Fertilisation

Question 1

How do sperm gain their fertilising capacity? (2)

Answer 1

• Interactions with the female reproductive tract.
• Sperm Capacitation

Question 2

Define Sperm Capacitation (4)

Answer 2

Further and final maturation process which confers the following functional
characteristics on to sperm and ultimately renders them capable of fertilisation:

1. A change to a state of hyperactivated motility.
2. The ability to bind to the oocyte’s zona pellucida and afterwards undergo the acrosome reaction.
3. The capacity to fuse with the oocyte

Question 3

Name the 4 female tract Interaction mechanisms (4)

Answer 3

Chemotaxis
Thermotaxis
Rheotaxis
Boundary-following navigation

Question 4

Define Chemotaxis (3)

Answer 4

— Chemical guiding mechanism.
— Sperm responding to a gradient of chemoattractant e.g. steroid hormones.
— Molecular and behavioural mechanisms yet to be fully understood

Question 5

Define Thermotaxis (3)

Answer 5

— Temperature guiding mechanism.
— Female tract consists of different areas with marginal differences in temperature.
— Sperm responding to changes in the extracellular temperature gradient.

Question 6

Define Rheotaxis (3)

Answer 6

Ability to respond to fluid currents in the female tract environment.

Question 7

Define Boundary-following navigation (4)

Answer 7

— Ability to turn corners in response to surface boundaries
— Human sperm with preference to follow boundaries on the left or right hand side have been shown to possess higher DNA integrity than straight swimming sperm

Question 8

The Sperm Capacitation Puzzle

Answer 8

image

Question 9

Sperm-oocyte interaction - 4 steps (6)

Answer 9

• Once a few capacitated sperm make it to the site of fertilisation, they come
  in contact with the cumulus-oocyte complex (COC) and a much higher
  concentration of follicular fluid (FF).
• FF in addition to secretions from the COC e.g. progesterone further modulate the spermatozoa to initiate the process of sperm-oocyte interaction and subsequent fertilisation:
  — Penetration of the cumulus oophorus
  — Sperm-zona binding
  — Acrosome reaction
  — Sperm-oocyte fusion and oocyte activation

Question 10

Penetration of the cumulus-oophorus

Answer 10

Once sperm come in contact with the cumulus-oophorus cell mass surrounding the oocyte, the enzyme hyaluronidase present on the surface of the sperm head dissolves hyaluronic acid (hyaluronan) – the major
cementing factor between cells that constitute the cumulus oophorus cell mass.

Question 11

Sperm-zona binding (3)

Answer 11

• Four zona pellucida (ZP) glycoproteins expressed – ZP1, ZP2, ZP3, ZP4.
• Studies across mammalian species have shown that sperm bind to ZP2 and ZP3 via receptors present on the sperm’s plasma membrane.
• Recent studies show that human sperm bind to ZP2.

Question 12

Acrosome Reaction (3)

Answer 12

• Triggered in response to sperm-oocyte interaction
• Fusion of the sperm’s plasma membrane with it’s outer acrosomal membrane.
• Results in the release of hydrolytic and proteolytic enzymes e.g. hyaluronidase and acrosin.

Question 13

Sperm-oocyte fusion & oocyte activation (4)

Answer 13

Intracellular calcium spike + Cortical reaction =
1)Release from meiotic arrest and completion of meiosis II.
2)Incorporation of sperm DNA and pronuclear formation.
3)Embryo cleavage

Question 14

How does the sperm trigger oocyte activation? (4)

Answer 14

Phospholipase C Zeta (PLC ζ). (PLC ζ + PIP2 = IP3 + DAG)
* Discovered in 2002 by Saunders et al. and widely accepted as the oocyte activation factor introduced by sperm.

• Acts by activating inositol trisphosphate (IP3) production, leading to repetitive calcium release (oscillations) from the oocyte’s calcium stores.
• Has been identified in all species studied so far, including human

Question 15

What 2 things do you look at for fertilisation? (2)

Answer 15

Pronuclei
Polar bodies

Question 16

Fertilisation + Pronuclei (4)

Answer 16

PN: Normal fertilisation. Normally fertilised oocytes should contain two juxtaposed and centrally located PN, with distinctly clear membranes and nucleolar precursor bodies (NPBs).

0PN: No fertilisation.

1PN: Abnormal fertilisation. Contains a haploid set of chromosomes. There are cases where a 1PN zygote contains a diploid set of chromosomes, resulting from errors in the synchrony of PN formation/fusion. Zygotes
in this category often have 2 polar bodies.

≥3PN: Abnormal fertilisation. Contains additional set(s) of chromosomes, which could be either digynic or diandric. Significantly linked to the formation of aneuploid embryos.

Question 17

Fertlisation + Polar Body (PB): (3)

Answer 17

0PB: Oocyte at the Metaphase I stage of meiosis.

1PB: Mature oocyte at the Metaphase II stage of meiosis with one polar body extruded. No fertilisation.

2PB: Oocyte has completed second meiotic division and extruded second polar body in response to fertilisation.

Question 18

Fertilisation + embro dev (9)

Answer 18

1) Fertilised egg ( 2PN)
2) 2 cell
3) 4 cell
4) 8 cell
5)Morula
6) Early blastocyst
7) expanded blastocyst
8) hatching blastocyst
9) Hatched Blastocyst