5 days.

Question 2

How long does the egg last once ovulated?

Answer 2

When the oocyte is ovulated, if it’s not fertilised in 24 hours, it will no longer be fertilized.

Question 3

Why is the age of the mother an important factor in fertilisation?

Answer 3

This is because after the age of 31, the fecundity rate decreases dramatically. When at age 42, there is a less than 0.2% chance of pregnancy. This is related to the fact that there is a limited set of oocytes.

Question 4

What route does the sperm take to be ejaculated?

Answer 4

The sperm cells are produced at the level of the testes, released from the seminiferous tubules and move along the epididymis to reach the urethra to be ejaculated.

Question 5

What is capacitation?

Answer 5

The sperm’s capability to fertilisation.

Question 6

Why do sperm cells need to acquire capacitation?

Answer 6

Capacitation gives sperm cells hyperactivation.

Question 7

What happens in the fast event for capacitation?

Answer 7

An influx of bicarbonate ions in the cytoplasm, which increases the pH.

Altogether this activates an adenylate cyclase which then increases the levels of cyclic AMP, a secondary messenger that activates protein kinase activities, leading to activation of sperm motility.

Question 8

What happens in the slow event for capacitation?

Answer 8

A loss of cholesterol from the PM of sperm cells. BSA is added in vitro to capacitate sperm. This is because BSA is an acceptor of cholesterol, so can release from the PM, thus allowing hyperactivation of the spermatozoa.

Question 9

Why is hyperactivity important?

Answer 9

Hyperactivity is important as sperm cells need to travel a very long path once released in the vagina.

Question 10

Described the speculated ability of sperms for chemotaxis.

Answer 10

It was discovered that sperm cells present a olfactory receptor (hOR17-4), which is able to recognize the scent of bourgeonal.

This proves that sperm cells may have receptors that respond to specific molecules released by the oocyte, but this has not yet been proven in humans.

Question 11

Described the speculated ability of sperms for thermotaxis.

Answer 11

Sperm cells may be able to sense temperature differences, preferring the higher one as the temperature is 2 degrees higher in the ampolla than the isthmus.

Question 12

Described the speculated ability of sperms for rheotaxis.

Answer 12

Sperm cells have the ability to align against a flow and swim upstream.

Question 13

How do sperm cells cross the cumulus oophorus?

Answer 13

The cells are maintained around the oocyte due to the deposition of a muculastic extracellular matrix particularly rich in hyaluronic acid.

This is a glycosaminoglycan able to make a mucus like extracellular matrix together with several proteins.

Sperm cells possess hyaluronidase activity. These activities contain several lytic enzymes, e.g. acrosin.

Question 14

What is the zona pellucida made up of?

Answer 14

This is an envelope of glycoproteins. There are 4 in humans, ZP1, ZP2, ZP3 and ZP4.

Question 15

How does sperm cross the zonal pellucida?

Answer 15

Only sperm cells of the same species are able to recognise and bind to particular sugar molecules present on the ZP proteins.

When the sperm cell binds to ZP3, there are several responses on the sperm cells which increases cAMP and activation of protein kinases as well as influx of Ca2+ resulting in a fusion of acrosomal membrane with the PM of the sperm cell.

This ends up in complete release of the contents of the acrosome in the environment. The sperm cell can penetrate the zona pellucida only after the acrosomal reaction has occurred.

Question 16

What does the acrosomal reaction result in?

Answer 16

Results in a fusion between the outer membrane of the acrosome and the sperm PM. At the end of the process the original inner membrane of the acrosome will become the membrane of the sperm.

Question 17

Which glycoprotein is considered the primary binding between the sperm and the oocyte?

Answer 17

ZP3 considered to be involved in primary sperm binding.

Question 18

What is ZP2 thought to be involved in?

Answer 18

Once the sperm cell is bound to and goes across the zona pellucida, ZP2 is modified to prevent other sperm cells from binding.

Question 19

Describe the evidence that the sperm may complete the acrosomal reaction before reaching pellucida.

Answer 19

Some studies demonstrate that there is an interaction between the cumulus and sperm cells, where the sperm cells release specific proteins to stimulate cumulus cells to release progesterone.

This may induce the acrosomal reaction. This might be needed to release the acrosomal hyaluronidases to help digest the hyaluronic acid of the extracellular matrix.

Question 20

What is fertilin?

Answer 20

Fertilin (PH-30) belongs to the family of the ADAM proteins and can bind to integrins. They are present on the head of the sperm cells and can bind to a specific integrin expressed on the egg membrane.

Question 21

Is fertilin indisposable to fertilisation?

Answer 21

Knockout studies (in mice) where expression of fertilin was completely abolished shows that the mice were still fertile.

Therefore fertilin is important to fertilisation, but not absolutely indispensable.

Question 22

Where is IZUMO1 located?

Answer 22

IZUMO1 has been shown to locate on the inner side of the acrosome. Though after the sperm has undergone acrosomal reaction, there is a broader distribution of IZUMO on the head of the sperm cell, especially on the equatorial side.

Question 23

Is the protein CD9 a confirmed partner of IZUMO?

Answer 23

Initially, the protein CD9 was identified as a partner to IZUMO. However if this expression is eliminated, the sperm cells can still bind but don’t fuse.

This shows that there may be another protein that interacts with IZUMO, but CD9 is still important for cell fusion.

Question 24

How was JUNO discovered?

Answer 24

JUNO was originally identified when performing a screening of oocyte cDNA, identifying this as a folate receptor in the process.

Question 25

What is JUNO’s function?

Answer 25

JUNO was identified to be the protein responsible for the adhesion of sperm cells to the oolemma.

Question 26

Why was JUNO’s function confirmed?

Answer 26

It was demonstrated that if the expression of this receptor is eliminated, the oocyte was not able to bind to sperm cells anymore.

Question 27

In humans, what happens once the nucleus of the sperm cell enters the oocyte?

Answer 27

There is a surge of Ca2+ in the cytoplasm. There is the activation of several signalling molecules, e.g. PLC and tr-c-kit. This Ca2+ surge is very important for the block of polyspermy.

Question 28

How does a Ca2+ surge block polyspermy in the oocyte once fertilised?

Answer 28

This Ca2+ is important as there are cortical granules in the oocyte and it allows the release of the fusion of the granules with the PM.

The content of the granules are released, they act on the zona pellucida, making it more impermeable to any other sperm cells and alters the structure of the PM so that no other cells can fuse.

Question 29

What is the fast block to avoid polyspermy?

Answer 29

Once there is an interaction between the sperm cell and the PM, there is an initial depolarization of the PM which blocks the binding of other sperm cells.

Question 30

What is the slow block to avoid polyspermy?

Answer 30

Release of the content of the granules which doesn’t allow other sperm cells to go through the zona.

Question 31

How can JUNO be involved in blocking polyspermy?

Answer 31

JUNO can be shed from the PM to abolish the chances of polyspermy which can also produce vesicles with JUNO on its surface, thereby binding with other sperm cells and neutralizing them.

Question 32

How is meiosis regulated?

Answer 32

Meiosis needs to be completed. A complex molecular mechanism regulated everything.

The oocyte was blocked in metaphase II by the inactivation of the APC complex, which is blocked by the action of MAP kinases and CSF complex.

A metaphase promoting factor was maintaining the oocyte arrested in metaphase II. The CSF blocks the APC. The arrival of Ca2+ activates the CaMKII able to degrade a factor that inactivated the action of CSF and MPF.

The fertilised oocyte is able to move from metaphase to anaphase and release the 2nd polar body.

Question 33

How is chromatin decondensed?

Answer 33

DNA in the sperm cell is usually highly condensed due to the action of protamine, which is substituted by the histones to achieve a decondensation of the chromatin. Then the 2 nuclei will fuse.

Question 34

Describe the process of FIVET.

Answer 34

Fertilisation In Vitro and Embryo Transfer - this is the procedure that is usually performed when there are problems, for instance sperm mobility.

This puts the sperm in direct contact with the oocyte to fertilise in vitro, then transferred back into the uterus.

Question 35

Describe the process of ICSI.

Answer 35

Intracytoplasmic Sperm Injection - if there are problems for example at the level of fusion proteins, then the sperm cell can be injected directly at the level of the oocyte.

Question 36

How can fertility be preserved?

Answer 36

If fertility would like to be preserved, then embryos or oocytes can be frozen at metaphase II.

Question 37

How does the age of the uterus play a role in fertilisation?

Answer 37

The older uterus is less fertile than the younger uterus.

Question 38

How does RU486 work as a medical abortion drug?

Answer 38

RU486 can be used as an abortion procedure. This drug blocks the action of progesterone, causing the cervix to soften, uterine lining to breakdown and bleeding to begin.

A few days later, a second drug is taken to expel any remnant fetal tissue.