01-15: Fertilization Flashcards
What are the steps of fertilization?
1- Penetration of corona radiata 2- Attachement of ZP 3- Acrosomal reaction & penetration of ZP 4- Binding & fusion of sperm and egg 5- Prevention of polyspermy 6- Metabolic activation of egg 7- Decondensation of sperm nucleus 8- Completion of egg meiosis 9- Development & fusion of male and female pronuclei
What happens after the fusion of the outer acrosomal membrane with the sperm plasma membrane?
portals are created for the acrosome contents to be released into
What causes the release of the acrosomal enzymes?
fragmentation of fused acrosomal membrane and plasmalemma of the sperm
What is one of the major acrosomal enzymes?
HA
What does HA do?
helps to breakdown the HA component of the intercellular matrix between the corona radiata cells
Swimming movements of spermatozoa helps in the penetration through the ___________-
corona radiata
What are the steps of sperm penetration into the corona radiata?
1- Sperm plasmalemma fuses with acrosomal membrane at numerous points
2- Portals to the exterior are created
3- HA diffuses out of the acrosome
4- HA dissolves follicle cells of corona radiata
What are the steps of the acrosomal reaction?
1- Sperm is attached to the Zona pellucida
2- ZP3 proteins mediates the attachment of sperm to the ZP
3- ZP3 protein stimulates acrosomal reaction in mammals (through G proteins in the sperm plasma membrane)
4- Massive influx in Ca through sperm plasma membrane
5- Na+ influx and H+ efflux
6- pH increased
7- Penetration of sperm through the ZP
What is the most important enzyme involved in the penetration of the zona pellucida?
Acrosin (a serine proteinase)
also called Zona Lysin
Is Acrosin released from the acrosome?
NO
remains attached to the portion of the acrosomal membrane that fuses to the remaining sperm plasmalemma and overlies the nucleus
What does acrosin do?
- digests a small hole though the ZP
- assists in the penetration of swimming spermatozoa through the ZP
What is the fast block to polyspermy?
rapid depolarization of egg plasmalemma
How fast is the fast block to polyspermy?
-70 to +10mV within 2-3 seconds
What does the fast block to polyspermdy do?
- temporarily prevents polyspermy
- allows time for the egg to establish the slow block
What is the slow block to polyspermy?
-release of polysaccharides from the cortical granules located just under the plasmalemma of the egg
What organelle forms cortical granules?
ER
During the slow block to polyspermy, where do the polysaccharides enter?
the perivitelline space (between the plasmalemma and the ZP)
What happens to the perivitelline space after entrance of the polysaccharides during the slow block to polyspermy?
the perivitelline space becomes hydrated which causes swelling of the space
How is sea urchin fertilization different from mammalian fertilization?
in sea urchins, there is no cellular coat that the sperm has to penetrate
What are the steps of sea urchin fertilization?
1- Sperm extends acrosomal filament
2- Filament makes contact with the jelly coat of the egg and penetrates the jelly coat
[This location determines the point of sperm entry]
3- Acrosomal filament makes contact with vitelline membrane and penetrates membrane with lytic enzymes
4- The lytic enzymes allow for gaining of contact with the hyaline cytoplasm just below the plasmalemma of the egg
5- Hyaline cytoplasm flowing towards sperm entry
6- Hyaline cytoplasm creates a fertilization cone (cortical granules have now moved closer to the plasmalemma)
7- In the region near the point of sperm entry, the cortical granules have fused to the plasmalemma and emptied their dark body contents into the space between the plasmalemma and the vitelline membrane
8- Dark body contents fuse to the undersurface of the vitelline membrane
9- Space between the vitelline membrane and the plasmalemma increases in size as water enters the space
10- Vitelline membrane is raised off the plasmalemma
11- The fertilization cone draws the sperm nucleus into the cytoplasm
12- Cortical alveoli fuse to the outside of the plasmalemma (creates hyaline plasma membrane)
13- Sperm nucleus and centrioles are almost completely within the egg cytoplasm
14- Fertilization membrane has sealed over the point of sperm entry
15- Sperm flagellum and other cytoplasmic organelles remain outside the fertilization membrane
16- Sperm nucleus is completely enclosed within the egg cytoplasm
17- Hyaline plasma membrane has sealed over the point of sperm entry
What is the tanned fertilization membrane?
-Egg surface changes color from white to yellow (tanned)
- dark bodies that have formed a continuous later fused with the undersurface of the vitelline membrane
- formation of the fertilization membrane
When can no more sperm penetrate?
formation of the tanned fertilization membrane
What are the 3 final steps in fertilization?
1-metabolic activation of egg
2-decondensation of sperm nucleus
3-completion of meiosis in oocyte
4-organization of first mitotic spindle by sperm centrosome
What induces the metabolic activation of the egg?
1- Sperm introduce phospholipase Czeta into the egg cytoplasm
2- Release of Ca within the egg cytoplasm
What happens when the Ca is released into the egg cytoplasm?
- initiates blocks to polyspermy
- stimulates increase in egg respiration and metabolism (via Na/H exchange mechanism)
- increases pH and oxidative metabolism
Na+ = moves in H+ = moves out
What makes sperm DNA so tightly packed?
disulfide cross-linkages among protamine molecules
What happens in the decondensation step of the sperm nucleus?
1- sperm nuclear membrane becomes more permeable
2- allows entry of egg cytoplasmic factors
3- disulfide crosslinkages are reduced to sulfhydryl groups
4- tightly packed sperm chromatin becomes more loosely packed
5- male pronucleus is established
What organizes the first mitotic spindle in sperm?
sperm centrosome
How is fertilization ended?
1- Meiosis II
2- Pronucleus of the male is surrounded by a membrane
3- Fuses with pronucleus of the female
4- End of fertilization
What is a pronucleus?
the nuclear material of the head of the spermatozoan or of the oocyte after the oocyte has been penetrated by the spermatozoan
What is the male pronucleus?
spermatozoan
What is the female pronucleus?
oocyte
What does each pronucleus normally carry?
a HAPLOID set of chromosomes
What does the male pronucleus consist of?
decondensed nuclear material
What is a pronuclear membrane?
- derived from the ER of the egg
- forms around the female chromosomal material (pronucleus)
What is a zygote?
- single celled stage
- male and female pronuclei have fused together
- share a common membrane
- establishes the diploid chromosome number
What are the steps of in vitro fertilization?
1- Hormonal stimulation of egg maturation
2- Removal of eggs by laproscopy
3- Collection of sperm sample and concentration of most active sperm
4- In vitro fertilization
5- Early cleavage in vitro
6- Extra embryos frozen
7- Reimplantation of up to 3 embryos
