8. FERTILISATION Flashcards
1
Q
What’s the importance of coagulation of sperm?
A
- 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
2
Q
How does the cervical mucus allow sperm to move through?
A
- 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
3
Q
What is the sperm capacitation reaction?
A
- 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
4
Q
How do sperm become hyper activated or hypermotile?
A
- 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
5
Q
What is the acrosomal reaction?
A
- 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
6
Q
What happens after the sperm & the oocyte membrane fuse?
A
- 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
7
Q
What are the two effects of an intracellular rise of Ca2+ caused by phospholipase zeta?
A
- 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
8
Q
What is the cortical reaction?
A
- 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
9
Q
What happens when Meiosis II is resumed?
A
- 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
10
Q
What is syngamy?
A
- Syngamy is the fusion of two cells or pronuclei
11
Q
What happens once the two pronuclei have formed?
A
- Once the two pronuclei have formed, they fuse known as syngamy
- Their membranes break down
- The chromosomes allign on the mitotic spindle
12
Q
What happens once the pronuclei have fused?
A
- 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)
13
Q
When are the polar bodies formed?
A
- 1st polar body = Meiosis I
- 2nd polar body = Meiosis II
- Form due to unequal distribution of cytoplasm
14
Q
What is the cumulus oophorus?
A
- Outer layer of the egg, responsible for protecting the egg & is derived from granulosa cells
15
Q
What is the corona radiata?
A
Inner layer of the cumulus, formed from granulosa cells which are in contact with the zona pellucida
16
Q
What is the zona pellucida?
A
- 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
17
Q
Give an overview of the steps of fertilistaion
A
- Sperm comes into contact with the zona-cumulus oocyte complex
- Sperm penetrate the zona pellucida & bind to ZP3
- Acrosome reaction - acrosome bursts & releases enzymes (acrosin & proteases)
- Enzymes digest the zona pellucida
- Sperm & oocyte membrane fuse
- Sperm is taken in by phagocytosis
- Phospholipase Zeta within sperm causes increase in intracellular calcium
- Influx of Calcium into oocyte triggers cortical reaction & resumes Meiosis II
- Cortical granules fuse with ZP to form a hyaline membrane, preventing polyspermy
- Meiosis II extrudes second polar body & forms female pronucleus
- Sperm nuclear membrane breaks down, chromatin decondenses & chromosome separates
- Two new membranes form around two sets of haploid chromosomes
- Two pronuclei have formed
- Pro nuclei fuse - syngamy, membranes around individual nuclei breakdown
- Metaphase mitotic spindle forms. Chromsomes align on equator
- Mitosis occurs forming a 2-cell embryo
18
Q
Describe the development of the zygote after day 1 of fertilisation
A
- 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)
