Fertilisation

Question 1

What are challenges of navigating the female reproductive tract?

Answer 1

• Avoid retrograde transport
• Transverse the cervix
• Travel through the cervix
• Travel through the oviduct
• Attain capacity to fertilise - capacitation, hyper-activation, acrosome reaction

Question 2

How much sperm is lost in cow and humans due to retrograde transport?

Answer 2

60% in cow
99% in humans
- intra uterine semen deposition (pig)
- semen squirted through cervix at copulation (horse, dog)
- Maintaining mating position, high pressure (dog)
- Viscous/gel fraction of seminal plasma acts to “plug” tract (horse, pig, rodent)

Question 3

Describe the structure of a sperm?

- moving from head to tail

Answer 3

In the head region 
- acrosome 
- nucleus 
In the mid-piece
- centriole 
-mitochondrion, surrounding the start of the flagellum 
In the tail
- flagellum 
- tail sheath

Question 4

Describe the structure of the head of the sperm from nuclear envelope outwards?

Answer 4

Nuclear envelope
Subacrosomal space 
Inner acrosomal membrane 
Acrosomal space
Outer acrosomal membrane 
Plasma membrane

Question 5

Why do sperm move through the reproductive tract?

Answer 5

• motility of sperm
• viscous fluid currents caused by uterine cilia
• uterine contractions
  Sperm tail
• energy production (mid-piece mitochondrial sheath supplies ATP)
• propulsive apparatus (axoneme)

Question 6

How is sperm motility evaluated?

Answer 6

Computer Assisted Semen Analysis (CASA)
Asses the quality of motility
Identify fertile sperm

Question 7

What is sperm co-operation?

Answer 7

Sperm do not attack rival ejaculates and instead display co-operation - i.e. sperm from a single male will co-operate to gain a competitive advantage over sperm from another male

• occurs in species where multiple males mate with the same female - inter-male sperm competition
  i. e. the common wood mouse

Question 8

What is the wood mouse sperm train?

Answer 8

Sperm head hook attached to neck region, deployed in the presence of rival males sperm, forms a train to increase motility.
Over 2mm long consisting of thousands of sperm
= 2x the speed of a single sperm

Question 9

What are echidna sperm bundles?

Answer 9

Female mates approx. 11 males
Sperm forms a bundle, 20-100 sperm per bundle, have advantage of faster motility
NO. of sperm per bundle correlates to how promiscuous the female has been

Question 10

What are sperm head abnormalities?

Answer 10

Nuclear vacuoles 
- increased no. 
Tapered heads
Ruffled acrosome 
Knobbled acrosome

Question 11

What are sperm tail abnormalities?

Answer 11

Coiled tails
Double mid-piece
Folded tail
Detached head

Question 12

Does abnornal morphology matter in natural mating?

Answer 12

Will be selected out by female reproductive tract as it affects the ability to fertilise
Natural mating induces sperm competition
Natural selection + survival of the fittest

Question 13

What is ICSI?

Answer 13

Intra Cytoplasmic Sperm Injection
Modified IVF
No natural selection
Used with abnormal sperm

Question 14

What are the 2 phases of sperm transport in the female?

Answer 14

Rapid transport phase 
- Sperm reaches the oviduct within minutes 
- Unable to fertilise oviduct
Sustained transport phase 
- sperm undergo
-- capacitation 
-- hyper-activation 
-- acrosome reaction

Question 15

How do sperm become capacitated?

Answer 15

Epididymis = surface molecules added (proteins + carbohydrates)
Ejaculation = Surface molecules coated with seminal plasma proteins (de-capacitation factors)
Capacitation = female tract strips some of the proteins leaving exposed areas for sperm - egg binding

Question 16

What is capacitation?

Answer 16

Progressive de-stabilisation of plasma membrane

• glycoprotein molecules coating sperm head removed
• exposure of zona pellucida binding proteins
• allows sperm to bind to oocyte at fertilisation
• surface charge altered - may attract sperm to oocyte, sperm have a net negative surface charge (NNC)
• membrane fluidity increased to help the breakdown of acrosome

Question 17

What is hyperactivity?

Answer 17

Capacitated sperm exhibit hyper-activated motility
Strong wide amplitude, whiplashing tail beats
Increased cellular calcium levels lead to elevated cAMP
Increased force required to swim through the viscous environment within the oviduct
In Vitro, hyperactivity = more head movement and less linearity

Question 18

What is the acrosome reaction?

Answer 18

Capacitation exposes zona pellucida binding proteins on sperm plasma membranes (ZP3), sperm can bind to zona pellucida via ZP3
This binding initiates the acrosome reaction
- fusion of sperm plasma membrane and outer acrosomal membrane
- release of enzymes to digest zona pellucida
- exposure of equitorial segment for oocyte fusion (leads to vascularisation producing pores) - release of hyaluronidase and acrosin
Then, the vesicles are lost and the inner acrosomal membrane and equitorial segment are exposed

Question 19

How is the Zona Pellucida exposed?

Answer 19

Acrosomal enzymes digest small hole in zona pellucida
Penetration of zona pelluicida is a rapid process
Sperm move into peruictelline space between zona and oocyte plasma membrane

Question 20

What is the 1st step in zona pellucida digestion?

Answer 20

Sperm in peruitelline space surrounded by plasma membrane microvilli
Cortical granules in oocyte cytoplasm

Question 21

What is the 2nd step in zona pellucida digestion?

Answer 21

Sperm equitorial segment fuses with oocyte plasma membrane
Sperm is engulfed
Cortical granules released into peruitelline space

Question 22

What is the 3rd step in zona pellucida digestion?

Answer 22

Zona back to polyspermy
Sperm nucleus within oocyte
Sperm nucleus de-condenses and forms the male pronucleus

Question 23

Describe the genetic content at fertilisation?

Answer 23

Gametes are haploid (1N), single copy of each chromosome
At fertilisation oocyte has 1 female pronucleus and 1 male pronucleus (1N) + (1N)
Following fertilisation the female and male pronuclei fuse to form a diploid (2N) zygote

Question 24

How is the early embryo developed?

Answer 24

Zygote undergoes mitotic cleavage
1st mitotic cleavage division creates a two-cell embryo
Cleavage to form 4,8,16 cell embryo
Each cell in early embryo is called a blastomere (undifferentiated and genetically identical)

Question 25

How is the blastocyst formed?

Answer 25

Cells form two distinct layers 
- inner ball of cells (ICM)
- cells at the periphery (trophoblast)
Cells in outer cell mass pump sodium into blastocyst creating an accumulation of fluid, blastocyte
ICM develops into embryo 
Trophoblast cells become the placenta

Question 26

How does the blastocyst hatch?

Answer 26

Blastocyst continues to move down oviduct towards uterus
Implantation is prevented by the zona pellucida, at the uterus the blastocyst hatches by digesting a hole in the zona pellucida and becomes a free-floating embryo on the uterus for survival