01-15: Fertilization

Question 1

What are the steps of fertilization?

Answer 1

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

Question 2

What happens after the fusion of the outer acrosomal membrane with the sperm plasma membrane?

Answer 2

portals are created for the acrosome contents to be released into

Question 3

What causes the release of the acrosomal enzymes?

Answer 3

fragmentation of fused acrosomal membrane and plasmalemma of the sperm

Question 4

What is one of the major acrosomal enzymes?

Answer 4

HA

Question 5

What does HA do?

Answer 5

helps to breakdown the HA component of the intercellular matrix between the corona radiata cells

Question 6

Swimming movements of spermatozoa helps in the penetration through the ___________-

Answer 6

corona radiata

Question 7

What are the steps of sperm penetration into the corona radiata?

Answer 7

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

Question 8

What are the steps of the acrosomal reaction?

Answer 8

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

Question 9

What is the most important enzyme involved in the penetration of the zona pellucida?

Answer 9

Acrosin (a serine proteinase)

also called Zona Lysin

Question 10

Is Acrosin released from the acrosome?

Answer 10

NO

remains attached to the portion of the acrosomal membrane that fuses to the remaining sperm plasmalemma and overlies the nucleus

Question 11

What does acrosin do?

Answer 11

• digests a small hole though the ZP

- assists in the penetration of swimming spermatozoa through the ZP

Question 12

What is the fast block to polyspermy?

Answer 12

rapid depolarization of egg plasmalemma

Question 13

How fast is the fast block to polyspermy?

Answer 13

-70 to +10mV within 2-3 seconds

Question 14

What does the fast block to polyspermdy do?

Answer 14

• temporarily prevents polyspermy

- allows time for the egg to establish the slow block

Question 15

What is the slow block to polyspermy?

Answer 15

-release of polysaccharides from the cortical granules located just under the plasmalemma of the egg

Question 16

What organelle forms cortical granules?

Answer 16

ER

Question 17

During the slow block to polyspermy, where do the polysaccharides enter?

Answer 17

the perivitelline space (between the plasmalemma and the ZP)

Question 18

What happens to the perivitelline space after entrance of the polysaccharides during the slow block to polyspermy?

Answer 18

the perivitelline space becomes hydrated which causes swelling of the space

Question 19

How is sea urchin fertilization different from mammalian fertilization?

Answer 19

in sea urchins, there is no cellular coat that the sperm has to penetrate

Question 20

What are the steps of sea urchin fertilization?

Answer 20

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

Question 21

What is the tanned fertilization membrane?

Answer 21

-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

Question 22

When can no more sperm penetrate?

Answer 22

formation of the tanned fertilization membrane

Question 23

What are the 3 final steps in fertilization?

Answer 23

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

Question 24

What induces the metabolic activation of the egg?

Answer 24

1- Sperm introduce phospholipase Czeta into the egg cytoplasm
2- Release of Ca within the egg cytoplasm

Question 25

What happens when the Ca is released into the egg cytoplasm?

Answer 25

• 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

Question 26

What makes sperm DNA so tightly packed?

Answer 26

disulfide cross-linkages among protamine molecules

Question 27

What happens in the decondensation step of the sperm nucleus?

Answer 27

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

Question 28

What organizes the first mitotic spindle in sperm?

Answer 28

sperm centrosome

Question 29

How is fertilization ended?

Answer 29

1- Meiosis II
2- Pronucleus of the male is surrounded by a membrane
3- Fuses with pronucleus of the female
4- End of fertilization

Question 30

What is a pronucleus?

Answer 30

the nuclear material of the head of the spermatozoan or of the oocyte after the oocyte has been penetrated by the spermatozoan

Question 31

What is the male pronucleus?

Answer 31

spermatozoan

Question 32

What is the female pronucleus?

Answer 32

oocyte

Question 33

What does each pronucleus normally carry?

Answer 33

a HAPLOID set of chromosomes

Question 34

What does the male pronucleus consist of?

Answer 34

decondensed nuclear material

Question 35

What is a pronuclear membrane?

Answer 35

• derived from the ER of the egg

- forms around the female chromosomal material (pronucleus)

Question 36

What is a zygote?

Answer 36

• single celled stage
• male and female pronuclei have fused together
• share a common membrane
• establishes the diploid chromosome number

Question 37

What are the steps of in vitro fertilization?

Answer 37

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