Fertilisation Flashcards
What is fertilisation?
Fertilisation is the process of sperm and oocyte binding to form a diploid zygote.
What is formed when sperm and egg meet?
Zgote
What cellular layer must surround an oocyte for it to be fertilised?
Expanded cumulus cell matrix: to aid passage into fimbria, to protect zygote, to provide an additional barrier to polyspermy (inner layer = corona radiata)
What is the inner layer of the cumulus cell matrix?
Corona radiata
What is the glycoprotein layer surrounding the oocyte?
Zona pellucida
What is the function of the ZP?
Provides a surface receptor for sperm, to prevent polyspermy, to protect preimplantation embryo and prevent it sticking to walls of oviduct, to prevent fusion of embryos
What is the sperm surface receptor?
ZP3
What initiates muscular activity in the oviduct?
High levels of oestradiol
How is the fallopian tube specialised?
Its fimbriated end becomes closely apposed to ovulation site
Its folded, ciliated epithelium wafts ovulated oocyte and corona into tube
Secretions from tubal epithelial cells help nourish the developing embryo
Sexual intercourse in both male and female shares a number of interlinked phases, which are…
Arousal (excitement) Plateau Orgasm Resolution Refractory phase (generally only in males)
What nerve inputs controls erection, emission and ejaculation?
Erection - parasympathetic
Emission - sympathetic
Ejaculation - somatic (pundendal)
What happens in ejaculation?
Rhythmic contraction of perineal striated muscle (bulbospongiosus) and anal sphincter
What happens in emission?
Contraction of vas, seminal vesicles, prostate; relaxation of urethral sphincter
What is the most difficult region for the sperm to pass through?
Isthmus (junction of uterus and oviduct)
How many sperm will reach the fimbria/ampulla?
10-20
Describe the pH gradient of the female reproductive tract
pH near the vaginal opening is lower (approximately 5) than the fallopian tubes (approximately 8).
Why do the sperm rely on the female reproductive tract pH?
The sperm-specific pH-sensitive calcium transport protein called CatSper increases the sperm cell permeability to calcium as it moves further into the reproductive tract.
Intracellular calcium influx contributes to sperm capacitation and hyperactivation, causing a more violent and rapid non-linear motility pattern as sperm approach the oocyte.
What is capacitation? What happens?
Penultimate step in the maturation of mammalian spermatozoa -required for ability to fertilize an oocyte
Destabilization of the acrosomal sperm head membrane which allows it to penetrate the outer layer of the egg
Chemical changes in the tail that allow a greater mobility in the sperm.
What facilitates the capacitation of sperm? What is the result
The changes are facilitated by the removal of sterols (e.g. cholesterol) from the sperm head and non-covalently bound epididymal/seminal glycoproteins.
The result is a more fluid membrane with an increased permeability to Ca2+.
What facilitates increased sperm motility?
An influx of Ca2+ produces increased intracellular cAMP levels and thus, an increase in motility.
Hyperactivation coincides with the onset of capacitation and is the result of the increased Ca2+ levels.
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What attracts the sperm to the egg ?
Chemotaxis
What releases chemoattractant chemicals?
Egg chemoattractant chemicals attract sperm and encourage penetration of cumulus.
What chemoattractant does the egg release?
Progesterone + Atrial natriuretic peptide (ANP) from cumulus cells stimulate motility and the acrosome reaction
ANP may also attract sperm
Other chemotactic signals such as formyl Met-Leu-Phe (fMLF) may also guide spermatozoa
Binding of the sperm head to ZP causes…
Acrosome reaction and whiplash-like motility.
What sperm receptor binds ZP3?
GaIT
Describe the acrosome reaction.
Releases the hyaluronidase that digests the matrix of hyaluronic acid in the vestments around the oocyte.
Heparin-like glycosaminoglycans (GAGs) are released near the oocyte that promote the acrosome reaction.
Fusion between the oocyte plasma membranes and sperm follows and allows the sperm nucleus, but not the mitochondria, to enter the oocyte.
Does the sperm mitochondria enter the oocyte?
No
What the Cortical reaction
The egg “activates” itself upon fusing with a single sperm cell and thereby changes its cell membrane to prevent fusion with other sperm.
How does sperm entry trigger the cortical reaction?
Sperm entry causes calcium release into the oocyte - proposed to be caused by the introduction of phospholipase C isoform zeta from the sperm cytoplasm
Activation of the ovum includes what four events ?
Cortical reaction to block against other sperm cells, activation of egg metabolism, reactivation of meiosis and DNA synthesis.
What is the fast block to polyspermy?
Electrical block to polyspermy.
The resting potential of an egg is -70mV. After contact with sperm, an influx of sodium ions increases the potential up to +20mV.
What is the slow block to polyspermy?
Cortical reaction
What happens in the cortical reaction
Cortical granules directly beneath the plasma membrane are released the perivitelline space.
Granules contain proteases, mucopolysaccharides, hyalin, and peroxidases.
The proteases cleave the bridges connecting the plasma membrane and the vitelline membrane and cleave the bindin to release the sperm.
The mucopolysaccharides attract water to raise the vitelline membrane.
The hyalin forms a layer adjacent to the plasma membrane
Peroxidases cross-link the protein in the vitelline membrane to harden it and make it impenetrable to sperm.
ZP3/ZP2 are degraded by enzymes from cortical granules
Before fertilisation what phase of meiotic cycle was the egg in?
Metaphase of the second meiotic division
What happens to the meiotic cycle after fertilisation?
Calcium oscillations reactivate the meiotic cycle. This results in the production and extrusion of the second polar body.
What happens to the zygote?
Cleavage
When do zygotes lose totipotency?
Morula (16 cell stage)
What cell cycle phase does the zygote initially miss?
G1/G2
What happens to the morula?
Tight junctions form between the cells, this process is called compaction and this separates the inner cells which communicate via gap junctions from the outer cells.
What is blastulation? Describe what happens
A fluid cavity then forms in the intracellular spaces of the inner cell mass.
Na+ pumps move sodium into the intracellular space which lower the water potential, this drives the osmosis of water through the zona pellucida, the increased pressure forms a cavity called the blastocele which flattens the inner-cell mass now called the embryoblast to one side of the cavity and those of the outer cell mass form the trophoblast.
The ZP is degraded.
How does oestrogen affect the endometrial growth?
Stimulates the proliferative phase of growth and triggers spiral artery development
How does progesterone affect the endometrial growth?
Stimulates its secretory phase
How do PGs affect the endometrial growth?
Trigger arterial spasm and uterine muscle spasm.
What is the decidual reaction?
Presence of blastocyst leads to further endometrial development.
