Fertilisation Flashcards
What happens in the epididymis? (3)
- Stored in the tail of the epididymis and undergo further maturation steps stimulated by androgens
- Further fluid absorption
- Epithelium secretions including fructose, proteins and glycoproteins aid in maturation and also make up part of the seminal fluid
What happens to the sperm plasma membrane during maturation? (2)
- Transfer of proteins into and onto the sperm plasma membrane to increase membrane fluidity
- This is essential for the future acrosome reaction which involves the fusion of the sperm plasma membrane with the acrosomal plasma membrane
What happens to the flagellum during maturation? (2)
- More rigid flagellum for stronger swimming
- cAMP content of tail increases
What is capacitation? (2)
- Further changes to the sperm within the female reproductive tract stimulated by presence of proteolytic enzymes, cholesterol ‘sinks’ and higher ionic strength
- Sperm are not able to fertilise an egg until they’ve undergone capacitation
What 2 characteristics are exhibited by fully capacitated sperm?
- Hyperactivated motility
- Changes in membrane properties that subsequently allow the acrosome reaction to happen if the sperm finds the egg
Why does capacitation happen? (2)
- Sperm more responsive to signals from oocyte
- Sperm able to undergo the acrosome reaction which is essential for fertilisation
What general changes happen in capacitation? (2)
- Stripping or modification of surface glycoproteins on sperm plasma membrane
- Changes in surface charge and reduction in membrane stability (more fluid) through general loss of cholesterol and formation of lipid rafts
What are the steps of capacitation? (5)
- Sperm cytoplasm becomes more alkaline
- Higher pH increases calcium permeability and hence intracellular calcium concentration
- Increased adenylate cyclase activity and cAMP production
- Activation of spermatozoal protein kinase A (PKA)
- Downstream phosphorylation (including flagellum proteins) and signalling cascades, resulting in the membrane and flagellum changes
What is hyperactivated sperm motility? (3)
- Involved high amplitude, asymmetrical beating of the flagellum
- Helps the sperm meet its target
- Helps sperm get through the cumulus oophorus and penetrate the zona pellucida
What is CatSper? (2)
- Voltage gated calcium ion channel specific to sperm
- Activated by alkaline pH
- Essential for hypermotility and fertility
What are the general steps of fertilisation? (6)
- After ovulation, the oocyte sits in the oviduct ampulla and waits for sperm
- Sperm come through the cervix and utero-tubal junction to enter the oviduct
- Sperm is attracted to the oocyte via chemoattractants released by the oocyte and the cumulus cells and cilia in the oviduct help it along
- Sperm can get trapped in the oviduct cilia or phagocytosed by immune cells in the female reproductive tract
- Fertilisation typically occurs in the oviduct whenever the sperm meets the egg
- Surface sperm hyaluronidase digests the cumulus cells so it can reach the zona pellucida
How long do the gametes survive for in the female reproductive tract? (2)
- Oocytes 6-24 hours after ovulation
- Sperm 24-48 hours
How many sperm get through to the egg?
In humans, over 99% spermatozoa don’t even enter the cervix
What are the steps of sperm-egg binding? (6)
- Sperm makes contact with the zona pellucida
- Acrosome reaction involving release of hydrolytic enzymes
- Acrosome reacts with perivitelline space
- Plasma membranes of egg and sperm fuse
- Sperm nucleus enters egg
- Cortical granules fuse with the egg plasma membrane which makes the vitelline layer impenetrable to further sperm
What is the perivitelline space?
Space between the egg plasma membrane and the zona pellucida
What is the zona pellucida made of?
4 glycoproteins: ZP1, ZP2, ZP3, ZP4
What is ZP1?
Structural protein which crosslinks the others
What is ZP2? (2)
- Contains a sperm-binding domain necessary for oocyte-sperm recognition and penetration of the zona pellucida
- Also responsible for the primary block to polyspermy
What is ZP3/ZP4? (2)
- Complexed together
- Involved in primary sperm-egg binding
What is the zona pellucida?
Extracellular matrix surrounding the egg plasma membrane and the early embryo
What does the acrosome reaction involve? (5)
- Described as the terminal phase of the capacitation process
- Occurs after the sperm head binds to the zona pellucida and the ZP proteins are responsible for inducing the reaction
- Sperm acrosome swells and the acrosome membrane fuses with the sperm plasma membrane
- Acrosomal vesicle undergoes exocytosis
- Release of hyaluronidase and exposure of acrosin
How does gamete binding occur? (6)
- Primary binding: sperm membrane binds zona pellucida via ZP3 (complexed to ZP4) and a species-specific complementary binding partner on the sperm
- Acrosome reaction happens and exposes acrosin, a protease on inner membrane of sperm
- Secondary binding: sperm inner acrosomal membrane binds to zona pellucida via ZP2 and acrosin
- Acrosin further digests zona pellucida and oocyte plasma membrane
- Adhesion of sperm equatorial region and oocyte membrane
- Penetration of sperm head into oocyte
How does oocyte activation happen? (6)
- Sperm PLC-zeta is released into the oocyte cytoplasm
- PLC-zeta facilitates hydrolysis of membrane bound PIP2 to DAG and IP3
- DAG activates PKC which phosphorylates downstream proteins
- PI3 triggers calcium release from intracellular ER stores leading to calcium induced calcium release
- Oscillations in calcium concentration triggers oocyte activation and resumption of the final meiotic division
- Also causes release of cortical granules into the perivitelline space for block to polyspermy
What is in the cortical granules?
Ovasticin enzyme
How is the block to polyspermy established? (2)
- Calcium stimulates fusion of zygotic cortical granules with the oocyte plasma membrane and release of the contents into perivitelline space
- Enzymes including ovasticin cause hardening of the zona pellucida and inactivation of sperm receptors via ZP2 cleavage
How is fertilisation completed? (3)
- Calcium pulses causes resumption of meiosis and the second polar body is discarded resulting in formation of the female pronucleus
- Sperm nucleus decondenses and protamines are replaced by histones to form the male pronucleus
- Pronuclei come together, DNA replication occurs, pronuclear membranes breakdown and replicated chromosomes align on the mitotic spindle ready for the first cleavage division (syngamy)
What is syngamy?
Combination of 2 genomes
What is the egg/sperm called immediately after fertilisation?
Zygote
What is the egg/sperm called after the first cleavage division?
Embryo
What occurs in pre-implantation development? (3)
- First few cell divisions are synchronous as the embryo travels down the fallopian tube
- Inner cell mass and blastocoel cavity form, still bound by the zona pellucida
- Blastocyst hatches out of the zona pellucida to allow embryo implantation into the uterus
What hormones are present during pregnancy? (3)
- High levels of progesterone (made by the corpus luteum and then the placenta) prepares the uterus for implantation and pregnancy
- Increased oestrogen and progesterone override monthly cycle and suppress the HPG axis
- Human chorionic gonadotrophin hormone (HCG) is produced by the placenta which is detected by pregnancy tests
How long is human gestation?
40 weeks (38 weeks +2)
