Fertilisation Flashcards
How do sperm gain fertilisation capacity
Sperm Capacitation - further and final maturation process which confers the following functional characteristics on to sperm and ultimately renders them capable of fertilisation:
A change to a state of hyperactivated motility.
The ability to bind to the oocyte’s zona pellucida and afterwards undergo the acrosome reaction.
The capacity to fuse with the oocyte
Female tract interaction mechanisms (4 types)
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.
Boundary-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
What are the different stimuli affecting caTsper channels in chemotaxis
Em pH Progesterone Prostaglandins Small organic molecules
Describe the sperm capacitation puzzle- how does increase in Ca2+ act on caTsper channels? State the pathway
Ca2+ acts on caTsper channels causing influx of calcium ions activates SACY cAMP pKA tyrosine phosphorylation hyperactive motility
Describe the sperm oocyte interaction
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 modulate the spermatozoa to initiate the process of sperm-oocyte interaction and subsequent fertilisation.
Penetration of the cumulus oophorus
Sperm-zona binding
Acrosome reaction
Sperm-oocyte fusion and oocyte activation
Penetration of 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) –
the major cementing factor between cells that constitute the cumulus oophorus cell mass.
Describe sperm-zona binding
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.
Describe the acrosome reaction
Triggered in response to sperm-oocyte interaction
Fusion of the sperm’s plasma membrane with it’s outer acrosomal membrane.
Results in the release of hydrolytic and proteolytic enzymes e.g. hyaluronidase and acrosin.
Describe sperm-oocyte fusion and oocyte activation
Intracellular calcium spike
Cortical reaction
Release from meiotic arrest and completion of meiosis II.
Incorporation of sperm DNA and pronuclear formation.
Embryo cleavage
Explain the oocyte activation process i.e how does the sperm trigger oocyte activation
Phospholipase C Zeta (PLC ζ).
Recently discovered in 2002 by Saunders et al. and widely accepted as the oocyte activation factor introduced by sperm.
Acts by activating inositol trisphosphate (IP3) production, leading to repetitive calcium release (oscillations) from the oocyte’s calcium stores.
Has been identified in all species studied so far, including human
Normal and abnormal fertilisation observed in ART in terms of number of pronuceli and polar bodies
Pronuclei:
2PN Normal fertilisation. Normally fertilised oocytes should contain two juxtaposed and centrally located PN, with distinctly clear membranes and nucleolar precursor bodies (NPBs).
0PN No fertilisation.
1PN Abnormal fertilisation. Contains a haploid set of chromosomes. There are cases where a 1PN zygote contains a diploid set of chromosomes, resulting from errors in the synchrony of PN formation/fusion. Zygotes in this category often have 2 polar bodies.
≥3PN Abnormal fertilisation. Contains additional set(s) of chromosomes, which could be either digynic or diandric. Significantly linked to the formation of aneuploid embryos.
0PB Oocyte at the Metaphase I stage of meiosis.
1PB Mature oocyte at the Metaphase II stage of meiosis with one polar body extruded.
No fertilisation.
2PB Oocyte has completed second meiotic division and extruded second polar body in response to fertilisation.
State the stages of embryo development in ART
1) Fertilised egg with 2PN
2) 4 cell
3) 8 cell
4)Morula
5)Early blastocyst
5/6)expanded blastocyst
5/6)expanded blastocyst
5/6)hatching blastocyst
6)hatched blastocyst
