fertilisation Flashcards
How do sperms gain their fertilising capacity?
- sperm capacitation
- interaction with the female reproductive tract
Sperm capacitation
- further and final maturation process in which sperms: happens in the female tract as the sperm get in.
1. change to a state of hyperactivated
motility.
2. gain ability to bind to the oocyte’s zona pellucida and afterwards undergo the acrosome reaction.
3. gain the capacity to fuse with the oocyte.
female tract interaction mechanisms of sperm
-Chemotaxis
-Thermotaxis
-Rheotaxis
-Boundary-following navigation
Chemotaxis
— Chemical guiding mechanism.
— Sperm responding to a gradient of chemoattractant e.g. steroid hormones.
— Molecular and behavioural mechanisms yet to be fully understood.
Thermotaxis
— Temperature guiding mechanism.
— Female tract consists of different areas with marginal differences in temperature.
— Sperm responding to changes in the extracellular temperature gradient.
Rheotaxis
— Ability to respond to fluid currents in the female tract environment.
Boundry- following navigation
— Ability to turn corners in response to surface boundaries
— Human sperm with preference to follow boundaries on the left or right hand side have
been shown to possess higher DNA integrity than straight swimming sperm (Huang et
What modulates the female tract interaction of sperms?
CatSper channels
What are CatSper?
- cation channel in sperms
- located in the flagella region
- responsible for calcium influx (calcium influx happens faster rate at body temp than room temp)
- responds to diverse stumuli : Em, pH, progesterone, prostaglandins, small organic molecules
What is the main change that occurs in sperm capacitation?
loss of cholesterol from cell membrane of sperm cells
=> Labs culture:
if you supplement culture media with substances that can take cholesterol away you see sperms switch to hyperactivity, which is a obvious sign they are undergoing cell capacitation.
- so female tract has cholesterol acceptors that takes away cholesterol from the sperm
- cholesterol leaving alters PL ratio in the cell membrane and also as a result affects the availability of soluble proteins in the membrane like soluble adenine cyclase become more available due to cholesterol loss further facilitating fertilisation.
What happens when progesterone(P4) binds to its receptors on cell membrane?
-P4 binds to ABHD2 receptor and stimulates CatSPer channels
-influx of Calcium allows for motility of sperm
What happens after capacitation?
- Once a few capacitated sperm make it to the site of fertilisation, they come in contact with the cumulus-oocyte complex (COC) and a much higher concentration of follicular fluid (FF).
- FF in addition to secretions from the COC e.g. progesterone further modulates
the spermatozoa to initiate the process of sperm-oocyte interaction and subsequent fertilization.
What are the events in Sperm- oocyte interaction?
Penetration of the cumulus oophorus
Sperm-zona binding
Acrosome reaction
Sperm-oocyte fusion and oocyte activation
- Penetration of the cumulus-oophorus
Once sperm come in contact with the
cumulus-oophorus cell mass surrounding the
oocyte, the enzyme hyaluronidase present on
the surface of the sperm head dissolves
hyaluronic acid (hyaluronan).
- Sperm-zona binding
ZP are located between the theca and granulosa in the oocyte , once the sperm penetrates through the cumulus it enters the oocyte :
- Four zona pellucida (ZP) glycoproteins
expressed – ZP1, ZP2, ZP3, ZP4. - Studies across mammalian species have
shown that sperm bind to ZP2 and ZP3
via receptors present on the sperm’s
plasma membrane. - Recent studies by Yauger et al. (2011)
and Baibakov et al. (2012) show that
human sperm bind to ZP2.
=> transgenic mice used