Exam 3: Ovulation & Fertilization Flashcards
What is ovulation
the timed release of a mature, developmentally
competent oocyte (egg) from the ovary into the oviduct, where fertilization occurs
Properties of ovulation
highly coordinated process
many genes related to ovulation are related to oocyte competence
Time of ovulation
After LH surge
Rabbit, mouse, rat: 12 hr
Cow & Sow: 40 hr
Sheep: 25 ht
Woman: 28-36 hr
Site of ovulation
Most species: random
Mare: ovulatory fossa
Monkey: alternate ovaries
What is the stigma
Location of rupture of follicular wall
Thinnest part of wall
Ovulation rate
Number of eggs fertilized per pregnancy
Ovarian events caused by preovulatory LH surge
Histamine release
Increase PGF2a
shift from E2 to P4 by dominant follicle (first)
Surface epithelium
Gap junction breakdown between granulosal cells & oocyte (second)
Result of histamine release
Increased follicular pressure
Ovulation
Result of increased PGF2a
Increased follicular pressure
Follicle wall weakening
Ovulation
Result of shift from E2 to P4 by dominant follicle
Follicle wall weakening
Ovulation
Result of surface epithelium
Follicle wall weakening
Ovulation
Result of gap junction breakdown between granulosal cells & oocyte
Fertilization
Steps that proceed ovulation
cumulus expansion
proteolysis
angiogenesis
inflammation
smooth muscle contraction
Post-capacitation events leading to fertilization
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
Binding to zona pellucida
Sperm contain specific proteins on their plasma
membrane surfaces overlying the acrosome that
bind specifically to zona pellucida proteins
What makes up the zona pellucida
Three glycoproteins: ZP1, ZP2, ZP3 (humans have ZP4 as well)
Provide structural integrity to zona pellucida
Mechanisms used by the sperm to enter the zona
Mechanical hypothesis
Enzymatic hypothesis
Mechanical hypothesis
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.
3 Reasons to support mechanical hypothesis
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
Enzymatic hypothesis
Enzymatic hypothesis: Unlike the above mechanical hypothesis, every step of sperm-zona interaction is enzyme-dependent. Sperm motility is of secondary
importance.
2 reasons to support enzymatic hypothesis
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
Acrosomal reaction
Actually occurs before contact with ZP
Fertilization
- 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
Steps of fertilization
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
Sperm-Egg fusion
Sperm reaches perivitelline space
Sperm head binds to egg plasma membrane: oolema
Whole spermatozoon is incorporated into egg cyptoplasm
Manner in which sperm is incorporated (in mammals)
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
IZUMO1 and JUNO
Proteins that facilitate binding of sperm and egg (respectively) at plasma membrane interface
non-fusogenic
Egg Activation
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
Corticol reaction
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
2 mechanisms to block polyspermy
Zona reaction
Plasma membrane block to polyspermy
Plasma membrane block to polyspermy
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
Zona reaction
The cortical granules release enzymes that modify the zona pellucida. This modification are important to avoid the binding of other sperm.
Resumption of meiosis II
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
The first visible change to occur in the sperm nucleus after its incorporation into the egg
Breakdown of nuclear envelope
Decondensation of the sperm nucleus
After contact with the ooplasma (egg cytosol), the nucleus begin to decondense
What follows decondensation
The replacement of protamines by histones as the
main protein to bind the sperm DNA
What structure forms the female pronucleus
After release of the polar body the egg nucleus is haploid
What structure forms the male pronucleus
sperm nucleus after decondensation
What is syngamy
The coming together of the sperm and egg pronuclei following mammalian fertilization
Steps of syngamy
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
Induced (artificial) egg activation
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
Parthenogenic activation
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
Species that frequently undergo parthenogenesis
Insects
Komodo dragons
