3b: egg and sperm cells general features and 3c Flashcards
what are the two fertilization functions
- fusion of genentic material
- activation of egg metabolism and initiation of development (fertilization triggers the beginning of emrbyononic development
Spermatogenesis
-sperm are highly specialized cells
-lose their cytoplasm
typical sperm structure
-haploid nucleus
-dna tightly compacted by protamines which replace histones
-nucleus in head of sperm
-acromosome infront of nucleus, derived from golgi and contains enzymes and sugars
-sometime between there is globular actin
-mitochondria before tail (flagellum)
-flagellum core 9+2 microtubule structure of axoneme is conxserved
exceptions: nematodes have aflagellate sperm (no tail)
Protamines
small arginine rich nuclear proteins that replace histones in sperm and allow for denser packing of dna in sperm head
Axoneme structure of flagellum
has dynein which attaches to axoneme for propelling of sperm action
-it uses atp to promote the sliding of the microtubules, causing flagellum to bend and move as waves of dynesins bend it along the length(read slide 31)
Egg structure
-ovum(egg)-female gamete capable of being fertilized
-oocyte: developing egg that cannot bind sperm or be fertilized yet
Enormous cells:
hsbvbe proteins
ribosomes
mrna
morphogenic factors
protective factors
-large nuclues
-at fertilization might still be diploid, in some stage of meiotic metaphase or already haploid pronucleus
-extracellular matrix (involved in sperm egg recognition, mat around egg
-egg cell membrane: fuse wtih sperm membrane , regulations of ions across
-beneath cell mem: cortex- enriched in globular actin and polymerize actin microfilaments which protude from egg surface
what stage is the egg at the time of fertiliztation for mammals
second meotic metaphase arrest
what is the difference between the extracellular matrix of invertebrates and mammalians
Invertebrates:
thin layer caleld vitelline envelope
-many have glycoprotein meshwork surrounding this which attracts sperm
Mammalia: thick layer called zona pellucida
-surrounded by CUMULUS, a layer of cells that nurture the egg at the tine of release from ovary
-innermost cumuls cells are called corona radiata
What are the substages of meiotic 1 prophase
1) leptonene: condensed chromosomes are seen
2) zygotene: homologous chromoseoms pair up and synaptonomeal complex gorms between homologous chrom
30 pachytene : recombination, crossing over occurs
4) diplotene: sister chromatids seperate and synaptonomeal complex breaks
5) diakenesis: further seperation of sister and condensing occurs but still together at chiasma (since this seperation occurs in meiosis 2)
what is homologous onthe egg tothe acromosome vesicle of sperm
cortical granules
General interaction of egg and sperm
1) sperm attracted to egg due to chemoattraction (molecules secreted by egg)
2) exocytosis of sperm acromosome (releasing enzymes)
3)sperm binds to ecm (either vitiline envelope or zone pellucida)
4) sperm makes its way thru ecm
5) egg and sperm membranes fuse
2 and 3 could exchnge
why are sea urchins good models for developmental biology
-sexually mature entire year
-can produce large transparent eggs in considerable quantities
Chemotaxis, chemoattractants
Chemotaxis: directed movement of a cell in response to a chemical gradient such as sperm following chemical secreted by egg,, from egg jelly
Chemoattractants are highly species specific- pepties released from on have receptors on other
-also called sperm-activating peptides since the chemoattracants activate the sperm to produce more atp to move more
what is resact
14 AA peptide released from egg jelly of sea urchin, only works on sperm of the same species, only they have te receptor for it
What is the acrosomal reaction in sea urchins
1) acrosomal mem and sperm call mem fuse (results ine exocysoe of acrosome content)
2) leads to extension of acrosomal process
-triggered by contact with egg jelly
-acrosomal enzmes released digest path through jelly layer
-once thorugh the jelly layer, acrosomal process adheres to vitiline envelope and can digest it where attached
what initiates the acrosomal reacton
-the receptors on sperm contact SPECIES SPECIFIC SULFATED POLYSACCHARIDES on egg jelly coat, iniates it
what on the egg jelly of egg is species specific highly
the sulfated polysaccharides
-binding of thisto the receptors on the sperm membrane leads to acrosomal reaction by activating other sperm membrane proteins
What are the species specfic reactions that happens on egg jelly with sperm
1) small peptides released from egg jelly lead to chemoattraction and sperm activation
2) sulfated polysaccharides on egg jelly can bind to sperm receptors and lead to acrosomal reaction
3)Acrosomal protein called BINDIN binds to dejellied eggs (eggs after acrosomal reaction and sperm digesting jelly) and will clump together
-also species specific
need specfici receptors for akk of these reactions
Bindin receptors
in vitiline envelope
-limit the number of sperm that can bind to the vitiline envelope
-not highly concentrated on envelope, so there is still exposed area when sperm binds
EBR1 glycoprotein is a bindin receptor
what are the species specific reactions simplified
1) find the egg (chemoattractins allow sperm to find and be activated)
2)penetrate coat (sulfated polysacharides allow sperm to bind and digest jelly coat)
3)adhere to viteline envelope (bindin ie EBR1 glycoprotein receptor allow sperm to bind to envelkpe)
After the three species specific stesp what happens
sperm and egg membrane must FUSE and form fertilization cone (strucutre rich in filamentous actin)
-requires specific proteins like binidin bc hard to fuse two lipid bilayers
-can occur anywhere on egg membrane surface for sea urchins
what is the two functions of bindin
1) attachment of sperm to vitiline envelope
2) fusion of sperm adn egg membrane due to the long stretch of hydrophobic AA’s near terminus called fusogenic region
How do we know bindin is necessary for fertilization
when we used cas9-mediated inactivation of bindin gene, it showed that in a sea urhcin a perfectly normal looking embryo BUT the bindin lacking sperm never fertilized the egg
What happens if more than one sperm enters the egg
polyspermy: more than one enters
leads to triploidy (three sets of chromosomes) then have two sets of spindles seperating three pairs of chromosomes, PROBLEM
-triploidy itself is not lethal but can cause lots of problems
How does egg prevent polyspermy
PREVENTS ENTRY OF SECOND
1) Fast block: electrical change of egg membrane
before sperm: lots of NA outside, less inside, has resting membrane potential of -70
after binding of sperm mem: shifts to +20mv whcih prevents other sperm bc fusogenic region of bindin doesnt work on pos charged surface
-occurs in sea urchins not mamals
-only lasts one min so sperm needs to be removed from surface
2) slow block:
-sperm entry causes cortical granules of egg to fuse with egg membrane releasing the contents and proteins called CORTICAL GRANULE SERINE PROTEASE
-those proteins cleave protein posts that connect viteline envelope to egg membrane
-mucopolysaccharides push away the viteline envelope as well and absorb water to move it away from egg membrane
-protein cross link enzymes stabilize the fertilization membrane
-hyalin forms coat around the egg, egg ectends long microvilli into hyaline layer so it provides structural support
What is apart of the cortical granules that is released after fast block
cortinal granules contain:
-cortical granule serine protease
-mucopolysaccharides
-portein crosslinking enzymes (TP and OVOP)
-hyaline
How does the coritcal granule reaction occur
1) Ca conc increases in egg cytoplasm after fertilization: brief entry of CA from outside to inside
2)calcium wave
3) ca increase triggers the cortical granule membrane fusion
calcium ionophore triggers increase
where does Ca come from
calcium stored in endoplasmic reticulum
How are calcium channels opened to be releaed to trigger cortical granule fusion
opened by IP3 (inositol 1,4,5-triphosphate )
Where does it come from:
PIP2
Membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) is substrate of phospholipase C (PLC) yielding products IP3 and diacylglycerol (DAG).
