Lecture 9: fertilisation and implantation

Question 1

What makes semen stick to female reproductive tract?

Answer 1

the coagulation of prostatic and seminal vesicle fluid

Question 2

Where is semen deposited in females for reproduction?

Answer 2

vagina

Question 3

How long does it take the coagulated ejaculate to disperse to allow the sperm to finally travel?

Answer 3

15 min

Question 4

What is the name for breaking down the coagulum?

Answer 4

liquefaction

Question 5

Whats a challenge that sperm face when displaced in the vagina?

Answer 5

1. pH of vagina: it can inhibit the motility of the sperm and cease it.BUT it can survive because seminal plasma that its bathed in buffers the pH.→ seminal plasma= 7.2→ Vagina= 4.2

Question 6

Whats a challenge that sperm face when at the cervix?

Answer 6

physical barrier of cervix: Cervical mucus prevents sperm transport!

• Only during pre-ovulation, cervical mucus is produced in bulk and its thin. Its mucus fibres are aligned, which assist in transporting sperm in columns through the cervix.
• Movement speed is 1.2 to 3 mm per minute
• cervical cancer filters out the poor sperm: the tails of normal sperm cause the fibres of the cervical mucus to vibrate in a rhythm. This allows it to break through the cervical mucus to travel.

Question 7

How many sperm in an ejaculate

Answer 7

200 million

Question 8

How long does it take sperm to move from cervix to the utero-tubal junction?

Answer 8

30 minutes

Question 9

What makes sperm move 15 cm from cervix to fallopian tube (clue: TCC)?

Answer 9

• Tail: it beats at a rate of 3mm/ minute
• Muscular contraction (stimulated by oxytocin) of the female reproductive tract.
• rhythmic uterine contraction: cervix stimulation by penis, causing oxytocin release

Question 10

What happens when sperm is at the uterus?

Answer 10

• Leukocytosis is initiated (innate immune response) → their job is to get rid of dying sperm
• the maternal immune system can be down-regulated if the women has had a long term partner
• half of the sperm will arrive at the wrong utero-tubal junction (i.e. the tube with no egg), therefore, they just die by leukocytosis.

Question 11

What happens when sperm is at the fallopian tube?

Answer 11

• tail beating slows down
• If they’re too early (there before ovulation- egg release) they will wait at the narrow end of the isthmus of tube.
• during ovulation, the sperm moves to the egg and extra sperm that has been stored in the crypts of the cervix can travel to tube too.
• sperm is able to sense when egg is coming, because of trophic signals from the ciliated and secretory cells of the fallopian tube.
• sperm can survive for a few days in the FRT

Question 12

What area of fallopian tube is the fertilisation site?

Answer 12

ampulla

Question 13

What are important processes sperm has to go through to activate sperm swimming capabilities (in order)?

Answer 13

1. Capacitation: important to induce hyperactivation swimming or tail and head. Sperm need this to manouver through nooks and crannies of the fallopian tube.→ Rapid: this occurs a few seconds after sperm in displaced inside female reproductive tract. SACY is activated by HCO3- and calcium. PKA signalling is stimulated which causes a change in membrane potential and upregulate intracellular calcium production ( thats to help increase the random swimming of sperm, basically like a preworkout for sperm)→ slow: cholesterol of membrane is removed so that sperm can maintain their high level of HCO3-
2. Acrosome reaction: the head of sperm and acrosomal membrane fuse together to release enzymes like proteinases and hyaluronidase.
   • It is triggered by high influx of calcium into the cell (high calcium from progesterone or ZP3 protein of zona pellucida)
   • This reaction HAS to occur near the oocyte so that sperm can be fertilising the egg naturally. Those enzymes released are needed to pierce through zona pellucida

Question 14

What are the steps in fertilisation?

Answer 14

1. Sperm undergoes capacitation
2. Sperm penetrates layer of cumulus cells that is surrounding the egg using the hyaluronidase
3. sperm digests through corona radiata and contacts the zona pellucida
4. contact with the ZP3 receptor on the zona pellucida triggers the activation of acrosome reaction, releasing its enzymes to make a shield for sperm and help it penetrate the zona pellucida into the perivitelline space (takes 10 minutes)
5. sperm attaches itself to vitelline membrane and they fuse. This fusion allows the nucleus of sperm to enter the ovum (egg). Cortical granules released from the ovum prevents the sperm from further penetration of zona pellucida.
6. sperm inside oocyte causes increase of intraceullular calcium, which alters protein phosphorylation patterns in oocyte. this activates the oocyte nucleus so that it can complete its second meiotic division.
7. Both oocyte and sperm form pronucleus that can undergo mitotic division
8. You have zygote now!

Question 15

What happens when zygote is made in fallopian tube?

Answer 15

• zygote begins cleavage in the tube and takes around 5 days to move from the tube to the uterus
• while its moving, the zygote is still continuing to divide from 2 → 4 → 8 → 16 cell stage.
• 16 cell stage is called morula

Question 16

How long does it take for implantation to occur?

Answer 16

7-10 days post fertilisation

Question 17

How long does it take for blastocyst development?

Answer 17

~6 days

Question 18

64 cell stage is called

Answer 18

blastocyst

Question 19

What are two cells that blastocysts have?

Answer 19

inner cell mass

trophectoderm

Question 20

What is decidualisation?

Answer 20

• the endometrial cells become plumpy and glycogen rich.
• it differentiates from endometrium → decidua
• it is not always required for a pregnancy

Question 21

What happens during implantation?

Answer 21

1. blastocyst still encapsulated by zona pellucida while free floating in uterine cavity.
2. blastocyst hatches and comes in physical contact with oligossacharide ligand on decidua, sticking via specific adhesion molecules (integrins). this process is called nidation
3. embryo “eats” through uterine wall and is completely embedded itself inside. the epithelium heals itself where the site of implantation is so its fully enclosed.
4. Trophectoderm starts to differentiate into (takes ~12-15 days)→ mono-nuclear cytotrophoblasts→ multinucleated syncytiotrophoblasts

Question 22

what is its structure?

Answer 22

• branching villous

- Made up of trophoblasts 1. cytotrophoblats 2. syncytiotrophoblasts 3. extravillous trophoblasts

Question 23

what is the point of a placenta?

Answer 23

1. Physical attachment of placenta to decidua

2. adaptation of blood vessels in uterus called SPIRAL ARTERIES to sustain pregnancy

Question 24

Whats the point of EVTs?

Answer 24

EVT runs away from decidua in columns and spreads out away from nidation site (where it stuck to get into decidua)

• Spiral artery remodelling: they are important for spiral artery remodelling, they replace endothelial cells of spiral arteries with itself (trophoblasts). They also digest the muscular wall and transforms them so they can be flaccid, wide tubes for blood to constantly flow through.

Question 25

what are trophoblast plugs?

Answer 25

• EVT form plugs in spiral arteries during the first trimester so theres not a lot of maternal blood reaching placenta. The low oxygen environment induces EVT development.
• Absence of maternal blood means that glandular secretions and yolk sac help nourish embryo.
• The plugs act like pressure reducing valves, stops the implanting placenta from getting hosed down by maternal blood at high pressure.
• The plugs are then broken down by second trimester so maternal blood can go into intravillous space.