Fertilisation (lecture) Flashcards
What is fertilisation?
Fusion of egg and ovum to create diploid offspring
What must happen for fertilisation to take place?
Sperm must make it to the ovum
Gametes must recognise each other
Sperm must penetrate ovum
Haploid egg nucleus and sperm nucleus must fuse to form diploid zygote (syngamy)
Ovum must block entry of additional sperm to avoid polyploidy
What are the key features of an egg cell?
Germinal Vesicle: Nucleus containing DNA
Germinal spot: nucleolus
Ooplasm: Cytoplasm containing nutrients
Vitelline Membrane: Cell membrane surrounding the ooplasm
Zona Pellucida: Extracellular matrix containing glycoproteins
Corona Radiata: layer of follicle cells
What are the key features a sperm cell?
Head: Contains nucleus with sperm DNA, capped with acrosome, which penetrates the egg membrane
Neck: Contains the centriole
Midpiece: contains the mitochondria which provides energy for movement
Tail: Drives the sperm forward
Sperm can only travel through liquid, how have terrestrial animals overcome this problem?
Sperm inseminated in seminal fluid (and oviductal fluid in female reproductive tract)
What are the five steps of sperm penetration of the ovum?
1 - Bonding of sperm to Zona Pellucida. Sperm penetrate cumulus cell layer via hyper activated motility
2- Sperm undergoes acrosome reaction to penetrate Zona Pellucida
3- Penetration through the Zona Pellucida, sperm and oocyte membranes (mediated by recognition proteins)
4 - Sperm nucleus enters ovum
5 - Membrane and Zona Pellucida rapidly block polyspermy
What happens when sperm comes into contact with the egg (example from sea urchins)?
Triggers exocytosis of the sperms acrosome
What is the acrosome?
An organelle that develops over the anterior half of the head in spermatozoa
What is the acrosomal reaction?
Hydrolytic enzymes are released from the acrosome and make a hole in the jelly coat. Growing actin filaments form the acrosomal process, which protrudes from the sperm head and penetrates the jelly coat. Proteins on the surface bind to receptors in egg plasma membrane
What is triggered in the egg when sperm fuses?
Depolarisation of the membrane, which acts as a fast block to polyspermy
What is the cortical reaction?
cortical granules in the egg fuse with the the plasma membrane, sperm binding receptors are clipped off and cause the fertilisation envelope to form. Acts as a slow block to polyspermy.
What is the perivitelline space?
Lies between plasma membrane and vitelline layer
What is the vitelline layer?
Mesh of extracellular matrix, through which the sperm receptor proteins extend
What is the clear evolutionary pattern in fertilisation across species?
Rapid diversification of reproductive molecules, particularly those involved in sperm-egg reproduction
