Fertilisation and embryo development Flashcards
What are the steps in fertilisation in mammals?
- Sperm capacitation
- Sperm binding
- Acrosome reaction
- Sperm membrane fusion
Describe sperm capacitation
- Removal of glycoprotein coat and alteration of sperm mitochondria
- Hyperacitve motility occurs
What receptor is required is required for sperm binding?
ZP3 on the zona pellucida
Describe the acrosome reaction
- Loss of acrosome contents (enzymes)
- Digests ZP
- Exposed sperm surface proteins bind to ZP3
- Once bound, sperm and egg can fuse
Describe sperm membrane fusion
- Causes depolarisation of egg membrane
- Sperm completely invaginated into egg
- Stimulates cortical reaction (release of cortical granules)
- Blocks polyspermy
- Membrane depolarised
- Oocyte activation
- Formation of ootid
- Tail of sperm lost
How does sperm membrane fusion depolarise the oocyte membrane?
- Granules induce calcium influx into egg from extracelular space
- Induces more calcium release from endoplasmic reticulum
- Wave of depolarising calcium
What is the function of membrane depolarisation in sperm membrane fusion?
- Blocks polyspermy
- Once depolarised, ZP impermeable to other spem
What occurs in oocyte activation following sperm membrane fusion?
- Completion of meiosis
- Extrusion of hte second polar body
Where does membrane fusion begine?
The equatorial segment of the sperm
Describe the chromosomal events that occur during sperm membrane fusion
- Immediately after fusion, chromatin of sperm completely decondensed (nucleus of sperm loses shape)
- Nucleus appears larger and fluffier
- 2 pro-nuclei now present in cell = ootid
What is syngamy?
Fusion of the 2 pro-nuclei to form a zygote
Where does fertilisation take place?
In the oviduct
What is the zygote?
Single celled embryo formed following syngamy
What is formed from the first cleavage division of the zygote?
- 2 cell embryo
- Each cell within is called blastomere
- ZP still present
Describe cleavage of the zygote
- No cytoplasm synthesis, only DNA replication
- So individual blastomere volume increases
- Eary stages, each blastomere has potential to develop into separate offspring
What happens to the blastomeres following the first cleavage division of the zygote?
- Further divisons resulting in 4, 8 and tehn 16 cell embryos
What is the morula?
Last point in the embryo where all cells are identical (16 cells)
(Is the last point at which a cell can be used for genetic analysis)
Define totipotent
Having the ability to form all cell necessary for normal development
In what stages of development are all cells in the embryo totipotent?
Up to and including the morula, not demonstrated beyond 16 cell stage
Describe the formation of the blastocyst
- Outer cells within morula more squashed than inner
- Outer cells form cell-cell adhesions (tight junctions, expression of proteins)
- Inner cells have gap-junctions
- Outer cells pump sodium into morula
- Increases ionic concentration, water in through ZP = fluid accummulation causing increased pressure
Define blastocyst
The embryological stage where there is a distinct cavity formed
What do the outer cells of the blastocyst become?
Trophoblasts
What do the inner cells of the blastocyst become?
Inner cell mass
What do the trophoblasts give rise to?
Chorion and placenta