Fertilisation Flashcards
During ovulation, how what mechanism is in place to move the egg down the fallopian tube?
muscular contractions of the fallopian tube
During ovulation, which part of the fallopian tube picks up the cumulus-oocyte complex?
ciliated fimbriae of the infundibulum
Cillia sweep the complex into the tube
Once ovulated how long is the oocyte viable for?
~24 hours
How long are sperm in the female tract capable of fertilisation for?
~5 days
Where in the fallopian tube does fertilisation occur?
ampulla
What are the sperms interactions with the egg?
- detects the oocyte-cumulus complex
- penetrates the cumulus
- binds zona pellucida
- acrosome reaction
- penetrates the zone pellucida
What substance/enzymes are involved in the coagulation of sperm in the upper vagina and it’s subsequent liquidification?
Within 1 min, semenogelin reacts with enzymes from the prostate - coagulum forms
~half an hour later, PSA cleaves semenogelin - becomes less viscous and flow out of vagina
Where is semenogilin secreted from?
seminal vesicles
How does the myometrium aid in fertilisation?
Myometrium contracts all the time during the menstrual cycle, varying in amplitude and frequency.
During late follicular phase when we’re most fertile the frequency of contractions increases, helping push sperm up to fallopian tubes.
How do sperm penetrate the cumulus
Sperm completely disperse the cumulus cells – they produce hyaluronidase enzyme which breaks down the hyaluronic acid holding the cumulus cells together around the oocyte.
Describe the acrosome reaction
The acrosome contains hydrolytic enzymes which break through the ZP.
Critical because release of these enzymes also exposes the internal acrosomal membrane on the sperm which has receptors on required for fusion with the egg
What is the name of the receptor on the sperm which is exposed by the acrosome reaction and is critical for oocyte binding?
Izumo
What is the name of the receptor on the egg which binds Izumo on the sperm?
Juno
What happens once the sperm has fused with the egg?
An increase in intracellular Ca sweeps across the egg followed by Ca oscillations lasting several hours
Leads to release of meiotic block at MII and completion of meiosis II.
How does the egg prevent more than one sperm fusing (polyspermy)?
Fast block - egg plasma membrane depolarises within mins
Slow block aka cortical reaction - rise in Ca causes exocytosis of cortical granules from the egg, released proteases diffuse through ZP inducing the zona reaction - proteolytic enzymes from the cortical granules change the structure of the ZP so sperm can no longer bind it
Also Juno is lost from egg plasma membrane after ~40 mins.
What does the sperm contribute to fertilisation?
Haploid male genome – sex of the baby
Centriole – the oocyte has none - forms spindle for first cell division
What does the oocyte contribute to fertilisation?
Haploid female genome
Cytoplasm
All organelles
Mitochondria – maternally inherited
How does the fertilised egg get down to the uterus?
Cillia in the fallopian tubes beat towards the uterus to transport the embryo down
Progesterone: oestrogen ratio relaxes the musculature so it is easier
Describe syngamy (the fusion of the male and female genetic material)?
Male and female pronuclei replicate their DNA then migrate to each other guided by the sperms aster (microtubules radiating from its centrosome)
Pronuclear membranes break down, M&F genetic material combine and a new nuclear envelope forms around the zygote nucleus which then starts dividing (“cleavage”) - end of fertilisation/start of embryogenesis
In the secretory phase of the uterine cycle what is the purpose of the secretions?
glycogen, glycoproteins and amino acids - to nourish the blastocyst in early development when the placenta is not developed enough to do so
How long does the window of implantation in the endometrium last?
4 days
