Fertilisation

Question 1

Where, and what is the axoneme?

Answer 1

It is located in the midpiece and is responsible for propulsion. It has a 9 + 2 arrangement. It is surrounded by outer dense fibres to stiffen and provide the tail beat.

Question 2

What is flagellum movement controlled by? Provide a specific example.

Answer 2

It is controlled by ion channels e.g. Calcium channel CatSper

Question 3

What is the possible role of sperm cytoplasmic factors?

Answer 3

Activation of the egg and development of the embryo.

Question 4

In some species, what is the role on the centriole (part of the axoneme)?

Answer 4

Regulating cell cleavage.

Question 5

What happens to sperm mitochondria?

Answer 5

They are eventually destroyed in normal embryo development, it is debatable if they have any other function.

Question 6

What is contained within the ovum?

Answer 6

A store of substrates of mRNAs, to sustain growth until true embryonic expression occurs.

Question 7

What surrounds the ovum?

Answer 7

The egg membrane (oolemma) and an extracellular matrix (zone pellucida). This initially evolved to protect the egg and early embryo, but it is also involved in gamete recognition.

Question 8

What state is the ovum in at ovulation?

Answer 8

Arrested in metaphase II, which the chromosomes aligned on the metaphase phase and attached to spindle fibres.

Question 9

What occurs during ovum maturation?

Answer 9

Asymmetrical division to eliminate one set of chromosomes (4n to 2n) and very little cytoplasm, via a vesicle called the first polar body.

Question 10

What are th role of cumulus cells?

Answer 10

To form a viscous matrix around the ovum and secrete hyaluronic acid. They increase the total volume, allowing the cilia to waft the whole complex from the proximal end of the oviduct into the oviductal lumen for fertilisation.

Question 11

What is the fertile life of an unfertilised egg?

Answer 11

24 hours

Question 12

What is behavioural oestrus?

Answer 12

Synchronised mating to before or near to the time of ovulation.

Question 13

What does sperm have to travel through to get to the egg?

Answer 13

Valine, cervix, and uterus (lower portion), then cross the utero-tubule junction. This is part achieved by smooth muscle action, and part by sperm motility (allowing the most motile ones to be selected).

Question 14

What is capacitation?

Answer 14

Further maturation processes the sperm undergoes after ejaculation. These usually take several hours and involve removing proteins/lipids from the sperm plasma membrane to make it more fluid for changes such as receptor aggregation.

Question 15

Where does the sperm go near the time of ovulation?

Answer 15

It leaves the storage sites in the isthmus region (near the uterine-tubule junction) to the ampullary region, where fertilisation will take place.

Question 16

What is hyper activated motility?

Answer 16

Even further changes the sperm undergoes to alter its motility and beat much more erratically, in order to penetrate the ZP. This occurs whilst the sperm is passing through the cumulus mass.

Question 17

What is the acrosome reaction (AR)?

Answer 17

A result of primary sperm egg binding and leads to the vesiculation and fusion of the outer acrosome membrane with the overlying plasma membrane. This results in the release of acrosomal contents and exposure of the inner acrosomal membrane.

Question 18

How is the ZP penetrated?

Answer 18

Acrosomal contents (serine proteases such as acrosin) and sperm head force. The sperm makes a narrow split in the ZP to reach the perivitelline space, and makes contact with the oolemma.

Question 19

What happens when the sperm reaches the oolemma?

Answer 19

The equatorial region membrane fuses with the oolemma to form a contiguous membrane. This causes the egg to release cortical granules, whose son products modify the oolemma/ZP to prevent further sperm entry (block to polyspermy).

Question 20

How is sperm genetic material expressed?

Answer 20

The chromatin decondenses and the protamines are replaced by histones.

Question 21

How if egg genetic material expressed?

Answer 21

Meiosis resumes and the chromosomes are separated, with one seat eliminated as the second polar body. The remaining set decondense and are surrounded by a nuclear membrane to form the female pronucleus.

Question 22

What is syngamy?

Answer 22

The male and female pronuclei migrating to the centre of the egg and coalescing. (Note: the cumulus will have now disappeared).

Question 23

How is paternal mitochondria destroyed after fertilisation?

Answer 23

Mitochondria membrane proteins are tagged with ubiquitin, a marker for protein degradation and recycling. This marker is masked until fertilisation, where after it is amplified.

Question 24

What are the main features of mammalian sperm?

Answer 24

Condensed nuclear chromatin + protamines; acrosome; equatorial segment; flagellum.