Lecture 6: fertilisation and implantation Flashcards
When are the two polar bodies ejected from the oocyte?
- When there is the LH surge. It is released and sits under the zona pellucida where is rarely undergoes meiosis 2
- Second mitotic division (meiosis II) in response to fertilisation sees the second polar body form being retained in the ZP
Process as sperm enters the vagina?
- Sperm leaves male tract bathed in seminal plasma
- As prostatic and seminal fluids mix a coagulum is formed that holds the ejaculate together and adheres the ejaculate to the femal reproductive tract.
What makes life hard for the sperm? Path of sperm?
- acidic vagina between 3.5-7.4 is inhibitory to motility + survival
- Seminal plasma buffers the pH to where sperm can be motile.
- Fertilisation occurs at the ampulla so the sperm must pass through the cervix in a short window where the cervical mucus is not hostile to sperm transport
- During the periovulation window mucus is produced in large amounts and is thin, fertile mucus aligns its fibres and allows the passage of sperm in columns through the cervix @ 1.2-3mm/min
- The tails of normal sperm cause fibres to vibrate, abnormal sperm fail to induce this retarding the progress of the abnormal sperm.
Sperm survival times?
vagina = 2.5h
cervix = 48h
uterus = 24h
fillopian tube = 48h
OVERALL = 48h+ (clinical advice indicates 5day survival)
(99% of spermatozoa do not enter the cervix
Transpoert time and speed of sperm?
2-7 hours to reach the ampulla
Swimming speed =55um/s = 20cm/hr
- speed is related to fertility and decreses with freezing/thawing
- speed decreased with time since ejaculation
What happens to ‘unused’ sperm?
- Sperm arriving in the uterus induce an infiltration of leucocytes
- These invading leucocytes phagocytose dead or dying sperm
- Potentially the ‘immune’ exposure helps with tolerance to foetus
What is capacitation?
- A process involving the removal of capacitation inhibitory substances from the sperm
- There is a loss of cholesterol from the sperm membrane
- Freshly activated sperm are not capable to fertilisation
- It hyperactivates sperm: increased bending of tail and swing of head, potentially facilitating movement through tortuous folds
- Capacitated sperm can undergo acrosome reaction
The acrosome reaction?
The acrosome is a membranous pouch containing proteinases and other enzymes such as hyaluronidase. The acrosomal and plasma membranes fuse allowing release of contents into environment.
This reaction is triggered by an influx of Ca into the cell:
- Progesterone (from the cumulus cells) or ZP3 can trigger
- The reaction must occur close to the oocyte
Normally 10% of sperm undergo spontaneous reaction distal to oocyte and so can not fertilise the egg.
Path for fertilisation?
Occurs in the ampulla and must occur within 24h of ovulation
The sperm must penetrate the layers of cumulus cells surronding the egg, assisted by hyaluronidase secreted from the acrosome which digests the basement membrane allowing sperm to squeeze between cumulus cells.
Next it encounters the corona radiata, a layer of tightly packed follicle cells and digests its way past these to encounter the ZP.
Penetration of the ZP and fertilisation?
The ZP is the final protein barrier surrounding the ovum.
There are three major proteins ZP1, 2 and 3. ZP-3 is the primary sperm receptor and contact between ZP-3 and the sperm ligand for ZP-3 induces the acrosome reaction completely.
They then penetrate the ZP to the perivitelline space (takes about 10min and is a race between sperm)
ONE sperm attaches to the oolemmal membrane (oolemma) at sites of microvilli and the two membranes fuse allowing sperm nucles to onter the ovum.
Post fertilisation reactions?
Entry of the sperm causes a release of intracellular calcium
This is followed by regular spikes of calcium in the oocyte. inducing the resumption of meiosis.
= Cortical reaction
Cortical granules released into the perivitelline space, release of enzymes (proteases and B-hexosaminidase) cleaves ZP2, digests ZP3 and cross-link the zona (involves ZP1)
= Blocks polyspermy by hardening the ZP
Resumtion of Meiosis after fertilisation?
Is dependent upon Ca rise and destabilizes a protein complex that held the chromosomes in metaphase
Loss of complex allows chromosomes to migrate to the poles
Second polar body is shed and the first may also divide (but not usually)
Zygote transport? Stages of development prior to implantation?
The zygote begins cleavage in the tubes and the blastomeres divide symetrically for approx 5 days whilst traversing the tubes.
The morula at 8-16 cells is when cells become tightly cohesive and compacted together.
The blastocyst?
Has two differentiated cell populations:
Trophectoderm on the outside forming the placental trophoblast
Inner cell mass that forms the embryo proper
- Remains free floating in uterine cavity for 2-3 days before hatching and attaching to the decidual epithelium = Nidation
- nitation occurs via specific adhesion molecules
