Initiation Of Pregnancy Flashcards
Why is the fertile window so short
Egg is only viable for 24 hours after ovulation
Sperm can survive in reproductive tract for 5 days
How far do sperm travel and how to they reach their intended destination
160 to 200mm to ampullary-isthmic junction
Combination own motility and uterine/oviduct contractions
99% lost through leakage and remaining 1% that make it bind to the oviduct/epithelium to “rest”
Why does sperm capacitation need to occur and how does it occur
What is this process dependent on
Freshly ejaculated sperm not fertile and must spend time away from inhibitory factors in seminal fluid
1) change in sperm activity = sperm gains corkscrew tail motion
2) Change in sperm head = acrosome reaction in which there is a change in membrane dynamics and releases proteolytic enzymes which will allow it to penetrate and enter ova
The process of capacitation is dependent on calcium
What is isogamy
Vast difference in size between egg and sperm
What enzyme digests through the cumulus and what happens when the sperm encounters an oocyte
- Hyaluronidase to digest through the ECM of the cumulus cells (not all can achieve this)
When sperm encounters an oocyte it interacts with glycoproteins ZP2/3 on the zona pellucida and triggers the acrosome reaction which hydrolyses this layer
Acrosin is released
Takes 5-20 mins
How does the successful sperm prevent polyspermy
Carries factor PLZ-zeta which causes release on intracellular calcium through activation of ryanodine receptors
This causes transient spikes of calcium which enables egg to resume meiosis from point of arrest (metaphase II)
High calcium concentrations cause breakdown of cell cycle regulators, creates maturation promoting factor MPF and so interrupts metaphase II arrest
How does the cortical reaction prevent polyspermy
Cortical granules near the membrane of the egg release their contents upon transcient calcium release into the perivitelline space.
Enzymes within the granules cause conformational changes to ZP2 and 3 preventing any further sperm from interacting with them
What happens on completion of second meiotic division
Extrusion of Half of the chromosomes through the second polar body. All that remains is the male and female pro nucleus
Female and male pro nucleus come together in process of syngamy. Now can be classed as a zygote
Cleavage process begins 20hrs ish after
Examples of when abnormal fertilisation can occur
- when polyspermy defences are breaches ( during IVF if they are 2 pro nuclei from male and 1 from female. Embryo will cleave due to calcium transience but wont develop past 3 days
- digynic fertilisation = errors in meiosis which result in 2 female pro nuclei and 1 male pro nuclei
- parthenogenic activation = egg activation without sperm ( 1 pronuclei from female). Will not develop past blastocyst stage
What occurs in early embryo development
Mitosis occurs but there is no growth of the cell itself (remains 1 cell), after 32-cell stage morula is formed and individual blastomeres cannot be seen because tight junctions form between the cells
Cells on the outside become the trophectoderm and other cells of the morula become compacted in one region. This is now known as the inner cell mass. At this the trophectoderm begin to secrete fluid leading to the formation of a blastocoel cavity. The inner cell mass differentiates into the epiplast and the hypoblast. Implantation occurs on day 6 ish in which the blastocyst burrows its way out of the zona pellucida and arrives in the uterus. Adjacent cells of the endometrial stroma in response to the presence of the blastocyst and progesterone secreted by the corpus luteum differentiate into metabolically active secretory cells call the decidual cells.
What molecular changes occur during early development
Sperm and egg are highly specialised forms of cells. Must become unspecialised and this occurs through detmethylation of all DNA
This creates a totipotent stem cell population that can later re-go methylation to create specific populations of cells (has to be done correctly though)
How are progesterone levels maintained in order to sustain implantation and pregnancy
As progesterone is critical for maintaining pregnancy its levels must be kept up and this is done through release of chorionic gonadotropin which binds to LH receptors on ovaries to maintain the corpus luteum by synchiotrophoblastic cells.
Progestogens can also be produced by the bio conversion of cholesterol
Once steroid synthesis begins do not longer need CG
Twins = how?
- monozygotic twins (identical twins)
1= early embryo splitting
2 = fused dichorionic
3 = ICM split
4 = embryonic disc split
5= incomplete split
Dizygotic twins = non identical twins
Process of IVF
1 = ovary is super ovulated by pharmacological intervention
2) via transvaginal ultrasound aspiration of the ovary, eggs are removed from the follicles
3) eggs are co-incubated with sperm on a Petri dish and allowed to fertilise the egg
4) at blastocyst stage fertilised egg is re-implanted into uterus to allow for pregnancy
Process of superovulation and down regulation
Superovulation = process of generating more than 1 egg from ovary
Down regulation = suppress growth in the ovarian follicles by prescribed agonist of GnRH which causes suppression of FSH and LH
Follicles mature to preantral stage
