Fertilisation and Implantation

Question 1

What are the essential components for fertilisation?

Answer 1

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

Question 2

Where is sperm deposited?

Answer 2

At the cervical os

Question 3

How is sperm transported into the oviduct?

Answer 3

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

Question 4

What happens at ovulation?

Answer 4

Cervical mucus changes from ‘sperm hostile’ to ‘sperm friendly’ and the secondary oocyte makes its way into the oviduct

Question 5

Describe what happens in fertilisation

Answer 5

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

Question 6

Describe the acrosome reaction

Answer 6

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

Question 7

How does the sperm fertilise the egg?

Answer 7

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

Question 8

What is the acrosome reaction?

Answer 8

It allows spermatozoa to penetrate the zona pellucida and fuse with the oocyte membrane.

Question 9

How do the sperm and egg find each other in the oviduct?

Answer 9

The sperm is attracted to the egg via chemotaxis, which only occurs in mature eggs and sperm

Question 10

How do sperm and egg cells fuse with each other in a specific way?

Answer 10

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

Question 11

How is polyspermy and excess of oocytes prevented?

Answer 11

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

Question 12

What is a morula?

Answer 12

16-32 cell stage where polarisation begins to form two cell populations; outer cells (trophoblast precursors) and inner cells (pluriblast cells)

Question 13

What is a blastocyst?

Answer 13

32-64 cell stage where polarised cells begin to differentiate into different cell types

Question 14

What is hatching?

Answer 14

When the blastocyst frees itself from the zona pellucida in a series of expansion-contraction cycles

Question 15

When the blastocyst attaches to the endometrium, how does the conceptus ‘talk’ to the mother?

Answer 15

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

Question 16

Describe the first differentiation in implantation

Answer 16

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

Question 17

Describe the apposition component of implantation

Answer 17

Where the blastocyst is positioned within the uterine cavity ~6-7 days post-fertilisation

Question 18

Describe the adhesion component of implantation

Answer 18

Trophoblast cells fix to maternal tissues and to each other through the use of cell adhesion molecules containing laminin and fibronectin

Question 19

Describe the invasion phase of placental development

Answer 19

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

Question 20

How is COX-2 thought to play a part in successful implantation?

Answer 20

Converts amino acids to PGE2 promotes invasion and decidualisation

Question 21

How is HB-EGF thought to play a part in successful implantation?

Answer 21

Involved in attachment and invasion

Question 22

How is VEGF thought to play a part in successful implantation?

Answer 22

Involved in angiogenesis

Question 23

How is HLA-G thought to play a part in successful implantation?

Answer 23

inhibits antigen-specific lymphocyte response & decreases natural killer cell function

Question 24

How is IDO thought to play a part in successful implantation?

Answer 24

regulated by IFNs to promote anti-proliferative effects

Question 25

How is TGFβ thought to play a part in successful implantation?

Answer 25

Regulates invasion and proliferation

Question 26

What happens if fertilisation and implantation do occur (maternal recognition of pregnancy)?

Answer 26

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

Question 27

Why are women told not to tell people they’re pregnant until after 12 weeks?

Answer 27

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.

Question 28

What is an ectopic pregnancy?

Answer 28

When the egg is fertilised outside of the uterus, such as in the fallopian tube (most commonly)

Question 29

What is placenta praevia?

Answer 29

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