Sperm Transport and Fertilisation Flashcards
Steps to fertilisation
Sperm deposition
Sperm transport
Sperm capacitation
Oocyte maturation
AR & Fertilisation
Sperm transport(2)
Once sperm are deposited in female tract, thus must reach oviduct – site of fertilisation
There are 2 phases of sperm transport to isthmus:
- Rapid & Sustained
Sperm loss & selection (2)
Following ejaculation, most sperm are lost from female tract by:
- Retrograde flow (backwards flow)
o Depends on site of deposition
o Sometimes minimised by ejaculate qualities (e.g. volume, highly
viscous, copulatory plug)
- Phagocytosis by neutrophils
o Estrus primes cervix, uterus for protective immune response o Sperm/SP/extender can all illicit responses
o SP has important immunomodulatory roles
o SP protects sperm from phagocytosis by female tract?
Why would post mating inflammation be important?
- Post mating inflammation, apart from clearance, in that semen can introduce pathogens too.
Physical barriers to sperm transport (3)
The cervix (left) and UTJ (right) also present physical barriers to sperm transport, limiting the number of sperm gaining access to isthmus
- I.e. cervix big issue in sheep
- Note: also have retrograde flow of mucous
Sperm transport through cervix
Sperm transverse the cervix by swimming through the bottom of cervical crypts, where low viscosity mucus favours transport = privileged pathways
Most relevant for species w/
- Vaginal semen deposition - Complex cervix
Sperm reservoir formation at UTJ
A reservoir of sperm forms just past the UTJ, in the isthmus Sperm head binds directly to ciliated oviduct epithelial cells
- Prolongs sperm viability and motility (up to several days in domestic species)
Extended storage in sperm reservoirs
Some species can store sperm for a very long time (months to years)
Fairly common in bats, reptiles and birds – sometimes w specific structures (e.g. hen has sperm storage tubules in the cervix equivalent)
Sperm capacitation
Capacitation is the final maturation event in sperm, triggered in uterus and completed in oviduct prior to fertilisation
Involves many molecular changes to sperm:
- Loss/rearrangement of membrane
proteins
- Loss of membrane cholesterol
- Increased intracellular calcium
Results in
- Release from oviductal reservoir (i.e. from the oviductal cells) - Hyperactivated motility
- Ability to fertilise
Capacitation typically takes several hours
*Can reverse capacitation changes by exposing sperm to seminal plasma *SP plays role in capacitation
If you were doing in vitro fertilisation, what would you do to try and stimulate capacitation in vitro?
Different for different species, because require different things to drive capacitation sperm capacitation
Oocyte maturation (2)
Oocyte undergo 2 different maturation steps before ovulation
- Resume meiosis to form an MII oocyte (1 polar body) – except in bitch
(post-ovulation)
- Cumulus expansion (inside follicle)
This process can be replication for aspiration oocytes by in vitro maturation (IVM)
The fertilisation process
The sperm and oocyte will meet in ampulla, and fertilisation process will begin
Zona pellucida binding and the AR
The sperm acrosome secretes hyaluronidase to help sperm get through the cumulus cells.
Once in contact with the oocyte, sperm binds to zona protein 3 (ZP3), triggering the acrosome reaction.
AR leads to:
- Release of enzymes to help ZP penetration
- Fusion of outer and inner acrosomal membranes - Exposure of fusion proteins on the sperm head
It is crucial that sperm are not acrosome reacted prior to this point, if they are, they die quickly.
Undergoing capacitation is a pre-requisite for undergoing acrosome reaction
Sperm-oocyte fusion
Following AR and penetration of ZP:
1. Sperm enter the perivitelline space
2. Oocyte microvilli bind to sperm fusion
proteins
3. the sperm nucleus engulphed, loses its membrane and decondenses
This results in:
- oocyte completing meiosis (2nd polar
body)
- formation of 2 pronuclei – 1 male
(paternal – from sperm)), 1 female (maternal)
Male and female pronuclei fuse (syngamy) to create a zygote (a 1 cell embryo)
Why is there a biological mechanism regulating the numbers of sperm that enter the ampulla around the time of ovulation?
i.e. barriers to get into oviduct
Too many sperm around oocyte will get trouble.
Polyspermy happens when multiple sperm fertilise the same oocyte
Artifical insemination – location & timing
Example timeline for AI of synchronised ewes (aka fixed time AI)
- if using frozen semen, highest preg rates with intrauterine insemination at 55 hours
- no heat detection required, all ewes insemination at same time. Example timeline for AI of cows based on heat detection
- requires at least twice daily heat detection
- cows will all be inseminated at different times
A sperm is undergoing AR. Which process has not yet occurred?
B.
Capacitation happens first, after we
have release of sperm from
oviductal reservoir (also see
hyperactivated motility), when
sperm get close to oocyte secrete
hyaluronidase (to break through cumulus cells around oocyte), only after acrosome reaction and after fertilisation will get syngamy (fusion of male and female pronuclei)
