Fertilisation Flashcards
What are challenges of navigating the female reproductive tract?
- Avoid retrograde transport
- Transverse the cervix
- Travel through the cervix
- Travel through the oviduct
- Attain capacity to fertilise - capacitation, hyper-activation, acrosome reaction
How much sperm is lost in cow and humans due to retrograde transport?
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)
Describe the structure of a sperm?
- moving from head to tail
In the head region - acrosome - nucleus In the mid-piece - centriole -mitochondrion, surrounding the start of the flagellum In the tail - flagellum - tail sheath
Describe the structure of the head of the sperm from nuclear envelope outwards?
Nuclear envelope Subacrosomal space Inner acrosomal membrane Acrosomal space Outer acrosomal membrane Plasma membrane
Why do sperm move through the reproductive tract?
- 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)
How is sperm motility evaluated?
Computer Assisted Semen Analysis (CASA)
Asses the quality of motility
Identify fertile sperm
What is sperm co-operation?
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
What is the wood mouse sperm train?
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
What are echidna sperm bundles?
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
What are sperm head abnormalities?
Nuclear vacuoles - increased no. Tapered heads Ruffled acrosome Knobbled acrosome
What are sperm tail abnormalities?
Coiled tails
Double mid-piece
Folded tail
Detached head
Does abnornal morphology matter in natural mating?
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
What is ICSI?
Intra Cytoplasmic Sperm Injection
Modified IVF
No natural selection
Used with abnormal sperm
What are the 2 phases of sperm transport in the female?
Rapid transport phase - Sperm reaches the oviduct within minutes - Unable to fertilise oviduct Sustained transport phase - sperm undergo -- capacitation -- hyper-activation -- acrosome reaction
How do sperm become capacitated?
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
What is capacitation?
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
What is hyperactivity?
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
What is the acrosome reaction?
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
How is the Zona Pellucida exposed?
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
What is the 1st step in zona pellucida digestion?
Sperm in peruitelline space surrounded by plasma membrane microvilli
Cortical granules in oocyte cytoplasm
What is the 2nd step in zona pellucida digestion?
Sperm equitorial segment fuses with oocyte plasma membrane
Sperm is engulfed
Cortical granules released into peruitelline space
What is the 3rd step in zona pellucida digestion?
Zona back to polyspermy
Sperm nucleus within oocyte
Sperm nucleus de-condenses and forms the male pronucleus
Describe the genetic content at fertilisation?
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
How is the early embryo developed?
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)
How is the blastocyst formed?
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
How does the blastocyst hatch?
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
