8. FERTILISATION

Question 1

What’s the importance of coagulation of sperm?

Answer 1

• Sperm is coagulated by the enzyme coagulase which is found in the seminal fluid
• Coagulation prevents the loss of sperm via the vagina
• The sperm can adhere to structures within the female reproductive tract such as the cervical crypts
• The sperm will be deposited in the cervical crypts, and will then make their way towards the cervix
• This acts as slow release of sperm later on, acting as a reservoir to increase the chances of fertilisation

Question 2

How does the cervical mucus allow sperm to move through?

Answer 2

• The cervical mucus becomes more viscous & forms glycoprotein channels to allow sperm to enter the cervix
• The cervical mucus also facilitates the capacitation reaction
• The cervical mucus helps to filter out seminal fluid, abnormally morphological sperm & cellular debris

Question 3

What is the sperm capacitation reaction?

Answer 3

• The sperm capacitation reaction involves physiological or biochemical changes that allow fertilisation to occur
• Capacitation is partly achieved by removing the sperm from it’s seminal fluid, cervical mucus facilitates this aswell as uterine & tubal fluid
• One of these changes involves removing the glycoprotein coat & the seminal plasma proteins covering the acrosomal region, to make fertilisation easier
• Another change is that sperm become hypermotile

Question 4

How do sperm become hyper activated or hypermotile?

Answer 4

• Ca2+ ions enter the sperm via Catsper channels (Calcium sperm channels)
• the influx of calcium allows forceful tai beating with increased frequency
• The sperm also become more motile once they reach the alkaline environment of the uterus compared to the acidic vaginal environment

Question 5

What is the acrosomal reaction?

Answer 5

• The acrosomal reaction occurs when the sperm comes into contact with the zona-cumulus oocyte complex
• The sperm penetrates the cumulus oophorus & binds to ZP3 receptors on the zona pellucida
• The binding to the zona pellucida triggers the release of enzymes such as acrosin & protease from the acrosome when it bursts
• The enzymes digest the zona pellucida
• The sperm & oocyte membrane fuse

Question 6

What happens after the sperm & the oocyte membrane fuse?

Answer 6

• The sperm is taken in by phagocytosis
• The sperm contains the enzyme phospholipase zeta
• Phospholipase Zeta converts PIP2 -> IP3 + DAG
• The IP3 causes an intracellular release of Ca2+ which causes Meiosis II to resume & the cortical reaction

Question 7

What are the two effects of an intracellular rise of Ca2+ caused by phospholipase zeta?

Answer 7

• There’s an intracellular rise of Calcium as PIP2 -> IP3 + DAG
• The influx of Ca2+ into teh oocyte has two main effects:
  1. Cortical reaction
  2. Resumes Meiosis II

Question 8

What is the cortical reaction?

Answer 8

• The oocyte cytoplasm contains granules
• The cortical reaction is when these granules fuse with the zona pellucida to prevent multiple sperm binding known as polyspermy
• These granules contain proteases, hyaline & peroxidases
• Fusion of these granules with zona pellucida form a protective hyaline layer to prevent other sperm binding

Question 9

What happens when Meiosis II is resumed?

Answer 9

• The correct haploid number of chromosomes is already achieved by the menstrual cycle when the oocyte was arrested in Metaphase II of Meiosis II
• Once Meiosis II is resumed, the second polar body is extruded
• The sperm nuclear membrane breaks down & the chromatin is decondensed and the chromosomes are separated
• Two new membranes form around the two sets of haploid chromosome forming two PRONUCLEI

Question 10

What is syngamy?

Answer 10

• Syngamy is the fusion of two cells or pronuclei

Question 11

What happens once the two pronuclei have formed?

Answer 11

• Once the two pronuclei have formed, they fuse known as syngamy
• Their membranes break down
• The chromosomes allign on the mitotic spindle

Question 12

What happens once the pronuclei have fused?

Answer 12

• As the chromosomes align on the mitotic spindle, assuming the position of the equator, Mitosis occurs
• Mitosis then produces a 2-cell embryo which can further divide (cleavage)

Question 13

When are the polar bodies formed?

Answer 13

• 1st polar body = Meiosis I
• 2nd polar body = Meiosis II
• Form due to unequal distribution of cytoplasm

Question 14

What is the cumulus oophorus?

Answer 14

• Outer layer of the egg, responsible for protecting the egg & is derived from granulosa cells

Question 15

What is the corona radiata?

Answer 15

Inner layer of the cumulus, formed from granulosa cells which are in contact with the zona pellucida

Question 16

What is the zona pellucida?

Answer 16

• The zona pellucida is a glycoprotein layer secreted by the egg. It changes it’s structure to become impenetrable to more than one sperm
• It consists of adhesion molecules (ZP3) to allow the sperm to bind

Question 17

Give an overview of the steps of fertilistaion

Answer 17

1. Sperm comes into contact with the zona-cumulus oocyte complex
2. Sperm penetrate the zona pellucida & bind to ZP3
3. Acrosome reaction - acrosome bursts & releases enzymes (acrosin & proteases)
4. Enzymes digest the zona pellucida
5. Sperm & oocyte membrane fuse
6. Sperm is taken in by phagocytosis
7. Phospholipase Zeta within sperm causes increase in intracellular calcium
8. Influx of Calcium into oocyte triggers cortical reaction & resumes Meiosis II
9. Cortical granules fuse with ZP to form a hyaline membrane, preventing polyspermy
10. Meiosis II extrudes second polar body & forms female pronucleus
11. Sperm nuclear membrane breaks down, chromatin decondenses & chromosome separates
12. Two new membranes form around two sets of haploid chromosomes
13. Two pronuclei have formed
14. Pro nuclei fuse - syngamy, membranes around individual nuclei breakdown
15. Metaphase mitotic spindle forms. Chromsomes align on equator
16. Mitosis occurs forming a 2-cell embryo

Question 18

Describe the development of the zygote after day 1 of fertilisation

Answer 18

• DAY 1 = Zygote - 1 CELL
• DAY 2 = 4-CELL
• DAY 3 = Morula - 16 CELL
• DAY 5 = BLASTOCYST
• The blastocyst differentiates to form two lineages: embryoblast & trophoblast
• DAY 7 = IMPLANTATION (blastocyst hatches out of ZP)