fertilisation and contraception Flashcards
Where does fertilisation occur
Ampulla (most dilated area of Fallopian tube)
What re granulose cells in oocyte suspended in and what does it produce
Hyaluron rich matrix - produces progesterone and chemo-attractants
How many chromosomes does the secondary oocyte have
23 (haploid)
How does the secondary oocyte obtain nutrients
From cytoplasm
What acts as a protective shell of oocyte
ZOna pellucida
Features of the plasma membrane of sperm head
-what can it secrete
Odorant receptors can react to chemo-attractants from oocyte
3 surface binding molecules- ADAM (a disintegrin and metalloproteinase domain) family; Fertilin alpha, beta and cyritestin
Can secrete Hyaluronidase
Acrosome of sperm
Surrounds head and contains the enzyme acrosin
What do the sperm have ti undergo to attain full fertilisation capacity
Capacitation
What happens when sperm gets to isthmus of Fallopian tube
Waits for egg to be released into the ampulla and send out chemoattractins so the sperm can swim to it
What is capacitation
1) Changes in movement characteristics
2) Changes in spermatozoal surface by stripping of glycoproteins
How is movement changed in capacitation
regular wave like (from vagina to oviduct) changes to wide amplitude whiplash beats needed to swim upstream from isthmus to ampulla
What direction do ciliated cells of oviduct flow and why
Against the sperm so sperm has to swim against the tie. This introduces selective pressure
What changes are there in spermatozoal surface in capacitation
- Changes in surface charge, macromolecular organisation, loss of cholesterol
- Reduced stability of plasma membrane and enhances fusibility
- Increased permeability to calcium
How does penetration of the corona radiate occur
Secretion of hyaluronidase
Digestion of extra cellular matrix
Active movement to reach zona pellucida
How does sperm attach to zona pellucida
Receptors for ZP proteins (ZP2R and zP3R) are present on different membrane components of the spermatozoa
the receptor for ZP3, ZP3R is on the surface of the sperm head
the receptor for ZP2, ZP2r, is on the inner acromosal membrane
Where is the receptor for ZP3, ZP3R
surface of sperm head
where isreceptor for ZP2, ZP2r
inner acromosal membrane
What does binding of ZP3 to its receptor on sperm head plasma membrane lead to
Calcium influx which causes depolymerization of the F-actin present between the acrosome and sperm-head plasma membrane. Acrosome now has room to expand and does so.
Increased calcium also leads to cAMP and increase in pH
Consequence of acrosome binding
- Acrosome swelling
- Acrosome fusion with overlying surface plasma membrane. Gives a vesiculated appearance.
- Exteriorisation of contents and inner acrosomal membrane (IAM) by exocytosis. to outside. This leads to exposure of inner ZP2 receptor.
- The revealed ZP2 Receptor on inner acrosomal membrane can now bind to ZP2 on zona pellucida.
- Mechanical propulsion along ZP2 filaments until oocyte plasma membrane surface is reached
- Release of acrosin aids digestion of a pathway through the zona pellucida
How does the intern layout of the oocyte prevent triploidy
Specific areas on egg surface are rich in interns to allow binding in correct areas
What happens post fusion
. At fertilisation, the oocyte is still arrested in second meiotic metaphase (M phase).
- A rise in Ca++ after fusion leads to exit from M phase.
- One set of chromosomes is dispatched as the 2nd polar body
- The other half set of 23 unpaired chromosomes remain behind in the female pronucleus and can unite with the 23 paternal chromosomes of the penetrating sperm
How Is gynogenetic triploidy avoided
Dispatching the second polar body
What is another way that triploidy can happen
How can this be avoided
If the exiting second polar body encounters the entering sperm there can be interference and all 3 haploid sets of chromosomes remain inside, this results in fetal death
Spermatozoa do not bind to oocyte membrane immediately overlying the second metaphase spindle. (This region doesn’t have binding proteins like integrins, CD9, and of microvilli).
This ensures avoidance of encounter between the exiting second polar body and the entering spermatozoa
How is polyspermy/ androgenetic triploidy avoided
Changes in electrical activity of zygote leads to Ca2+ wave from point of sperm entry . There are Ca2+ spikes.
this leads to cortical reaction which is release of cortical granules into perivitelline space. enzymes cleaves ZP2 and hydrolyses binding region of ZP3 so ZP2 and 3 are no t available anymore for more sperm binding
What does sperm contribute to in cytoplasmic inheritance
Centriole- essential for Karyo- and cytokinesis
What does the oocyte contribute to cytoplasmic inheritance
Cell membrane, cytoplasm, cell organelles and mitochondria (maternal cytoplasmic inheritance)
how does silencing of gene occur
Through addition of methyl groups
How does the combined oral contraceptive pill work
Oestrogen: inhibits FSH release and therefore development of follicle
Progesterone: inhibits LH release, prevents ovulation, alters cervical mucus, motility in Fallopian tube
