4 - Fertilisation and Implantation

Question 1

What are the essential ingredients for fertilisation?

Answer 1

Functioning hypothalamic-pituitary-gonadal axis
Normal oogenesis & spermatogenesis
Normal structure and function of reproductive tract
Trouble free transport of gametes
Gamete fusion
Trouble free transport of embryo

Question 2

Where is sperm deposited?

Answer 2

Cervical os

Question 3

What feature does the cervical os have that helps the sperm towards the cervical canal?

Answer 3

Cilliated surface

Question 4

What is the distance for the sperm to get into the uterus and oviducts and how long does it take?

Answer 4

13-15cm

2-7 hours

Question 5

What is capacitation?

Answer 5

Sperm removes it’s glycoprotein coat and becomes hyperactive and sensitive to surrounding signals

Question 6

What happens to the cervix mucus during ovulation?

Answer 6

The cervix mucus changes from sperm-hostile to sperm-friendly

Question 7

What happens during ovulation?

Answer 7

The oocyte makes its way into the oviduct

Question 8

How is an egg fertilised?

Answer 8

Sperm finds egg

Sperm recognises egg.

Sperm has acrosome reaction to penetrate extracellular layer

Sperm cell membrane fuses with egg cell membrane and triggers Ca2+ wave in egg.

Polyspermy is blocked.

Fertilisation cone forms around sperm head.

Movement and fusion of pronuclei

Question 9

What happens during polyspermy blocking?

Answer 9

Triggered by production of second messengers
Release of calcium wave triggers cortical granule
Removes all the sperm receptors and hardens the zone pellucida
Cell division is also triggers – protein synthesis, DNA replication

Question 10

How do the sperm and the egg find each other?

Answer 10

They need to recognise one another from a distance
Sperm attracted to egg via chemotaxis
Only found in mature eggs and sperm
Mechanisms vary depending on species

Question 11

How do the cells fuse with each other in a species-specific way?

Answer 11

In mammals:
A glycoprotein called ZP3
Found in the zona pellucida
Binds to β1, 4 galactosyl transferase receptor on the sperm plasma membrane
Capacitated sperm are species-specific in their binding to ZP3
Binding triggers changes in Ca2+ and pH in sperm and acrosome reaction

Question 12

How is the number of cells fusing restricted?

Answer 12

Production of second messengers on sperm/egg binding triggers polyspermy blocking.

Slow block: release of calcium in a wave
calcium triggers cortical granule
release and activation of cell division

Question 13

What is the movement of the fertilised oocyte?

Answer 13

• Travels down the oviduct
• Begins to divide
• At 16-32 cell stage there is a polarisation of 2 cells populations
• At 32-64 cell stage the polarised cells begin to differentiate into different cell types
  Around day 5 the blastocyst frees itself from the zona pellucida = hatching
  -7-9 days post-ovulation, the blastocyst will attach to the uterine wall

Question 14

What are the inner and outer cells at the 16-32 cell stage?

Answer 14

Outer cells from one population and are trophopblast precursors
Inner cells are pluriblast cells
This is called the morula

Question 15

What is placentation?

Answer 15

establishment of physical and nutrictional contact to get a supply of nutrinets for growth

Question 16

What is maternal recognition of pregnancy?

Answer 16

signalling presence to the the mother to prevent luteal regression

Question 17

What occurs during the first differentiation stage of implantation?

Answer 17

Outer cell layer – trophoectoderm – will become the placenta
Inner cell layer will become the foetus

Question 18

What occurs during the apposition phase of implantation?

Answer 18

Positioning of the blastocyst within the uterine cavity

Inner cell mass must be pointing down, towards the endometrium

Question 19

What occurs during the adhesion phase of implantation?

Answer 19

Cells of the trophoblast fix to the maternal tissues and to each other
Achieved via a group of adhesion molecules including laminin and fibronectin

Question 20

What happens in the invasion phase of implantation?

Answer 20

Trophoblasts form villous structures
Cytotrophoblasts penetrates into the maternal decidua (endometrium) through the trophoblast cell
Trophoblast reach maternal endometrial spiral arteries
Spiral arteries are converted from narrow to wide vessels
Villous trophoblast is the barrier between maternal and foetal circulation

Question 21

What is the luteal-placental shift?

Answer 21

When HCG and progesterone levels cross for the second time (at 8-10 weeks)

Question 22

What are the hormone effects of implantation?

Answer 22

Corpus luteum does not degenerate because hCG binds to LH receptors
Progesterone levels do not fall as the corpus luteum is maintained
Progesterone maintains the endometrium and becomes the decidua
Once the placenta has fully developed it can produce progesterone
At 8-10 weeks it takes over the role of the corpus luteum = luteal-placental shift
hCG is no longer produced and the corpus luteum degenerates and forms the corpus albicans

Question 23

What is an ectopic pregnancy?

Answer 23

Implantation occurring at an incorrect site – often the fallopian tubes

Question 24

What is placenta praevia?

Answer 24

Placenta blocks the exit point of the baby

C-section required for delivery before the woman goes into labour