Fertilisation and early embryo development Flashcards
Define the fertilisation period
-The time when oocytes are available to be fertilised by sperm, normally few hours after ovulation.
(N.B. the fertile period is the time when mating could result in pregnancy - hence fertile period is longer than fertilisation period)
What is the fertile life span of sperm in different species?
Bitch= 5-11 days Cow= 1-2 days Mare= 4-5 days Camelids= 4-5 days Some reptiles= 4-5 days
Discuss why the timing of ovulation in the dog is different to other species
- Ovulation occurs at prophase 1. Completion of meiosis 2 and formation of second polar bodies occurs after fertilisation
- The fertilisation period in the bitch is shifted so it is longer
Describe fertilisation stages (sperm)
- Sperm capacitation: removal of glycoprotein coat and alteration of sperm mitochondria (makes them hypermotile)
- Sperm binding: ZP3 (zona pellucida protein) acts as a receptor for sperm
- Acrosome reaction: loss of acrosome contents, enzymes digest ZP, exposed sperm surface proteins bind to ZP2
Describe fertilisation stages (egg)
- Sperm membrane fusion: results in depolarisation of egg membrane (due to influx of Ca2+). Causing cortical reaction which blocks polyspermy. Oocytes is activated allowing completion of meiosis 2.
- Sperm oocyte fusion: membranes between sperm and egg fuse, allowing sperm nuclei to pass into egg cytoplasm. (sperm becomes completely invaginated in the cytoplasm of the egg and the chromatin of the sperm begins to decondense.
- Sperm nucleus decondenses, egg and sperm nucleus fuse (syngamy)
When does the embryo become a zygote?
When the two pronuclei fuse together
What are cleavage divisions?
When the zygote undergoes a series of mitotic divisions, each cleavage division generates cells which are called blastomeres.
Describe cleavage of the zygote
- Cleavage occurs with NO CYTOPLASMS SYNTHESIS
- Hence with each individual blastomere the volume decreases.
What is a 16 cell embryo called?
A morula
What is a totipotent cell?
Has the ability to form all cells for normal development
N.B totipotency is not demonstrated beyong the morula stage
After the morula stage, what can’t happen?
The cell within the embryo cannot give rise to a new individual any longer.
How does a blastocyst form?
The outer cells within the morula are more squashed than the outer cells. (Tight junctions form between outer cells and gap junctions form between inner cells)
- The outer cell pump sodium into morula, ionic concentration therefore rises and water then diffuses through the ZP (fluid accumulation)
- Once a distinct cavity (blastoceole) is formed the embryo is called a blastocyst.
What do the 2 layers of the blastocyst becomes and what will these layers develop into?
Outer cells= Trophoblast which become chorion and placenta
Inner cells= Inner cell mass which will become the embryo
Describe the process of the hatching of the blastocyst
- The blastocyst continues to undergo mitosis and fluid continues to be produced. The fluid causes an increase in pressure.
- Proteolytic enzymes are produced by trophoblast
- ZP weakens
- ZP splits and blastocysts emerges.
In which species is progression continuous and in which is it discontinuous?
Continuous: rabbit/ sow
Discontinuous: cow/ ewe
