Fertilisation Flashcards
How many sperm are produced in an ejaculate?
20 - 300 million
What are the different mechanisms to make sure the egg only gets the best sperm?
Acidic pH and leukocytes in the vagina
Cervical mucus traps poor motile sperm
Rheotaxis were the sperm are swimming against the current
Chemotaxis trying to destroy the sperm and makes it warmer
What is the role of the oviduct in fertilisation - the different parts of the oviduct?
Utero-tubul junction - sperm selection and reduction
Isthmus - stores sperm
Ampulla - hold oocyte and site of fertilisation
How is sperm selected in the uterotubul junction in mice?
The protein ADAM3 must be recognised in order for sperm to pass through. This is found on the surface of the sperm and we think it binds to the junction.
The sperm needs to be healthy e.g. motile, have acrosome, uncapcicated,
What happens to sperm in the isthmus region?
They can rest by binding to the cillia
What are the 3 stages of fertilisation?
Sperm preparation
Sperm binding and fusion
Corticol reaction
Sperm preparation - what is capacitation?
Ejaculated sperm need to go through this to fertilise. Takes place in the female reproductive tracts and is activated by its alkaline environment.
Brings about physiological and molecular changes including loss of cholesterol, phospholipids and glycoproteins. Increases ROS generation, calcium influx, tyrosine phosphorylation
What happens to sperm after capacitation?
They become less stable but have increased motility and can respond to chemoattractants (they are hyperactive).
What triggers capacitation?
Endocrine signal at ovulation induces change in sperm
Why do sperm need to be hyperactive?
Helps pull the sperm away from oviduct epithelium (isthma)
Penetrate dense cumulus complex
Sperm preparation - what happens during the acrosome reaction?
Progesterone induces sperm plasma membrane and outer acrosomal mebrane fuse to create pores which releases enzymes and exposes new set of surface antigens which can bind to the oocyte
What enzymes are in the acrosome?
Proteolytic enzymes, acrosin, trypsin, hyaluronidase and proteases which are needed until this point to get through the uterus
Can uncapacitated sperm undergo acrosome reaction?
No
What does the zona pellucida do?
Mediated species-specific recognition, prevents polyspermy and protect preimplantatin embryo from reabsorption.
Made of Zp1, ZP2, ZP3, and ZP4 in humans
How does the sperm get through the zona pellucida?
ZP2 is the primary ligand on the egg but we dont know about the sperm. It is however more likely to be a multimolecular event.
Once bound it digests its way through using the acrosome enzymes
Once the sperm gets through the zona pellucida what happens (sperm binding to oolemma)?
In the perivitelline space the izumo1 (ligand exposed after acrosome reaction) on the sperm binds to the Juno receptors on the oolemma.
CD9 and CD81 stabilise this complex
How does the sperm and oocyte fuse?
Microvilli on the oolema engulf the sperm, the anterior in engulfed by pinocytosis and the spermatozoon stops moving. The fertilisation cone forms at point of fusion and the sperm head passes into the oocyte cytoplasm.
How does the oocyte stop polyspermy? (cortical reaction)
Corticol granules fuse with plasma membrane and contents are released into the perivitelline space. These contain proteases, peroxidase, polysaccharides and zinc.
What are the blocks set up by the corticol reaction?
Zinc sheild - short transient sheild hardens the zona pellucida and disregulates the zinc signalling in the sperm.
Membrane block - juno vesicles are shed into the perivitelline space and any other sperm bind to it.
Zona block - zona pellucida hardens by cleaving one ZP
These all take different amounts of time
How does the membrane block work?
Juno receptor is shed and relocated. The microvilli are reorganised and there is a decreased in CD9 production
How does the zona pellucida block work?
In the cortical granules there is a protein - ovastacin - which cleaves ZP2 hardening the zona.
Oligiosaccharides removed from ZP3 and adjacent ZP moleculars are cross-linked.
What inhibits ovastacin until gamete fusion?
Fetuin-B to keep the ZP permeable because premature ZP hardening can cause infertility (when fertilisation happens the ovastacin released in huge so over rides this mechanism).
What is polyploidy?
Multiple copies of the genome which is embryonic lethal.
This occurs from mismanagement of centrosomes (multiple spindles form resulting in asynchronous cell division).
Extremely rare in semi-indentical twins (egg fertilised by 2 sperm and split).
Can some animals cope with polyploidy?
Yes mainly birds
