Fertilization

Question 1

where does fertilization occur

Answer 1

in the ampulla region of the oviduct adjacent to the ampullary-isthmic junction

Question 2

at ovulation, what stage is the oocyte in

Answer 2

meiosis has been re-arrested at metaphase II

Question 3

what is the perivitelline space

Answer 3

space between vitelline membrane and zona pellucida

Question 4

what is the corona radiata

Answer 4

internal layer of cumulus cells radiating out from the zona pellucida

Question 5

what happens to cumulus

Answer 5

undergoes expansion –> changes from tightly packed cell mass to one with a looser association in which the cumulus cells are held together by a viscous hyaluronic acid matrix

Question 6

what is required for transport of oocyte after ovulation

Answer 6

• epithelial cell ciliary beating
• currents of oviductal fluid
• oocyte must be contained within its expanded cumulus matrix
• temporary adhesion between complex and apex of cilia on fimbria

Question 7

when does the sperm acquire motility and potential for egg binding/fertilization

Answer 7

during maturation in the epididymis (but can’t fertilize until later)

Question 8

what is capacitation

Answer 8

the maturational changes that sperm undergo in the female tract (capacity to bind and fertilize the egg) –> modification of sperm plasma membrane and associated molecules –> exposure of surface receptors involved in egg recognition, increases fusogeniticy of membrane, alters mechanisms of calcium homeostasis

Question 9

what does capacitation involve (5)

Answer 9

• removal of previously applied decapacitation factors
• lowering of cholesterol/phosopholipid ratio of the sperm plasma membrane
• hyperpolarization of the membrane resting potential in the region overlying the acrosome
• aggregation of lipid microdomains of the plasma membrane that contain zona pellucida-binding protein complexes
• phosphorylation of surface tyrpsine residues

Question 10

what does the loss of cholesterol in capacitation do

Answer 10

• allows increased intracellular pH, which increases sensitivity of sperm for induction of acrosome reaction
• may also thin plasma membrane, exposing receptors involved in sperm-egg interaction

Question 11

what is capacitation mediated by changes in

Answer 11

• hormones (increasing progesterone/prostaglandin from lutenizing follicle)
• contents of oviductal lumen from ovulated follicular fluid
• increasing oviductal fluid bicarb concentration

Question 12

hyperactivated motility

Answer 12

• at the same time that sperm become capacitated, they develop a hyperactivated motility
• increased amplitude of tail movement
• reversible (can go back to activated/progressive motility)
• role in detaching sperm from the oviductal reservoir

Question 13

first theory about whether a female controls the timing of sperm capacitation to coincide with ovulation

Answer 13

• events surrounding ovulation signal oviductal epithelium in sperm storage area –> secretes something that triggers capacitation and hypermotility –> decreases binding affinity for oviductal epithelium –> hyperactivated motility detaches it so it can move to site of fertilization
• tl;dr: female controls capacitation so that it is closely timed with the arrival of the oocyte

Question 14

second theory about whether a female controls the timing of sperm capacitation to coincide with ovulation

Answer 14

• during estrus there is a fairly constant release of sperm from storage site to ampullary-isthmic junction (fertilization area)
• sperm become capacitated by changing conditions as they move up the oviduct
• if oocyte isn’t there in a reasonable time, these sperm die and are replaced by more
• ensures that there are always a population of capacitated sperm present when the oocyte arrives
• minor role for female control of capacitation timing only

Question 15

what has longer life cycle - oocyte or sperm

Answer 15

sperm

Question 16

once cumulus-oocyte complex enters ampulla, what happens

Answer 16

firmly adheres to oviduct epithelium and remains attached until after fertilization

Question 17

why is hyperactivated pattern of motility following capacitation important

Answer 17

important in ability of sperm to detect and hone in on the oocyte –> hyperactivated sperm are better able to swim through the mucus found in the isthmus of the oviduct, turn and swim out of crypts in oviductal wall, and have an effective search pattern within confines of the oviduct

Question 18

how sperm find oocytes (non-mammals)

Answer 18

• sperm chemotaxis –> products of the egg serve to attract the sperm
• most species: small peptides/proteins, lipids –> control intracellular calcium levels
• increasing levels of attractant cause membrane hyperpolarization, preventing Ca increase in the sperm –> low Ca causes linear swimming, sperm continue up attractant concentration gradient to egg
• decreasing attractant levels activate Ca channels –> increase intracellular Ca –> thrashing motility that gives non-linear progression –> reorientation

Question 19

how sperm find oocytes (mammals)

Answer 19

• at longer distances, sperm display thermotaxis (swim up temp gradients)
• closer range: chemotactic responses
• follicular fluid is chemotactic for sperm
• products of oocyte/cumulus complex also act in short range, odorant receptors too

Question 20

penetration of oocyte vestments

Answer 20

-once sperm has located oocyte, needs to penetrate cumulus oophorous, zona pellucida, vitelline membrane

Question 21

penetration of cumulus oophorous

Answer 21

• only capacitated sperm, with hyperactivated motility reflex, are capable of penetrating cumulus cells and their hyaluronic matric
• hyaluronidase bound to sperm plasma membrane helps

Question 22

penetration of zona pellucida

Answer 22

• capacitated sperm binds to ZP –> undergoes acrosome reaction
• zona and zona-binding proteins are species-specific

Question 23

what is the acrosome reaction

Answer 23

• following capacitation in female tract, sperm are capable of undergoing the acrosome reaction
• involves fusion of the outer acrosomal membrane with the plasma membrane of the sperm
• results in exposure of the acrosomal content to the extracellular environment

Question 24

what 2 substances induce acrosome reaction

Answer 24

• protesterone: sperm selection (induce AR?), prime sperm

- zona pellucida: sperm binding to it causes AR

Question 25

zona pellucida and acrosome reaction

Answer 25

• ZP made up of 3 glycoproteins: ZP1, ZP2, ZP3
• ZP consists of pairs of ZP2 and ZP3 –> bound together in long chains, crosslinked by ZP1
• only acrosome-intact sperm bind to ZP

Question 26

what happens when sperm binds to zona pellucida

Answer 26

• sperm plasma membrane fuses at multiple points with outer acrosomal membrane causing membrane vesiculation
• acrosomal contents released, exposing the inner acrosomal membrant
• inner acrosomal membrane binds to ZP2 (secondary binding) –> orients sperm

Question 27

apart from penetration of ZP, why does AR occur?

Answer 27

• only acrosome reacted sperm are capable of binding to, and fusing with, oocyte plasma membrane (passing next barrier)
• sperm selection

Question 28

fusion with the vitelline membrane (oocyte plasma membrane)

Answer 28

• head of sperm initially contacts vitelline membrane end on and then reorients for lateral attachment
• sperm plasma membrane overlying equatorial region fuses with vitelline membrane
• vitelline membrane and ooplasm adjacent to this area surround sperm –> membrane fuses over sperm
• sperm incorporated into ooplasm
• fused membrane break down, releasing sperm nucleus into egg cytoplasm

Question 29

contributions of sperm to next generation

Answer 29

• paternal haploid genome
• signal to initiate egg metabolism/activation
• centrosome

Question 30

what does activation of the egg involve

Answer 30

increase in intracellular Ca levels

Question 31

how does sperm increase Ca in oocyte

Answer 31

• penetrating sperm induces soluble sperm-derived factor “oscillin” or “oscillogen” which enhances the Ca induced Ca release from intracellular stores
• oscillogen is sperm-specific phospholipase C

Question 32

resumption of oocyte meiosis

Answer 32

• second meiotic arrest of oocyte is maintained by cytostatic factor
• elevated Ca in oocyte destroys CSF –> meiosis continues

Question 33

formation of male and female pronuclei

Answer 33

• upon entering cytoplasm, sperm head swells and head/tail separate
• sperm nuclear membrane degenerates, exposing nuclear material to ooplasm
• protamines are removed, histones are bound –> decondensation of paternal chromatin
• new nuclear envelope formed –> male pronucleus
• following completion of second meiosis, female chromatin is decondensed and acquires a nuclear envelope –> female pronucleus

Question 34

pronuclear migration

Answer 34

• sperm centriole directs formation of sperm aster, a growing microtubule spindle-like structure
• growing structure pushes male pronucleus towards center of the cell
• when aster has grown enough to reach female pronucleus, it draws to the center adjacent to male pronucleus

Question 35

syngamy

Answer 35

• chromatin in each pronucelus duplicates and condenses, nuclear membrane breaks down
• syngamy as defined in invertebrates does not occur in mammals

Question 36

block to polyspermy

Answer 36

• limiting number of sperm at site of fertilization
• zona reaction/hardening to prevent penetration of further sperm after oocyte activation
• vitelline or membrane block - sperm unable to bind to or fuse with vitelline membrane

Question 37

how does triploidy occur

Answer 37

• failure of the block to polyspermy (single pronucleus fertilized by 2 male pronuclei)
• failure to extrude second polar body (2 female pronuclei, 1 male)