Lecture 6 – Fertilisation Flashcards
At which stage of the menstrual cycle is cervical mucus least viscous?
Day 9-16 of the cycle
Where does the oocyte slow down in the female tract for fertilisation?
The oocyte slows down in the Ampulla for fertilisation
How does sperm get to the oocyte?
Through their own motility and through movement of fluid by cilia through the ovarian tube assisting the sperm.
What is the advantage of the folded nature of the walls of the ovarian tubes?
It traps sperm so that it is spread out along the length of the tube.
What is the order of penetration of the oocyte by the sperm?
First it penetrates the cumulus that is a loose layer of cells surrounding the zona pellucida (corona radiata is innermost layer of cumulus). The sperm can disperse this layer and then the acrosome reaction allows it to penetrate the zona pellucida. Once it fuses with the oocyte within there is a rapid increase in free ca2+ across the egg.
What are Izumo and Juno?
Izumo is the sperm membrane receptor for fusion.
Juno is the membrane receptor on the oocyte.
What is the pattern of free Ca2+ after the sperm fuses with the oocyte?
Triggered by a sperm specific phospholipase C. A large rise in Ca2+ sweeps across the cell and lasts 3 mins. There are then recurrent oscillations every few minutes that lasts up to a few hours.
What prevents polyspermy after the free Ca2+ osciallations have stopped?
The Ca2+ triggers a cortical reaction which prevents further polyspermy. It releases a series of enzymes from corticol granules. The enzymes cause a change in structure in the zona pellucida. Also causes loss of Juno protein so there is no membrane receptor for sperm.
How does fertilisation cause release of the meiotic block?
MPF (maturation promoting factor) is activated by release of cytostatic factor which causes progression through the cell cycle.
How does the sperm and egg nuclei combine in the zygote phase?
The nuclei replicate themselves to form two pronuclei. The sperm centriole allows these two to be drawn together.
How does the zygote move to the uterus and what assists this?
The increased progesterone to oestrogen ratio causes relaxation of the ovarian tubes particularly the sphincter of the isthmus. The cilia also waft in the opposite direction to move to zygote to the uterus.
What are the different cleavage stages from the zygote?
Zygote cleaves into two blastomeres.
Morula stage is 16-32 cells stage.
Why does the blastomere cell size decrease with each division?
As there is no cytoplasmic synthesis.
What is the embryo dependent on for growth up to the blastocyst stage?
Maternally derived proteins.
What is compaction and when does this occur?
It occurs from the 8 cell stage onwards and is the formation of inside out polarity. The outer cells form the trophoblast layer and inner cells begin to form the inner cell mass.
