Lecture 7 - 2018 Flashcards
What happens to meiosis following the LG surge?
Meiosis is resumed and the first division is completed. The process stops again, however, at second metaphase.
When will meiosis resume again at the second metaphase?
At fertilisation.
Describe meiosis after the LH surge?
There is an oocyte (2n) that will undergo meiotic division and will create an oocyte (n - 23 chromosomes) and the 1st polar body. The polar body is retained within the zona pellucida.
What happens to the sperm as it goes through the vas deferens?
As the sperm leaves the vas deferens and the urethra it is based in seminal plasma.
What happens to the seminal fluids and the prostatic fluids when they reach the vagina?
They come together in the vagina and form a coagulum - this is to hold the sperm together in the female reproductive tract.
What happens to the coagulum after 2-15minutes of ejaculation?
The coagulum is dispersed allowing for individual sperm to begin their quest for an egg.
What is the breakdown of the coagulum called?
Liquefaction - proteases break down the coagulum.
What happens is the sperm stays stuck in the coagulum?
They are not able to reach the fallopian tubes.
How does the sperm combat the acidic pH of the vagina (median 4.7)?
The seminal plasma forms a buffer, which drops the vaginal pH to 7.3 allowing the sperm to be motile.
What happens when the sperm reach the cervix?
Sperm move through the cervix at a rate of 1.2-3mm/min. Sperm have to get through the cervical mucous. The tails of normal sperm cause the fibres of the cervical mucus to vibrate in rhythm - if sperm is not normal it will not be able to induce this vibration in the mucous.
What happens to the cervical mucous around the LH surge?
It allows for a window of opportunity for sperm - the mucous has thinned out and allows for sperm to move through it.
What is the survival time for sperm?
Vagina - 2.5 hours.
Cervix - 48 hours.
Uterus - 24 hours.
Oviduct - 48 hours.
Clinical advice - sperm can last in the female reproductive tract for up to 5 days.
What is the transport time of sperm?
Sperm takes roughly 2-7 hours to reach the ampulla of the oviduct.
What is the swimming speed of sperm?
20cm/hr. Speed is related to fertility - speed decreases with time, freezing and thawing.
How much sperm can be found in a human ejaculate?
Up to 200,000,000 sperm, and of that only 1,000,000 will enter the uterus.
What do sperm do if they enter the fallopian tube prior to ovulation?
They slow down the beating of their tails and wait in the narrow isthmus of the tube.
Where does fertilisation typically occur?
At the isthmus/ampulla junction.
What happens when sperm arrive in the uterus?
They induce an infiltration of leukocytes - the invading leukocytes will phagocytose the dead or the dying sperm.
Describe capacitation in fertilisation?
A freshly ejaculated sperm are not capable of fertilising an egg. There is removal of the capacitation inhibitory substances from the sperm, there is also loss of cholesterol from the sperm membrane. Capacitated sperm now become hyperactivated.
What happens when sperm become capacitated?
They become hyperactivated:
- tail movement changes with an increase in bending of tail and swinging of head, which allows for sperm to move around objects in their way.
This allows for sperm to move through the tortuous folds of the fallopian tubes.
Can a non-capacitated sperm undergo the acrosome reaction?
No - only capacitated sperm can.
What is the acrosome?
Membranous pouch that covers the sperm.
What does the acrosome contain?
Digestive enzymes (hyaluronidase and acrosin). These enzymes break down the zona pellucida.
What happens during the acrosomal reaction?
The acrosomal and plasma membranes of the sperm head fuse - this allows release of the acrosomal contents to the environment around the sperm.
What triggers the acrosome reaction?
Influx of calcium into the cell, this can be triggered by many factors e.g. progesterone, ZP3.
How does calcium trigger the acrosome reaction?
Calcium comes into the cell which causes the plasma membrane to fuse with outer membrane. it causes the release of the acrosomal contents into the environment outside of the sperm, which helps to break down the ZP.
What happens if the acrosome reaction occurs distally from the oocyte?
The sperm is unable to fertilise the egg.
Describe progesterones role in the acrosome reaction?
It will trigger the breakdown of the plasma protein and it fuses to the acrosomal membrane - it creates pores and releases acrosomal content into the environment. The inner acrosomal membrane is exposed, this membrane contains receptors for the zona pellucida.
How else does progesterone help with sperm breaking down the egg membrane?
The cumulus oophorus will partially trigger an acrosome reaction - the sperm will release hyluronidase which allows for digestion of the cumulus cells and the basement membrane. This allows for the sperm to squeeze its way through the tightly packed cells of the corona radiate (outside ZP).
What is the zona pellucida?
Protein coat that surrounds the egg.
What happens when sperm come into contact with ZP?
Triggers complete full acrosome reaction. The sperm have a ZP3 ligand which attaches to the ZP3 protein on the ZP. This allows for sperm to get through the ZP via proteinases.
What happens when sperm have made their way through the ZP?
They come across the perivitilline space and then come to the oolemmal membrane of the egg (membrane the surrounds the egg). The sperm attaches side on to the membrane and the two membranes fuse and the sperm nucleus enters the oocyte.
What happens in the cortical reaction?
This is when the sperm nucleus and the oocyte nucleus have met. Once this occurs there is a release of intracellular calcium. This causes there to be a release of cortical granules (these sit on the oolemmal membrane) into the perivitelline space. These granules then attack the ZP and cause it to harden.
What do these cortical granules contain?
Proteases and beta-hexosaminidase.
What is the purpose of hardening the ZP after the sperm and egg have fused?
This is to prevent other sperm entering the zona - basically to stop polyspermy.
When does meiosis resume?
This occurs post-fertilisation.
What causes meiosis to resume post-fertilisation?
The rise in intracellular calcium will cause destabilisation of a protein complex which held the chromosomes in metaphase. Loss of complex will allow the chromosomes to migrate to the poles for meiosis to resume.
How long does it take the zygote to reach the uterus from the fallopian tube?
5 days.
Describe zygote transport?
Zygote begins cleavage in the tube and travels the oviduct and enters the uterus.
What happens to the blastomere as it travels down to the uterus?
It divides symmetrically and continued doubling gives rise to the morula at the 8-16 cell stage. The cells of the morula become tightly cohesive and compacted.
What happens in nidation?
The hatched blastocyst comes into physical contact with the receptive decidua. The blastocyst attaches to the endometrial/decidual epithelium via specific ashesion molecules.
When does implantation occur?
Approximately day 21.
