Fertilization

Question 1

What is fertilization?

Answer 1

the event that sees a single sperm nucleus unite with the egg nucleus within the activated oocyte cytoplasm

Question 2

What surrounds the oocyte?

Answer 2

• vitelline membrane
• perivitelline space
• zona pellucida
• cumulus oophorus (granulosa cells)

Question 3

Explain the transport of the oocyte

Answer 3

• upon ovulation, oocyte picked up by fimbria of oviduct and transported through the infundibulum to the ampulla
• on entry to the ampulla, it immediately attaches to the oviductal epithelium

(vital that oocyte is still contained within its expanded cumulus matrix during transport)

Question 4

What is capacitation?

Answer 4

maturational changes sperm undergo in the female tract (acquire capacity to bind to and fertilize the egg)

Question 5

What is involved with capacitation?

Answer 5

1. removal or modification of proteins that were acquired by the sperm during epididymis passage
2. lowering of cholesterol/phospholipid ratio of sperm plasma membrane
3. hyperpolarization of membrane resting potential in region overlaying acrosome
4. aggregation of lipid microdomains of plasma membrane that contain zona pellucida-binding protein complexes
5. phosphorylation of surface tyrosine residues

Question 6

Capacitation is speculated to be mediated by changes in what?

Answer 6

1. hormone milieu
2. contents of the oviductal lumen from the ovulated follicular fluid
3. increasing oviductal fluid bicarbonate concentration

Question 7

What is hyperactivated motility?

Answer 7

increased amplitude of tail movement which also causes side-side movement of the head

Question 8

What is the function of hyperactivated motility?

Answer 8

detaching sperm from oviductal reservoir
locating the egg
penetration of outer egg coats

Question 9

What are the 2 thoughts on why capacitation occurs?

Answer 9

1. events surrounding ovulation signal oviductal epithelium to secrete something that triggers capacitation
2. capacitation occurs because of changing conditions as they move up the oviduct

Question 10

Do sperm have a shorter lifespan once they go through capacitation?

Answer 10

yes

Question 11

How does the sperm find the oocyte in non-mammalians?

Answer 11

chemotaxis

oocyte secretes substances that effect intracellular calcium levels in the sperm (low calcium causes linear swimming and sperm continue up attractant concentration gradient towards egg)

Question 12

How does the sperm find the oocyte in mammals?

Answer 12

thermotaxis at long distances

some chemotaxis at short distances

Question 13

How do sperm penetrate the cumulus oophorus?

Answer 13

only capacitated sperm can penetrate the cumulus oophorus because of hyperactivated motility

also have hyaluronidase, which helps break down the hyaluronic acid matrix of the cumulus oophorus

Question 14

How do sperm penetrate the zona pellucida?

Answer 14

capacitated sperm bind to ZP and undergo acrosome reaction

Question 15

What is the acrosome reaction?

Answer 15

fusion of outer acrosomal membrane with the plasma membrane of the sperm resulting in exposure of acrosomal content to extracellular environment

Question 16

What 2 substances are thought to induce the acrosome reaction?

Answer 16

progesterone

the ZP itself

Question 17

Binding to the ZP causes a rapid increase in cytosolic calcium in the sperm resulting in what?

Answer 17

1. sperm plasma membrane fuses at multiple points with outer acrosomal membrane causing membrane vesiculation
2. acrosomal contents released, exposing inner acrosomal membrane
3. inner acrosomal membrane binds to ZP2 (glycoprotein on ZP)

Question 18

What happens to the sperm after the acrosome reaction and binding to the ZP?

Answer 18

• sperm plasma membrane fuses with vitelline membrane, which wells up to surround the sperm
• sperm is now incorporated into the ooplasm
• membrane then breaks down and the sperm nucleus is released into the egg cytoplasm

Question 19

What are the contributions of the sperm to the next generation?

Answer 19

1. paternal haploid genome
2. signal to initiate egg metabolism/activation
3. centrosome

Question 20

What happens when the egg is activated?

Answer 20

1. escapes 2nd meiotic arrest

2. undergoes the cortical reaction

Question 21

How does the sperm increase calcium in the oocyte?

Answer 21

introduces a soluble sperm-derived factor (oscillin/oscillogen) that enhances the calcium-induced calcium release from intracellular stores

(calcium triggers entry into meiosis)

Question 22

What is the male pronucleus?

Answer 22

upon entering the ooplasm, the sperm head separates from the tail and the nuclear membrane degenerates and a new nuclear envelope is formed around it

Question 23

What is the female pronucleus?

Answer 23

following completion of 2nd meiosis, the female chromatin is decondensed and acquires a nuclear envelope

Question 24

What is pronuclear migration?

Answer 24

male and female pronuclei are directed towards the center of the cell

Question 25

What occurs in syngamy?

Answer 25

chromatin in each pronucleus duplicates and condenses and the nuclear membranes break down

Question 26

Name 3 methods of avoiding polyspermy

Answer 26

1. limiting the number of sperm at the site of fertilization
2. cortical granules of the ooplasm fuse with the vitelline membrane, discharging their enzymatic contents into the perivitelline space (cause zona hardening)
3. vitelline or membrane block (sperm cannot bind or fuse to vitelline membrane)

Question 27

How does triploidy occur?

Answer 27

1. failure of polyspermy block

2. failure to extrude the 2nd polar body