Fertilization Flashcards
Pre-Fertilization Processes
- Meiosis I & Meiosis II
- Capacitation - happens inside female reproductive tract
(IVF uses capacitating media) - Migration
In oogenesis, meiosis is (slow, fast). In spermatogenesis, meiosis is (slow, fast).
slow; fast
Parts of an Ovum
- corona radiata (HA)
- zona pellucida (glycoproteins)
- plasma membrane
- germinal vesicle (nucleus)
- germinal spot (nucleolus)
- ooplasm
Mythological Gametic Models:
- Sperm as Suitors: The egg chooses which sperm she likes
- Egg as sleeping beauty: sperm kiss/activates egg
- Founder myth: hero sperm and princess egg
- Conquering Warrior: sperm wars and warriors
Truth about fertilization:
- Sperm capacitation
- Egg activates sperm
- Egg spoon feeds sperm (in direction)
- Sperm is hyperactivated
- Eggs not passive
- Sperm do not race
- Reproductive tract is not passive
Freshly released sperms are not capable of fertilizing eggs. They still need to mature through a process called _______
capacitation
Where does capacitation happen?
Female reproductive tract
Before Capacitation: the membrane of the sperm is rigid, and won’t fuse with the egg
cell membrane
After Capacitation: the membrane is more fluid, and can now fuse
What can capacitation do?
- It can change the flow of calcium ions into the sperm, for the sperm to move quickly, and to undergo the acrosome reaction
- Equips the sperm head with molecules that can better sense the presence of the egg and follow the signal to the egg
- Adenylyl cyclase enzyme activates protein kinase A, which enhances sperm movement
- Cholesterol removal from the sperm membrane further activates ion channels, boosting motility and fertilization capability
Uterus: capacitation initiated
Oviduct: capacitation completed
hyperactive motility
Site of fertilization
Ampulla
What causes the migration of oocytes
Contraction of fimbriae
Stages of fertilization
- Binding of sperm to zona pellucida
- Acrosome reaction
- Penetration through zona pellucida
- Fusion of plasma membranes
- Sperm nucleus enters egg cytoplasm
Four Major Events of Fertilization
- Contact and recognition between sperm and egg
The chemoattraction of the sperm to the egg by the soluble molecules secreted by the egg
The exocytosis of the acrosomal vesicle to release its enzymes
The binding of the sperm to the extracellular envelope of the egg
The passage of the sperm through the extracellular envelope
Fusion of the egg and sperm cell membranes
- Regulation of sperm entry into the egg
Species-specific recognition
Presence of sperm “bindin” in sea urchin; “fertilin” in mammals; egg CD9
Prevents polyspermy
- Fusion of genetic material from the two gametes
- Activation of egg metabolism to start development
Another outcome of the sharp rise in Ca2+in the egg’s cytosol :
increase in the rates of cellular respiration
protein synthesis by the egg cell
Upon contact and fusion of the gamete plasma membranes, the membrane becomes depolarized, resulting in the ___________
fast block to polyspermy
Essential Proteins for Sperm-Egg Interaction
Sperm-zona binding
ADAM3 (fertilin in mammals) (bindin in sea urchin)
ZP proteins
Sperm-egg fusion
CD9
Juno
IZUMO1
Mechanism of egg sperm interaction
- Zona binding
Sperm: ADAM3 - Acrosome reaction
- Penetration
Fusion
Sperm: IZUMO1
Egg: CD9
Fusion of the gamete membranes triggers an increase of Ca2+ in the egg’s cytosol, causing cortical granules in the egg to fuse with the plasma membrane and discharge their contents, and hardening of the vitelline layer——FERTILIZATION ENVELOPE
Oocyte membrane becomes impenetrable to other spermatozoon
Membrane block
ZP alters its structure and composition to prevent sperm binding and penetration
Zona reaction
Possible mechanism of mammalian zona pellucida (ZP) hardening
Compaction of the supramolecular structure of the egg coat
sperm-derived genome and egg derived genome may be functionally different and play complementary roles during certain stages of development.
Genomic imprinting
Fertilization in humans is completed within _____________ of ovulation
24 hours
Approximately 400 to 600 MILLION sperms are deposited at cervical opening during ejaculation.
Only about 200 sperms reach the fertilization site; most degenerate and are absorbed by the female genital tract
Human fertilization
Fertilization @ 20 mins
Fertilization @ 28 mins = spermatozoa penetrates zona pellucida
Fertilization @ 31 mins = spermatozoa penetrates oocyte
Clinical Correlates: Twinning
- A fertilized, single egg splits into two developing zygotes at a very early stage.
- Identical twins; same sex.
Monozygotic (monoovular)
Clinical Correlates: Twinning
- Result from the fertilization by two sperm of two separate ova that have reached maturation at the same time.
- Not identical twins; can be different sexes
- Incidence increases with the age of the mother
Dizygotic (polyovular)
Clinical Correlates: Treatment of Infertility by IVF and ET
*ART/MAR/MAP
*GIFT (gamete intrafallopian transfer)
*ZIFT (zygote intrafallopian transfer)
*ICSI (intracytoplasmic sperm injection)
Fast block = _______________
Slow block = __________
Change in membrane permeability
Hardening of vitelline membrane
in vitro fertilization and embryo transfer procedure in humans
- Hormonal stimulation of egg maturation
- Removal of eggs by laparoscopy
- Collection of sperm sample and concentration of most active sperm
- In vitro fertilization
- Early cleavage in vitro
- Extra embryos frozen
- Reimplantation of up to 3 embryos
Results of Fertilization
✓ Completion of meiosis II
✓ Restoration of diploidy
✓ Chromosomal sex determination
✓ The embryo contains only maternal mitochondria
✓ Genetic and evolutionary implications (variation)
✓ Initiation of cleavage
Fertilization in Flowering Plants
A. Microspore mother cell -> microspores -> vacuolization -> (Generative cell & vegetative cell) 1st asymmetric mitotic division -> 2nd mitotic division and pollen tube growth
B. Megaspore mother cell -> female meiosis -> 8-nucleate stage cell -> cellularization -> nuclei migration
two small cells in a flowering plant’s embryo sac that help guide pollen tubes to the egg
synergids
After gamete fusion (___________) the migration of gamete nuclei is realized by ______ in animals and _________ in plants
plasmogamy; microtubules; actin
micropyle function
restrict the entry site of sperm to egg
