Exam 3: Ovulation & Fertilization

Question 1

What is ovulation

Answer 1

the timed release of a mature, developmentally
competent oocyte (egg) from the ovary into the oviduct, where fertilization occurs

Question 2

Properties of ovulation

Answer 2

highly coordinated process
many genes related to ovulation are related to oocyte competence

Question 3

Time of ovulation

Answer 3

After LH surge
Rabbit, mouse, rat: 12 hr
Cow & Sow: 40 hr
Sheep: 25 ht
Woman: 28-36 hr

Question 4

Site of ovulation

Answer 4

Most species: random
Mare: ovulatory fossa
Monkey: alternate ovaries

Question 5

What is the stigma

Answer 5

Location of rupture of follicular wall
Thinnest part of wall

Question 6

Ovulation rate

Answer 6

Number of eggs fertilized per pregnancy

Question 7

Ovarian events caused by preovulatory LH surge

Answer 7

Histamine release
Increase PGF2a
shift from E2 to P4 by dominant follicle (first)
Surface epithelium
Gap junction breakdown between granulosal cells & oocyte (second)

Question 8

Result of histamine release

Answer 8

Increased follicular pressure
Ovulation

Question 9

Result of increased PGF2a

Answer 9

Increased follicular pressure
Follicle wall weakening
Ovulation

Question 10

Result of shift from E2 to P4 by dominant follicle

Answer 10

Follicle wall weakening
Ovulation

Question 11

Result of surface epithelium

Answer 11

Follicle wall weakening
Ovulation

Question 12

Result of gap junction breakdown between granulosal cells & oocyte

Answer 12

Fertilization

Question 13

Steps that proceed ovulation

Answer 13

cumulus expansion
proteolysis
angiogenesis
inflammation
smooth muscle contraction

Question 14

Post-capacitation events leading to fertilization

Answer 14

Hyperactive motility
Binding to zona pellucida
Acrosomal reaction
Penetration of zona pelllucida
Sperm-oocyte membrane fusion
Sperm engulfed
Decondensation of sperm nucleus
Formation of male pronucleus

Question 15

Binding to zona pellucida

Answer 15

Sperm contain specific proteins on their plasma
membrane surfaces overlying the acrosome that
bind specifically to zona pellucida proteins

Question 16

What makes up the zona pellucida

Answer 16

Three glycoproteins: ZP1, ZP2, ZP3 (humans have ZP4 as well)
Provide structural integrity to zona pellucida

Question 17

Mechanisms used by the sperm to enter the zona

Answer 17

Mechanical hypothesis
Enzymatic hypothesis

Question 18

Mechanical hypothesis

Answer 18

The sperm penetration through the zona is purely mechanical, Acrosomal enzymes do not play a role in the zona penetration by spermatozoa
The sole function of the acrosome reaction is to
expose proteins in the equatorial segment involved in fusion with the egg’s oolema.

Question 19

3 Reasons to support mechanical hypothesis

Answer 19

1) The force generated by sperm is sufficient to break the zona pellucida.
2) A fertilizing spermatozoon leaves a penetration slit in the zona with very sharply defined borders. It looks as though the spermatozoon physically cuts the zona rather than dissolves it.
3) Proteases inhibitors block the binding of the sperm to the zona, but once the binding is established, they can not longer prevent the sperm from passing through the zona

Question 20

Enzymatic hypothesis

Answer 20

Enzymatic hypothesis: Unlike the above mechanical hypothesis, every step of sperm-zona interaction is enzyme-dependent. Sperm motility is of secondary
importance.

Question 21

2 reasons to support enzymatic hypothesis

Answer 21

1) The acrosome is present in all mammalian species. The acrosome reaction is an event necessary for the penetration of the zona pellucida in all the cases studied
2) Sperm acrosomes are loaded with powerful zona-hydrolyzing enzymes

Question 22

Acrosomal reaction

Answer 22

Actually occurs before contact with ZP

Question 23

Fertilization

Answer 23

• Once released from the follicle or the epididymis tail, the egg and the sperm will perish in minutes/hrs/days depending on the species. If they find each other and fuse, fertilization & initiate embryo development
• Egg then activated & begins developmental
  program, the gametes nuclei come together to form the
  genome of the new organism
• Of the 100 million human sperm ejaculated during coitus, about 200 reach the site of fertilization in the oviduct. Once there, the sperm migrate through the shell of follicular cells surrounding the ovulated egg and then bind to and traverse the ZP. Finally, one binds and fuses with the egg’s plasma membrane

Question 24

Steps of fertilization

Answer 24

Fusion of sperm & plasma membranes
Increase in Calcium inside the egg
Cortical granule exocytosis
Cyclin destruction & inactivation of cdk
Block polyspermy: plasma membrane block & ZP block
Sperm chromosome decondensation
Male & female pronuclear formation
Syngamy & first mitosis

Question 25

Sperm-Egg fusion

Answer 25

Sperm reaches perivitelline space Sperm head binds to egg plasma membrane: oolema Whole spermatozoon is incorporated into egg cyptoplasm

Question 26

Manner in which sperm is incorporated (in mammals)

Answer 26

Sperm head’s (equatorial segment plasma membrane) fuses first with the oolema Posterior region of the sperm head and the tail are subsequently incorporated via membrane fusion. Anterior region of the head with the inner acrosomal membrane exposed is engulfed by the egg in a phagocytic manner

Question 27

IZUMO1 and JUNO

Answer 27

Proteins that facilitate binding of sperm and egg (respectively) at plasma membrane interface non-fusogenic

Question 28

Egg Activation

Answer 28

After fusion with sperm, egg undergoes series of morphological and biochemical events that lead to cell division and differentiation Rise in intracellular calcium > cortical granule exocytosis & resumption of meiosis

Question 29

Corticol reaction

Answer 29

Cortical granules are formed from the Golgi complex and migrate to the periphery of the egg Cortical granule exocytosis begins 2 min after sperm-egg fusion and the majority (more than 95 %) of the cortical granules are exocytosed in less than 5 minutes Regulated by calcium

Question 30

2 mechanisms to block polyspermy

Answer 30

Zona reaction Plasma membrane block to polyspermy

Question 31

Plasma membrane block to polyspermy

Answer 31

The incorporation of the cortical granule membrane to the oolema change the property of the egg plasma membrane and block other sperm to fuse with the egg

Question 32

Zona reaction

Answer 32

The cortical granules release enzymes that modify the zona pellucida. This modification are important to avoid the binding of other sperm.

Question 33

Resumption of meiosis II

Answer 33

MPF is a complex formed by CDK (cyclin-dependent kinase) and cyclins. It is high during the metaphase II arrest, as soon as the egg is fertilized the activity of MPF declines. The inactivation of MPF regulates the formation of the male and the female pronucleus

Question 34

The first visible change to occur in the sperm nucleus after its incorporation into the egg

Answer 34

Breakdown of nuclear envelope

Question 35

Decondensation of the sperm nucleus

Answer 35

After contact with the ooplasma (egg cytosol), the nucleus begin to decondense

Question 36

What follows decondensation

Answer 36

The replacement of protamines by histones as the main protein to bind the sperm DNA

Question 37

What structure forms the female pronucleus

Answer 37

After release of the polar body the egg nucleus is haploid

Question 38

What structure forms the male pronucleus

Answer 38

sperm nucleus after decondensation

Question 39

What is syngamy

Answer 39

The coming together of the sperm and egg pronuclei following mammalian fertilization

Question 40

Steps of syngamy

Answer 40

Aposition of the pronuclei Interdigitation of proximal surfaces of pronuclei Complex folding of proximal surface of pronuclei Vesiculation of pronuclear envelopes and formation of mitotic spindle First metaphase mitotic apparatus

Question 41

Induced (artificial) egg activation

Answer 41

Mammalian eggs can be activated by a wide variety of physical and chemical stimuli Most of these activating stimuli cause a rise in intracellular calcium concentration

Question 42

Parthenogenic activation

Answer 42

Activation of the egg without the sperm Eggs parthenogenically activated could develop into fetuses but they don’t arrive to term Happens very frequently in rats

Question 43

Species that frequently undergo parthenogenesis

Answer 43

Insects Komodo dragons