Lecture 5: Fertilisation and Implantation

Question 1

What are the 2 changes in ejaculate after it is deposited in the vagina

Answer 1

1. As prostatic and seminal fluids mix in the vagina, a coagulum is formed to hold the ejaculate together and adhere it to the female reproductive tract
2. Within 15 min, coagulum is dispersed via liquefaction due to prostatic proteases allowing individual sperm to start migration

Question 2

What main role does the seminal fluid play to help sperm

Answer 2

It buffers the acidic pH of the vagina (~4.7) which is inhibitory to sperm motility and survival to better 7.2 pH allowing motility of sperm

Question 3

Why does the sperm count reduce from 200 mil in semen to a few hundred at the site of fertilisation? and what are some natural reasons why sperm are unable to fertilise the egg.

Answer 3

1. From deposition in the vagina only 1 mil (1%) will pass through cervix which is hostile to sperm transport except around ovulation and filters out abnormal tailed sperm
2. Half of the sperm arriving at the uterotubal junction go to the wrong tube without the egg.
3. sperm require capacitation to undergo acrosome reaction and fertilise egg which can take variable amount of time.
4. 10% of sperm undergo spontaneous acrosome reaction distal to the oocyte and cannot fertilise the egg due to progesterone produced by corona radiata (but help contribute to others chance of fertilisation)

Question 4

How does cervical mucus filter sperm

Answer 4

1. Outside of the periovulatory window, mucus has cross hatched fibres and is compact- acting as a barrier
2. In the periovulatory window, cervical mucus is produced in large amounts, is thin and watery. The fibres are aligned to allow channels for sperm to swim through
3. tails of normal sperm cause the fibres of cervical mucus to break down as they vibrate together in rhythm. abnormal sperm can’t produce this rhythm so have harder time getting through

Question 5

How long can sperm survive in the female reproductive tract and why

Answer 5

Sperm can be stored and then released if enter fallopian tube prior to ovulation
they can survive 2.5 hrs in vagina,
24 hrs in uterus and
48 hrs in the cervix (stored in the crypts)
and fallopian tube (stored in microvilli or slow down and wait in the isthmus of the tube)

Generally total survival time is around 5 days

Question 6

How long does it take for sperm to get to the ampulla of fallopian tube. What factors affect its speed of transport

Answer 6

Takes 2-7 hrs

1. Sperm swimming speed decreases with time and with freezing and thawing.
2. Stimulation of cervix with penis causes release of oxytocin from pituitary, resulting in rhythmic uterine contractions which help sperm speed.
3. Cilia in the fallopian tube also help to move the sperm along towards the ampulla

Question 7

What is the innate immune response triggered by sperm and where. why is this good

Answer 7

When sperm arrive in the uterus they induce infiltration of leukocytes which phagocytose dead and dying sperm

This exposure to paternal antigens helps to tolerise the maternal system to paternal antigens- less risk of pre eclampsia

Question 8

What is capacitation and what does it cause

Answer 8

A process that prepares sperm for acrosome reaction- and therefore fertilisation, only initiated in the female reproductive tract

Capacitated sperm show hyperactivation in which the tail movement changes with increased bending of the tail and swinging of the head.

This facilitates movement of sperm through tortuous folds of fallopian tube nearer to site of fertilisation

Question 9

What is the acrosome reaction and what triggers it

Answer 9

The acrosome (membranous pouch containing proteinases and hyaluronidase) fuses with the plasma membrane of the sperm head.

This causes release of acrosomal contents to environment around the sperm which if near, helps to breakdown zona pellucida

It is triggered by influx of Ca2+ into cell which can be triggered by progesterone or ZP3 protein on zona pellucida + other factors - eg. calcium ionophore in vitro.

Question 10

What are the 4 steps to sperm entering the egg starting at the ampulla

Answer 10

1. sperm penetrates the cumulous cells surrounding the egg by squeezing between cells as hyaluronidase secreted from partial acrosomal reaction (due to progesterone) eats the BM of these cells.
2. sperm digests past the corona radiata - innermost layer of tightly packed cumulous cells- and contacts the zona pellucida- ZP3 induces complete acrosomal reaction.
3. sperm penetrates through the zona pellucida to the perivitelline space with help from acrosomal enzymes in 10min.
4. sperm attaches to vitelline membrane and two membranes fuse allowing sperm nucleus to enter the ovum

Question 11

What does entry of sperm into the vitelline membrane cause?

Answer 11

Release of intracellular calcium followed by spikes of calcium in the oocyte

1. a) cortical reaction: cortical granules are released by the ovum into the perivitelline space that further prevents sperm from penetrating the zona
   b) Release of proteases and b-hexosaminidase cleave ZP2, digest ZP3 sperm binding sites and cross link the zona to block polyspermy
2. Ca2+ influx activates the oocyte nucleolus to complete second meiotic division in response to fertilisation

Question 12

How many meiotic divisions does a follicle undergo from primary/primordial follicle to fertilisation- is this equal

Answer 12

Primordial follicle starts meiosis 1 and is arrested.
It completes it after LH surge- producing two cells with n from 2n. However one of these forms a small polar body retained in the zona pellucida which may divide but not usually

At fertilisation it is stimulated to do meiosis 2- (still has n) making a second polar body which is usually shed.

Question 13

What event is required for formation of the zygote

Answer 13

the pronuclei of sperm and oocyte break down and mitotic division begins

Question 14

How long does it take for zygote to enter the uterus and what does it change into through that time.

Answer 14

5 days:
30 hrs: Zygote: 2 cells
3days: Morula: 8-16 cells
4days+ : Blastocyst: 64 -100 cells

Cells get more cohesive and compacted as the zona pellucida is staying the same size.

Question 15

Describe the structure of the blastocyst- cell types and what they will form to

Blastocyst remains free in the uterine cavity for 2-3 days before implanting in the uterine wall.

Answer 15

Two differentiated cell populations:

a) Trophectoderm (epithelial cells that form placenta)
- mural trophectoderm are thin cells surrounding the inner cell mass
- polar trophectoderm are fat cells near the inner cell mass that adhere to the endometrium

b) Inner cell mass (form embryo proper)

In between the mural trophectoderm and ICM is the Blastocystic cavity

Question 16

What is nidation

Answer 16

Process of implantation of Blastocyst that has hatched out of the zona pellucida comes into physical contact with the receptive decidua

It attaches to the endometrial/decidual epithelium via specific adhesion molecules while rolling, then the polar trophectoderm start to eat into the wall.