Fertilisation Flashcards
What is required for fertilisation to occur?
- A sperm: Needs to undergo maturation in the epididymis and capacitation as it travels through the female reproductive tract.
- An egg: Needs to have completed meiosis 1 and arrested at metaphase II.
- Sperm and egg need to meet each other in the same place, at the same time.
- Fertilisation needs to be synchronised with a receptive endometrium (uterus).
What is released during ovulation?
Cumulus-oocyte complex (cumulus oophorus cell surrounding the egg).
What picks up the cumulus-oocyte complex?
Picked up by ciliated fimbriae on the infundibulum at the end of the uterine tube.
What aids the fimbriae to pick up the cumulus-oocyte complex?
- Follicular fluid acts as a chemoattractant.
- Muscular contractions of uterine tubes create negative pressure to help suck up the cumulus-oocyte complex.
What happens to sperm numbers as they move through the female tract?
As the sperm move through the female tract, their numbers decrease rapidly.
Describe the environment in the upper vagina and what protects the sperm from it.
The vagina is an acidic environment. Seminal plasma is alkaline, providing short-term buffering and coagulates within a minute of entering the vagina.
What secretes seminal plasma?
Accessory sex organs in males (prostate & seminal vesicle).
What is found in the cervix?
The cervix is filled with mucus called cervical mucus.
When is cervical mucus the thickest vs the thinnest and what is the significance of this?
Cervical mucus is very thick for much of the cycle, making the cervix virtually impenetrable to sperm. It is least viscous during days 9-16 (time of ovulation) due to the influence of oestrogen.
What promotes cervical mucus to be less thick?
Oestrogen, which increases during pre-ovulation.
Describe the mechanism of the contraceptive pill.
The oral contraceptive pill increases the viscosity of cervical mucus to stop sperm from getting through into the female tract.
How many sperm from the ejaculate actually make it into the uterus?
Approximately 100,000 sperm get through the cervix and enter the uterus.
What are the helpful mechanisms that aid sperm in traveling towards the oocyte?
Pro-ovarian contractions of the myometrium in the late follicular phase help the sperm on their journey towards the ovaries.
How many sperm from the ejaculate actually make it into the uterine tube?
Approximately 1,000 sperm make it into each uterine tube.
How do sperm know which tube to go for?
There is a 50-50 chance that the sperm will choose the right uterine tube, but evidence of chemotaxis suggests progesterone and compounds in follicular fluid may guide the sperm.
Where does the sperm need to travel to precisely?
The sperm must travel to the ampulla (top 1/3rd of the uterine tube).
How does sperm overcome the highly viscous secretions in the female reproductive tract?
The sperm are streamlined and optimised to move through this, acting as a selection pressure.
Where does fertilisation occur?
Fertilisation occurs in the ampulla region of the uterine tube.
How many sperm from the ejaculate actually make it into the ampulla?
Approximately 10’s to 100’s of sperm make it to the ampulla.
How long does the sperm remain capable of fertilisation within the female tract?
Approximately 5 days.
How long does the egg remain viable for?
Approximately 24 hours.
When is the chance of fertilisation the lowest?
Intercourse the day after ovulation, as the egg is only viable for approximately 24 hours.
What do ovulation prediction kits test for (OPKs)?
Ovulation prediction kits detect LH, as there is an LH surge about 24 hours before ovulation.
What happens once sperm reach the ampulla?
When sperm enter the ampulla, they encounter the cumulus-oocyte complex.
What needs to happen for sperm to reach the egg?
- The sperm has to get through cumulus cells.
- Bind to zona pellucida.
- Undergo acrosome reaction.
- Get through zona pellucida.
What is the cumulus?
Cumulus mass: Approx. 3,000 cells embedded in a gelatinous matrix made of hyaluronic acid.
How does sperm penetrate/disperse the cumulus mass?
Sperm have a hyaluronidase enzyme on their surface which can break down the hyaluronic acid.
What does the sperm do after penetrating the cumulus?
After penetrating the cumulus, the sperm bind to the zona pellucida, inducing the acrosome reaction.
What is the acrosome reaction and why is it vital?
The acrosome reaction permits penetration of the zona pellucida, allowing fertilisation of the egg.
What is the zona pellucida?
An extracellular protein matrix surrounding all mammalian eggs, important for sperm-egg binding and induction of the acrosome reaction.
What happens after sperm has penetrated the zona pellucida?
The sperm penetrates the zona pellucida and occupies the perivitelline space, leading to fusion with the oocyte plasma membrane.
What are the 3 key proteins in sperm-egg fusion?
Izumo (on sperm), Juno (on oocyte), Maia (on oocyte).
What is the protein present on the sperm head after the acrosomal reaction?
Izumo, required for fusion; it is only detectable on the sperm surface after the acrosome reaction.
What are the receptors for Izumo on the oocyte plasma membrane?
Juno & Maia, which interact with Izumo.
What are the receptors for Izumo on the oocyte plasma membrane?
Juno & Maia
Juno binds first → conformational change → Maia binds.
What happens after the fusion of egg and sperm?
Within 1-3 mins of fusion, there is a large rise in [Ca2+]i which lasts 2-3 mins, followed by Ca2+ oscillations every 3-15 min for several hours post-fertilisation.
What triggers the increase in [Ca2+] post-fusion?
Phospholipase C zeta (sperm-specific PLC) triggers the increase, leading to the release of IP3 and DAG - PKC.
What are the key effects of increased intracellular Ca2+ levels?
- Release of meiotic block
- Block to polyspermy
At what phase is the egg currently arrested?
The egg is currently arrested at metaphase 2.
What contributes to the meiotic block?
- M-phase Promoting Factor (MPF) = cyclin-dependent kinase (cdk1) & cyclin B
- Cohesion protein complexes.
How does calcium help release the meiotic block?
Raised calcium levels suppress CSF activity and destroy cyclin B by activating anaphase-promoting complex/cyclosome (APC/C), which also removes cohesion rings.
What is polyspermy?
Polyspermy is when more than one sperm fertilises an egg.
Describe the mechanism by which polyspermy is prevented.
- Fast block: Egg plasma membrane depolarises immediately after fertilisation.
- Slow block: Cortical granules fuse and release contents, triggered by increase [Ca2+]i.
What triggers the cortical reaction and what does it induce?
Triggered by increase [Ca2+]i, it induces the zona reaction and leads to the loss of Juno.
What is the zona reaction?
The alteration in the structure of the zona pellucida catalysed by proteases from cortical granules, leading to hardening and preventing sperm binding.
What is the loss of Juno?
Juno protein is shed from the oocyte plasma membrane post-fertilisation, preventing further sperm fusion.
What does the sperm contribute?
The sperm brings its haploid male genome and a centriole for spindle formation.
What does the oocyte contribute?
The oocyte contributes a haploid female genome, lots of cytoplasm, all organelles, and mitochondria.
Which germ cell contributes to the determination of the sex of the baby?
Sperm.
What is required for decondensation of the sperm DNA?
It requires protamine being replaced by histones.
What happens after sperm DNA is decondensed?
The male and female pronuclei replicate their DNA and migrate towards each other, guided by the sperm aster.
What is sperm aster?
Sperm aster is formed by the sperm centriole and consists of microtubules radiating from the centrosome.
What is syngamy?
Syngamy is the combination of male and female genetic material.
When does syngamy occur and what happens after?
Syngamy occurs 18-24 hours after sperm fusion, leading to breakdown of pronuclear membranes, chromatin intermixing, and nuclear envelope reforming.
What marks the end of fertilisation/beginning of embryogenesis?
The cleavage stage.
What is a zygote?
A fertilised egg.
Summarise the pathway from ejaculation to zygote formation.
Sperm travels to ampulla to fertilise egg, pronuclei combine, first cleavage occurs within 24hrs, embryo travels down uterine tube, reaches uterus around day 4, and implants around day 7 post-fertilisation.
Which other system in the body supports the transport of the embryo to the uterus?
The endocrine system
Which hormone is high at this level of the menstrual cycle?
Progesterone.
What’s the significance of increased progesterone levels at this time?
The increased progesterone:oestrogen ratio relaxes musculature of the female reproductive tract, allowing the embryo to pass through the uterine tube.
What structure helps transport the embryo through the uterine tube?
The embryo is mostly transported through the uterine tube under the action of cilia.
Stages in order of embryogenesis?
Fertilisation → cleavage → blastocyst formation → hatching → implantation.
What happens to the embryo as it moves through the uterine tube?
As the embryo moves through the uterine tube, it undergoes cleavage.
What is cleavage?
A period after fertilisation when a 1-cell embryo starts developing into a multicellular organism.
These cleavage-stage cells are called blastomeres.
What happens during cleavage?
The zygote cleaves to form two blastomeres, then goes through the 4-cell stage and 8-cell stages.
What is a morula?
The embryo is known as the morula at the 16-32 cell stage.
What happens to the size of the embryo as it divides and why?
Each blastomere size decreases with each division due to no cytoplasmic synthesis.
Why is the ZP significant?
The ZP helps to keep all the cells in tight contact to support embryo development.
When is the ZP lost?
During the hatching stage.
What are the first stages of blastomere development dependent on?
Up to the 2-cell stage, development is dependent on oocyte cytoplasm.
What happens during the 4 to 8-cell stage?
A major burst of transcription occurs as the embryo genome is activated.
Why is the quality of the egg/oocyte important?
Many maternally derived proteins persist until the blastocyst stage, highlighting the importance of egg quality.
What stimulates embryo metabolism and growth?
Embryo metabolism and growth are stimulated by various growth factors, both autocrine and paracrine.
What is the first stage of differentiation called?
Compaction.
What is compaction?
Starting from the 8-cell stage, inside-outside polarity begins to develop.
What does inside-out polarity result in?
Outer cells form the trophoblast, and inner cells form the inner cell mass.
How do we get blastocoel formation?
In the late morula stage, a cavity starts to fill with fluid due to intercellular junctions between trophoblast cells.
When do we get to blastocyst development?
Day 5.
What is a blastocyst made up of?
A blastocyst is made up of a blastocoel, distinct inner cell mass, and a single-layered epithelial trophoblast layer.
What do trophoblast cells produce and what is its function?
Trophoblast cells produce HCG for the maintenance of the corpus luteum.
What are the two poles of the blastocyst?
The blastocyst has an embryonic pole and an abembryonic pole.
What is the significance of this stage in IVF?
This is the point in IVF where embryo transfer is done, allowing for the selection of good quality embryos.
Stages in order of embryogenesis?
Zygote → blastomere → morula → blastocyst.
What is the hatching stage and when does it happen?
The final stage before implantation, occurring late day 6 onwards into day 7.
What happens during hatching?
The ZP is lost, and the blastocyst expands out of a hole in the ZP.
Which side does the embryo blast hatch out of?
The abembryonic side.
What is the rate of twin pregnancies?
1 in 67 pregnancies is a twin pregnancy (~1.5%).
What are the types of twins?
There are two types of twins: monozygotic (identical) and dizygotic (non-identical).
What does the form of monozygotic twins depend on?
It depends on the stage of development at which separation happens.
At which stage of the embryo is cleavage most common?
Cleavage at the blastocyst stage.
What has caused an increase in dizygotic twins in recent times?
Increased maternal age and fertility treatments.
What are the risks of twins to mother and baby?
Increased risks include twin-twin transfusion syndrome, premature birth, low birth weight, and maternal complications.
What is the cost of twins on the NHS?
Twins are a greater cost during pregnancy, especially if born early or with complications.
Describe the layers of the uterus.
The uterus has an outer serosa membrane, a muscular myometrial layer, and the endometrium.
What are the two layers of the endometrium?
The upper functional layer and the lower basal layer.
Which layer of the endometrium undergoes shedding during menstruation?
The upper functional layer.
How is a new functional layer made after it has shed?
A new functional layer is reconstituted from the underlying basal layer every month.
What cells are found in the endometrium?
Stromal cells and luminal epithelial cells.
Describe the blood supply of the endometrium.
The stroma contains a rich supply of blood vessels supplied by the spiral arteries from the ovarian artery.
What supplies blood to the endometrium?
The stroma contains a rich supply of blood vessels supplied by the spiral arteries from the ovarian artery.
What is the order of blood supply to the endometrium?
Ovarian → uterine → arcuate → radial → spiral.
What are the layers of the endometrium?
A: Functional layer of endometrium, B: Basal layer of endometrium, C: Myometrium.
What are the components of the endometrium?
1: Luminal epithelium, 2: Glandular epithelial extensions, 3: Stromal cells/stromal matrix, 4: Rich supply of blood vessels from spiral arteries.
What are the stages of the menstrual cycle in order?
Menstruation → proliferative (follicular) → ovulation → luteal (secretory).
What drives the replacement of the functional layer of the endometrium after menstruation?
Endometrial cells of the basal layer proliferate to form a new functional layer, driven by rising oestrogen in the first half of the cycle.
What happens at the end of the proliferative phase?
Further thickening due to stromal cell proliferation and oedema, increase in surface area and metabolic activity of surface epithelium, increase in glandular invaginations, and expression of intracellular progesterone receptors.
What hormone do the ovaries secrete after ovulation?
Progesterone.
What is the function of progesterone?
Progesterone stimulates synthesis and excretion of secretory material by the glands, which are rich in glycogen, glycoproteins, and amino acids to provide nutrition for the blastocyst.
What happens during the luteal phase?
Stromal cells become larger, spiral arteries fully develop, cellular secretions are released into the glandular lumen, and pinopodes appear.
What is a receptive endometrium?
The ability of the endometrium to successfully attach the blastocyst, nourish it, and keep it alive.
How does the endometrium become receptive?
Through the combined action of oestrogen and progesterone.
What is the window of implantation?
The period in each menstrual cycle where implantation is possible.
How long does the window of implantation last?
It lasts four days (days 20 – 24).
What characterizes the appearance of the endometrium during the window of implantation?
The appearance of small elevations at the apical pole of the epithelial endometrium cells, known as pinopodes.
What is the function of pinopodes?
Pinopodes are involved in the absorption of uterine fluid, bringing the blastocyst closer to the endometrium and immobilizing it for better contact.
What causes menstruation to occur after the window of implantation?
If there has been no implantation, the corpus luteum fails, leading to a fall in progesterone levels.
What is the effect of decreased progesterone levels?
Spiral arteries constrict, leading to collapse and necrosis of the functional layer, resulting in menstruation.
What are the clinical conditions relevant to this lecture?
Infertility, miscarriage, and ectopic pregnancy.
What is defined as a miscarriage?
Loss of pregnancy prior to 24 weeks.
What is the rate of miscarriage?
1 in 4 pregnancies.
What is the most common cause of miscarriage?
Approximately 60% are due to chromosomal problems, with 25% of embryos being aneuploid.
What is the rate of ectopic pregnancies?
1 in 100 pregnancies (1%).
What is an ectopic pregnancy?
Implantation occurring in the wrong place.
Where can ectopic pregnancy occur?
Anywhere outside the womb, including the abdomen and cervix.
Why is ectopic pregnancy life-threatening to the mother?
Rupture of blood vessels during implantation can lead to hemorrhage.
What are the risk factors for ectopic pregnancy?
History of pelvic inflammatory disease (PID), tubal surgery, failed sterilization, and IUD in place.
