3. fertilization Flashcards
1
Q
fertilization
A
- fusion of egg and sperm
- biological process at the basis of sexual reproduction
- restores diploid chromosomes
- underlies genetic determination of the sex of embryo
- triggers embryonic development (beginning of segmetation)
- occurs in the ampullary (distal) portion of fallopian tube
2
Q
timeline
A
- 24 h
1. passage of sperm through corona radiata
2. penetration of the zona pellucida
3. fusion of cell membranes
4. completion of second meiotic division of oocyte
5. formation of pronuclei
6. fusion of the pronuclei and formation of the zygote
3
Q
seminal fluid: maturation of spermatozoa
A
- acquisition of motility and fertilisation competence in epididymis (days)
- formation of semen
- capacitation (activation): conditioning the sperm membrane in female genital tract
- ejaculation: 200 million sperm deposited in vagina
- chemotaxis in tubes
- sperm storage site
- pH of semen protects from acidity of vagina
4
Q
capacitation
A
- vaginal secretions cause molecular changes in plasmalemma
- removal of decapacitating factors results in increased fluidity
- 4-5 hours
- sperm used for IVF must be artificially capacitated (incubation with progesterone)
- increased metabolic rate
- flagellum beats more rapidly: more motile
- plasmolemma changes allow sperm-egg binding
- motility change (not morphology): hyperactivation
5
Q
thermotaxis and chemotaxis
A
- drive sperm migration
- thermotaxis in tube until egg
- chemoattractants: progesterone, heparin, N-formyl peptides, chemokines…
6
Q
spermatozoa movement along tube
A
- reduce in number
- active motility
- contractions of smooth muscle of female genital tract
- most sperm never reach egg (only 20-200)
7
Q
viability of gametes
A
- determines fertile window
- ovulated oocyte 24h
- spermatozoa up to 120 h
7
Q
structure surrounding oocyte
A
- follicle cells: corona radiata
- zona pellucida
- oolemma
8
Q
interaction between sperm and cumulus cells
A
- sperm releases NYP-SP8 which binds to cumulus surface
- intracellular calcium in cumulus leads to increased progesterone release
- prog triggers acrosome reaction (releases hylauronidase and etc) to facilitate sperm penetration
- cumulus may assist AR by blocking egg movement
9
Q
binding to and getting through zona pellucida
A
- ZP3 receptor present on sperm head
- ZP3 activated AR
- aerosin and neuroamidase enzymes released dissolve ZP
10
Q
initial attachment between sperm and oocyte oolemma
A
- AR also exposes proteins on sperm surface that can bind to egg
- adhesion molecules
- ADAMs family of transmembrane proteins belong to zinc protease superfamily and have multidomain structure
- integrins, cadherins
- fusion of plasma membranes
- following membrane fusion the sperm nucleus sinks into the oocyte cytoplasm
11
Q
oocyte activation
A
- occurs in response to fusion with sperm
- frees oocyte from constraints inhibiting development
- does not require sperm nucleus
- can be activated by touching with a needle or changing pH
- triggers completion of meiosis
12
Q
changes in egg membrane at fertilisation that block polyspermy
A
- fast block: inversion of resting potential of oolemma from -70 to +20
- slow block: increase in Ca2+ triggers exocytosis of enzymes contained in cortical granules which harden the zona pellucida
13
Q
formation of pronuclei
A
- male nucleus enters egg cytoplasm: loses nuclear membrane and decondenses: male pronuclei
- sperm head provides centrioles for the formation of new microtubules
- Ca2+ release: oocyte nucleus completes division giving rise to another polar body (2-4h) and becomes female pronucleus
- pronuclei move toward each other, nuclear membranes dissolve
- first DNA replication :) and cell division
14
Q
what is a zygote
A
a cell formed by the union of two gametes
