Fertilisation and Implantation Flashcards
What are the essential components for fertilisation?
Functioning HPG axis, normal oogenesis/spermatogenesis to have occurred, presence of normal reproductive tract, unimpeded transport of gametes, ability for gametes to fuse, unimpeded transport of embryo
Where is sperm deposited?
At the cervical os
How is sperm transported into the oviduct?
Ciliated surface of the cervical os helps to propel the sperm towards the cervical canal –> transport into the uterus and oviducts is performed by the sperm’s own propulsion in addition to the fluid currents caused by the uterine cilia –> during this journey from the cervical os to the oviduct, sperm undergo capacitation (removal of the glycoprotein coat) and the sperm become hyperactive and sensitive to surrounding signals
What happens at ovulation?
Cervical mucus changes from ‘sperm hostile’ to ‘sperm friendly’ and the secondary oocyte makes its way into the oviduct
Describe what happens in fertilisation
Sperm locates and recognises egg –> undergoes acrosome reaction–> penetrates extracellular layer of ovum –> sperm cell membrane fuses with ovum membrane –> Ca2+ wave in the ovum –> blockage of polyspermy –> fertilisation cones form around the sperm head –> movement and fusion of the pronuclei from egg and sperm
Describe the acrosome reaction
Acrosome membrane of sperm fuses with plasma membrane of egg –> enzymes released from sperm allow entry into zona pellucida (hyaluronidase dissolves intercellular matrix between cumulus cells) –> sperm finishes journey between zona pellucida and oocyte membrane (oolemma) –> fusion between sperm and egg –> large increase in intracellular Ca2+ wave across the egg –> prevents polyspermy
How does the sperm fertilise the egg?
Sperm undergoes acrosome reaction and binds with sperm receptors on the ovum –> cortical vesicles within the oolemma fuse with the plasma membrane of the sperm and release their contents into the peri-vitelline space (exocytosis) –> modified fertilisation membrane detaches and starts to rise –> sperm pulled into egg (fertilisation cone aids) –> microtubules radiating from centrosome associated with male pronucleus then guide pronucleus to centre of egg –> fusion of pronuclei –> zygote
What is the acrosome reaction?
It allows spermatozoa to penetrate the zona pellucida and fuse with the oocyte membrane.
How do the sperm and egg find each other in the oviduct?
The sperm is attracted to the egg via chemotaxis, which only occurs in mature eggs and sperm
How do sperm and egg cells fuse with each other in a specific way?
There is a glycoprotein known as ZP3 which is present on the zona pullicida of the oocyte. This ZP3 glycoprotein binds to the β1, 4 galactosyl transferase receptor on the capacitated sperm plasma membrane. It is this binding which triggers the changes in Ca2+ influx and pH in the sperm, triggering the onset of the acrosome reaction
How is polyspermy and excess of oocytes prevented?
On the binding of the sperm to the egg cell, there is a production of second messengers which triggers the blocking of polyspermy (no more sperm can bind to that egg). This occurs in the form of a release of calcium (in a wave) which triggers the release of cortical granules, and the activation of cell division in the, now, zygote
What is a morula?
16-32 cell stage where polarisation begins to form two cell populations; outer cells (trophoblast precursors) and inner cells (pluriblast cells)
What is a blastocyst?
32-64 cell stage where polarised cells begin to differentiate into different cell types
What is hatching?
When the blastocyst frees itself from the zona pellucida in a series of expansion-contraction cycles
When the blastocyst attaches to the endometrium, how does the conceptus ‘talk’ to the mother?
Placentation - establish a physical and nutritional contact (required for growth)
Maternal recognition of pregnancy - cells will signal their presence to the mother in order to prevent luteal regression
Describe the first differentiation in implantation
Occurs about 6 days post-fertilisation, and cells of blastocyst have become outer layer (trophectoderm) and inner cell mass; trophectoderm will become the placenta and the inner cell mass will become the foetus
Describe the apposition component of implantation
Where the blastocyst is positioned within the uterine cavity ~6-7 days post-fertilisation
Describe the adhesion component of implantation
Trophoblast cells fix to maternal tissues and to each other through the use of cell adhesion molecules containing laminin and fibronectin
Describe the invasion phase of placental development
Trophoblasts (via proteolytic processes) penetrate the maternal decidua and endometrial spiral arteries –> trophoblasts form villous structures –> cytotrophoblasts break through trophoblast shell (syncytiotrophoblasts) and invade endometrial tissue to reach spiral arteries and widen them –> villous trophoblast layer forms the barrier between maternal and foetal circulation
How is COX-2 thought to play a part in successful implantation?
Converts amino acids to PGE2 promotes invasion and decidualisation
How is HB-EGF thought to play a part in successful implantation?
Involved in attachment and invasion
How is VEGF thought to play a part in successful implantation?
Involved in angiogenesis
How is HLA-G thought to play a part in successful implantation?
inhibits antigen-specific lymphocyte response & decreases natural killer cell function
How is IDO thought to play a part in successful implantation?
regulated by IFNs to promote anti-proliferative effects
How is TGFβ thought to play a part in successful implantation?
Regulates invasion and proliferation
What happens if fertilisation and implantation do occur (maternal recognition of pregnancy)?
Corpus luteum doesn’t degenerate as sycytiotrophoblasts secrete hCG –> progesterone levels don’t fall –> oestrogen levels don’t fall either –> endometrium is maintained and becomes known as the decidua
Why are women told not to tell people they’re pregnant until after 12 weeks?
At 12 weeks there is the luteal:placenta shift where corpus luteum stops producing oestrogen, and relies on its production from the placenta itself. Very vulnerable stage during pregnancy, as if transfer of control is not seamless, endometrium may degenerate –> miscarriage of the foetus.
What is an ectopic pregnancy?
When the egg is fertilised outside of the uterus, such as in the fallopian tube (most commonly)
What is placenta praevia?
This is when the placenta develops at the entrance to the birth canal, and therefore obstructs the passage for delivery, and as a result C-sections are required for these individuals
