Topic.2.Development.powerpoint2.0.Fertilization Flashcards
What is fertilization?
formation of a diploid zygote from a haploid egg and sperm
What are the key features of fertilization?
- Recognition at a distance
- Contact recognition and binding
- Egg and sperm fusion
- blocks polyspermy
- Egg activation
T/F: Sea Urchins have external fertilization
True,
Sea urchins, resact is released from the jelly layer into the surrounding seawater. Where sperm of the same species can bind to it.
Explain how Sea Urchins recognize at a distance
Chemotaxis: The migration of cells towards a soluble concentration gradient of a stimulant.
Ex: Sea urchins-resact
Resact
soluble glycoprotein derived from the jelly layer of the egg
Explain contact recognition in sea urchins:
The egg jelly layer binds to receptor on the sperm plasma membrane through a carbohydrate molecule called fucose sulfate
Acrosomal reaction in sea urchin:
acrosomal vesicle fuses with the plasma membrane causing the extracellular release of digestive enzymes that penetrate the jelly
coat.
Explain the acrosomal process:
It’s a thin extension that grows out from the sperm to meet the vitelline envelope on the egg. The thin extension is made up of actin monomers that lengthen into actin filaments through a process of polymerization. The acrosomal process is covered by proteins, called bindin, that attach to receptors on the egg. The interaction is bindin species specific.The binding of the sperm to the egg completes the acrosomal reaction and allows the sperm’s plasma membrane to fuse with that of the egg.
Explain egg and sperm binding in sea urchins
In the acrosomal process, the extension is covered by protein molecules called bindins that are attached to sperm-binding receptors on the vitelline layer of the egg. This binding is species-specific.
Explain Blocks to polyspermy: Fast transient block
In sea urchins
Na sodium channels on the egg vitelline layer open, causing an inward influx of Na+ causing depolarization. Unfertilized eggs have a negative charge but become more positive upon fertilization. Thus having a positive charge, blocks other sperm from fusing.
A fertilized sea urchin egg can be expected to be more negative or positive?
positive
Explain the cortical reaction/ slow blocks to polyspermy in sea urchin
triggered by intracellular calcium release which causes cortical granules (vesicles) to fuse with the membrane. Enzymes released from granules clip receptors, lifting the vitelline layer, which then hardens to form the fertilization envelope. Sperm nucleus enters.
in sea urchin cortical reaction, what triggers cortical granule fusion?
increase in intracellular calcium levels
Cortical Granules fuse with the egg plasma membrane resulting in the release of:
-Proteinases & glycosidases
-mucopolysaccharides
-peroxidases
-hyalin protein
Proteinases and Glycosidases
separate vitelline layer
from plasma membrane
Mucopolysaccharides
produce an osmotic gradient that causes water to rush into the space between the plasma membrane and the vitelline envelope, causing the envelope to expand and become the fertilization envelope
Peroxidases
crosslinks macromolecules of the vitelline membrane in order to harden the fertilization envelope
Hyalin protein
modifies the extracellular matrix of the egg to help block sperm entry; coats outer surface of the egg
Explain egg activation
-Early events
-late events
Early Events - increase in cell metabolism
Late Events - initiation of protein and DNA
synthesis in preparation of first cleavage (division)
Zona pellucida; mammals
extracellular matrix of the egg
T/F capacitation occurs in sea urchins
False, it occurs in mammals
Explain capacitation:
substances secreted by uterus or fallopian tubes removes glycoprotein coat and seminal proteins from acrosome.
Explain some key characteristics of capacitation:
– Increases sperm metabolism and motility
– Necessary for future sperm and egg binding
– Triggered by bicarbonate ions (HCO3–) in the vagina
– Requires about 5–6 hours in humans.
What is the role of microvilli?
Microvilli in egg plasma membrane
help facilitate fusion
Mammalian fertilization aims to make what step of oogenesis possible
Aims to explain the necessary steps in order for secondary oocyte to resume meiosis II, which is when the sperm penetrates the secondary oocyte
Explain in general terms fertilization steps in mammalian
1) capacitation removes proteins from acrosome so sperm can be more motile and bind; triggered by basic HCO3- in vagina (5-6 hrs)
2) acrosomal enzymes dissolve zona pellucida (ECM of egg) and Ca+ flows in
3) sperm bind to zp3; zp2 and zp3 are cleaved and causes BLOCK
4) oocyte has 2nd meiotic division forming n ovum and polar body (12-36 hrs)
5) sperm and egg fuse DNA
in mammalian fertilization, what is the acrosomal reaction?
when sperm comes into contact with the
oocyte’s zona pellucida (zp). Acrosomal enzymes begin to dissolve zp; actin
filament comes into contact with the zp.
In mammalian fertilization, how does Ca+ influx affect cell?
It causes cortical granules inside the second oocyte then fuse to the outer membrane.
The Zona Pellucida is
composed of three
proteins:
ZP1, ZP2 and ZP3
Contact and Recognition in mammals:
Sperm plasma
membrane receptors
bind to what protein on Zona Pellucida?
ZP3
What is the cortical reaction/slow block to polyspermy in mammalian fertilization?
released contents of the
cortical granules both
remove carbohydrate from
ZP3 so it no longer can bind to
the sperm plasma
membrane and partly cleave
ZP2, hardening the zona
pellucida
How does the timing of fertilization between humans and sea urchins differ?
-First cell division in sea urchins occurs ~ 90 minutes after
fertilization
-In humans: 12-36hrs
Explain sea urchin fertilization
1 sec: sperm contacts jelly egg coat
2 sec: acrosome breaks down jelly coat and bindin binds to the vitelline layer of the egg
20 sec: FAST block where Na+ enters egg; CORTICAL/SLOW block where Ca+ is released and vitelline layer hardens
5-40 min: egg is activated and increases metabolism to prep for first division at 90 min
