Fertilisation, Early Embryo Development

Question 1

Where does fertilisation occur in the female reproductive tract?

Answer 1

At the ampullary-isthmic junction of the oviduct.

Question 2

How long can sperm and eggs survive in the female reproductive tract?

Answer 2

Sperm ~5 days; Egg ~24 hours.

Question 3

What is capacitation?

Answer 3

A process where sperm become capable of fertilising an egg; involves hyperactivation and removal of the acrosome.

Question 4

What enzymes are found in the acrosome cap?

Answer 4

Acrosin and hyaluronidase.

Question 5

What is the first barrier sperm must penetrate to fertilise the egg?

Answer 5

The cumulus cells surrounding the egg, digested by hyaluronidase.

Question 6

What happens during the acrosome reaction?

Answer 6

Sperm interacts with ZP2/3 glycoproteins on zona pellucida, triggering enzyme release.

Question 7

What initiates egg activation?

Answer 7

Sperm’s phospholipase C gamma triggers calcium oscillations, leading to meiosis completion.

Question 8

How is polyspermy prevented?

Answer 8

Cortical granules release enzymes that modify zona pellucida, degrading ZP2/3.

Question 9

Which hormone dominates the follicular phase?

Answer 9

Oestrogen.

Question 10

What triggers ovulation?

Answer 10

A surge in LH due to positive feedback from high oestrogen levels.

Question 11

What hormone dominates the luteal phase?

Answer 11

Progesterone.

Question 12

What is cleavage?

Answer 12

Rapid mitotic divisions without growth, forming blastomeres and morula.

Question 13

What is the blastocyst composed of?

Answer 13

Trophoblast (outer layer), inner cell mass, and blastocoel cavity.

Question 14

What is required for implantation?

Answer 14

Hatching from zona pellucida.

Question 15

What is IVF?

Answer 15

In vitro fertilisation: eggs and sperm are co-incubated for 24 hours.

Question 16

What is ICSI?

Answer 16

Intracytoplasmic sperm injection: a single sperm is injected into an egg.

Question 17

When is ICSI used?

Answer 17

Male-factor infertility (e.g., low sperm count, azoospermia, spinal injury).

Question 18

What hormone maintains the corpus luteum in early pregnancy?

Answer 18

hCG (human chorionic gonadotropin).

Question 19

Which factor is critical for implantation in uterine epithelium?

Answer 19

LIF (Leukaemia Inhibitory Factor).

Question 20

What is the difference between monozygotic and dizygotic twins?

Answer 20

• Monozygotic = identical, from one zygote;
• Dizygotic = non-identical, from two fertilised eggs.

Question 21

What’s the risk with mono-chorionic mono-amniotic twins?

Answer 21

High risk of twin transfusion syndrome due to shared blood supply.

Question 22

How do sperm reach the site of fertilisation?

Answer 22

Through a combination of their own motility, uterine/oviduct contractions, and being pushed by cilia.

Question 23

What happens during sperm capacitation?

Answer 23

Removal from seminal fluid allows membrane changes, enabling sperm to become hyperactive and undergo the acrosome reaction.

Question 24

Why are most sperm lost before fertilisation?

Answer 24

99% are lost due to leakage or fail to navigate the female tract; only a few bind to the oviduct epithelium for nourishment and support.

Question 25

What initiates the acrosome reaction?

Answer 25

Binding to ZP2/3 glycoproteins on the zona pellucida of the egg.

Question 26

What does the enzyme hyaluronidase do?

Answer 26

It digests the hyaluronic acid in the cumulus cell matrix around the egg.

Question 27

How long does it take for sperm to penetrate the zona pellucida?

Answer 27

Approximately 5–20 minutes.

Question 28

What is fusion in fertilisation?

Answer 28

The sperm binds to the egg's plasma membrane, and its head and tail are engulfed by the egg.

Question 29

What triggers calcium oscillations in the egg?

Answer 29

Sperm introduces phospholipase C gamma, initiating a calcium signalling cascade.

Question 30

Why is calcium elevation crucial for the egg?

Answer 30

It allows the second meiotic division to complete and destroys cyclin B in the MPF complex, enabling the egg to begin embryonic development.

Question 31

How does the egg prevent polyspermy?

Answer 31

Cortical granules release enzymes that degrade ZP2/3, making the zona pellucida impermeable to additional sperm.

Question 32

What is syngamy?

Answer 32

The fusion of male and female pronuclei (they align but do not merge into a single nucleus).

Question 33

What marks successful fertilisation under a microscope?

Answer 33

Presence of two pronuclei and a polar body.

Question 34

What is cleavage in embryo development?

Answer 34

Rapid mitotic divisions without growth, forming smaller cells called blastomeres.

Question 35

What is a morula?

Answer 35

A solid ball of 16–32 blastomeres formed by compaction after cleavage.

Question 36

When does the blastocyst form?

Answer 36

Around 120–168 hours (5–7 days) post-fertilisation.

Question 37

What structures form in the blastocyst?

Answer 37

- Trophoblast (trophectoderm): outer layer that initiates implantation - Inner Cell Mass: becomes the fetus - Blastocoel: fluid-filled cavity

Question 38

What is essential before implantation?

Answer 38

The embryo must hatch from the zona pellucida.

Question 39

What hormone rises during the follicular phase, triggering ovulation?

Answer 39

Oestrogen; it stimulates an LH surge from the pituitary gland.

Question 40

What hormone dominates the luteal phase and maintains the endometrium?

Answer 40

Progesterone, secreted by the corpus luteum.

Question 41

What two cells are required for fertilisation?

Answer 41

A mature egg (oocyte) and a sperm cell.

Question 42

How long can sperm survive in the female reproductive tract?

Answer 42

Up to 5 days.

Question 43

How long is an egg fertilisable after ovulation?

Answer 43

Around 24 hours.

Question 44

How far do sperm travel to reach the egg?

Answer 44

160–200 mm to the ampullary-isthmic junction.

Question 45

What aids sperm travel?

Answer 45

Their own motility, uterine contractions, and oviduct cilia.

Question 46

What happens to most sperm after ejaculation?

Answer 46

99% are lost via leakage.

Question 47

What happens to the sperm that reach the oviduct?

Answer 47

They "rest" by binding to oviduct epithelial cells, allowing them to be nourished.

Question 48

Can freshly ejaculated sperm fertilise an egg?

Answer 48

No, they need to undergo capacitation.

Question 49

What is capacitation?

Answer 49

A process involving: - Time away from seminal plasma - Activation of sperm motility (whiplash tail) - Membrane changes for fertilisation (acrosome reaction) - Calcium-dependent processes

Question 50

What is the acrosome?

Answer 50

A cap-like structure on the sperm head containing digestive enzymes (e.g. acrosin, hyaluronidase).

Question 51

What is the role of hyaluronidase?

Answer 51

It digests hyaluronic acid in cumulus cells surrounding the oocyte.

Question 52

What triggers the acrosome reaction?

Answer 52

Binding of the sperm to zona pellucida glycoproteins ZP2 and ZP3.

Question 53

How long does it take sperm to penetrate the zona pellucida?

Answer 53

5–20 minutes, species-specific.

Question 54

What occurs after sperm penetrates the zona pellucida?

Answer 54

Fusion of the sperm and oocyte membranes allows the sperm's genetic material to enter the egg.

Question 55

What parts of the sperm enter the egg?

Answer 55

The head (containing DNA); the tail and midpiece are either left out or degraded.

Question 56

What is the egg's state before fusion?

Answer 56

Arrested at metaphase II (MII).

Question 57

What triggers completion of meiosis in the oocyte?

Answer 57

Calcium oscillations triggered by phospholipase C from the sperm.

Question 58

What marks the completion of meiosis?

Answer 58

Formation of a second polar body and alignment of male/female pronuclei.

Question 59

What is cleavage?

Answer 59

Series of mitotic divisions without growth, producing smaller blastomeres.

Question 60

What is the morula?

Answer 60

A compact cluster of 16–32 blastomeres.

Question 61

What is the blastocyst?

Answer 61

A hollow structure formed 5–7 days post-fertilisation containing: Trophoblast (outer layer) Inner cell mass Blastocoel cavity

Question 62

What hormone prevents the regression of the corpus luteum?

Answer 62

Chorionic gonadotrophin (CG/hCG).

Question 63

What does hCG do?

Answer 63

Mimics LH to maintain progesterone output from the corpus luteum.

Question 64

When does CG production decline?

Answer 64

After the first trimester (~12 weeks), as the placenta takes over progesterone production.

Question 65

What happens to the genome of the embryo?

Answer 65

Epigenetic marks are erased and re-established to allow totipotency.

Question 66

What is a digynic error?

Answer 66

Two female pronuclei and one male; embryo is non-viable.

Question 67

What is parthenogenesis?

Answer 67

Activation of an egg without a sperm; embryo is non-viable.