Chapter 7 - Fertilization

meiosis

what process produces gametes

the oocyte and sperm fuse together to form the zygote, giving a combination of genes from both parents

what occurs during fertilization

sea urchins → external fertilization

mammals → internal fertilization

what type of fertilization do sea urchins and mammals use

1) Sex → combine and pass on genes derived from 2 parents

2) Reproduction → generate a new organism

what are the 2 functions of fertilization

1) movement of the gametes toward one another

2) sperm and egg contact and recognize one another

3) fusion of sperm and egg membranes

4) regulation of sperm entry into egg

5) genetic material fuses

6) activation of egg metabolism

what are the 6 major events of fertilization

quiescent state

before fertilization, what type of state is the egg in

1) their egg and sperm are present in high numbers

2) their eggs are transparent

3) they undergo external fertilization

4) it’s very similar to mammalian fertilization

what are 4 reasons why we study sea urchins for fertilization

morphogenesis

during gametogenesis, what process occurs to get mature sperm

it is a packet of enzymes that are important in the sperm making it to the egg → front of sperm

what is the acrosomal vesicle in sperm

they are before the tail to provide energy needed to allow the tail to move the sperm

where are mitochondria located in sperm and why

some sort of propulsion needed to move the sperm

what is the tail of the sperm

the nucleus present in the sperm and egg before they have come together (haploid)

what is a pronucleus

egg cell membrane → vitelline envelope → jelly coat

what are the 3 layers of the sea urchin egg in order from inside to outside

ECM that surrounds the sea urchin egg and is important in sperm-egg recognition

what is the vitelline envelope

ECM molecules (different than those in the vitelline envelope) that are also important in sperm-cell recognition

what is the jelly coat

1) nutritive proteins

2) ribosomes

3) tRNAs

4) mRNAs

5) morphogenetic factors

6) protective chemicals (protect against environmental factors like UV)

what 6 things do the eggs contain that are needed for immediate development after fertilization

part of the sea urchin egg that contains enzymes important in preventing polyspermy

what is the cortical granule

cumulus → a whole bunch of cells (follicle cells) surrounding the egg that help support it

what do mammalian eggs have instead of the jelly coat and what is it

zona pellucida → still composed of ECM molecules

what do mammalian eggs have instead of the vitelline envelope

1) sperm contacts jelly layer (initiation next step)

2) acrosome reaction → sperm releases # of enzymes that allows it to digest the thick jelly layer

3) digestion of jelly layer

4) binding to vitelline envelope → acrosomal process was put out after acrosome reaction which can bind to vitelline envelope

5) fusion of acrosomal process membrane and egg membrane

summary of the 5 main events leading to the fusion of sperm and egg cell membranes in sea urchin fertilization

1) species-specific sperm attraction → chemical released by egg and sperm recognizes it

2) species-specific sperm activation → sperm only get activated by eggs of the same species

how are the problems of sea urchin gametes meeting and preventing cross-species mating addressed

the sperm moving towards specific chemicals released by the egg

what is sperm chemotaxis

a sperm-activating peptide secreted by eggs

what is resact

over time the sperm aggregated in the location where it was released

when a pipette released resact in a container filled with sperm what occurred

the ECM of the jelly coat is composed of different polysaccharides in different species, so acrosome reaction only occurs when jelly coat is from the same species as the sperm is

species-specific induction of the acrosome reaction is done through what in sea urchins

enzymes are released from the acrosome resulting in the extension of the acrosomal process which is covered in bindin

what occurs when sperm connects to the jelly coat

used immunohistochemistry and antibodies to label it

how did we determine bindin was located all over the acrosomal process

the binding of it to the vitelline envelope of the egg

what does the bindin on the acrosomal process allow for

only egg and bindin that were from the same species stuck together

when mixing different bindins with eggs that already had the jelly coat removed what occurred

1) sperm attraction and activation (sperm activating polypeptide)

2) acrosome reaction (polysaccharides in egg coat)

3) sperm attaching to egg surface (bindin)

what are the 3 levels of species specificity for fertilization

shape the egg makes to surround the sperm and pull it inside the egg

what is the fertilization cone

the electrocharge across membrane that is due to differences in ions outside the egg vs inside the egg

what is membrane potential

when the sperm binds to the egg Na+ rushes into the cell causing the membrane potential to jump from -70 mV to 20-25 mV → sperm can’t bind to eggs with a positive membrane potential, blocking addition sperm from binding

explain the fast block to polyspermy

they changed the concentration of Na+ the egg is sitting in → noted that at lower concentrations of Na+, polyspermy occurred in larger % of eggs because not enough Na+ present to change the membrane potential to positive

how did they prove the fast block to polyspermy

cortical granule reaction → involves them releasing their enzymes which leads to a fertilization envelope that is essentially the vitelline envelope modified slightly and pulled away from the egg → this is a physical block that prevents sperm from entering the egg

explain the slow block to polyspermy

proteins and enzymes inside the granules induce proteases that cleave away any sperm that’s hanging onto outside → as they are doing this, it causes the envelope to be lifted

what causes the envelope to be lifted during the slow block to polyspermy

initially extracellular Ca2+ comes in when Na+ does, causing it to be everywhere in the egg then it leaves or is stored → where sperm entered a Ca2+ wave starts from intracellular Ca2+ released from the ER → this wave moves down the cell and the cortical granules release their components as it does so

what initiates the cortical granule reaction that causes the slow block to polyspermy

fast block → within 3 seconds

slow block → 15-60 seconds

how quick after sperm-egg binding does fast block and slow block occur

1-2 minutes

how quick after sperm-egg binding does sperm enter the egg

within 2 hours

how quick after sperm-egg binding does it take for 1st division to occur

egg activation and metabolism for 1st division to occur → protein synthesis, DNA replication, and cytoplasmic movements of morphogenetic material

in addition to slow block to polyspermy, what is Ca2+ also responsible for

internuclear bridge

what forms between egg and sperm pronuclei

microtubules radiating around them

what is important in sperm and egg pronuclei coming together

ovaries are located in the end of the oviduct and during ovulation an egg is released from them and swept into the oviduct/fallopian tube by fimbriae

in mammalian fertilization how does the egg get to where it needs to be prior to fertilization by the sperm

very low down in the oviduct

where does fertilization occur in mammals

2 cell-stage → morula → blastocyst → implantation of egg on posterior side of uterus

after mammalian fertilization what are the stages of the egg

5-7 days

how long does it take for fertilized egg to be implanted during mammalian fertilization

cumulus

what part of the mammalian egg is critical for movement of it into the oviduct

translocation

what is the process of the gametes moving into the oviduct known as

it undergoes final maturations once in oviduct which are necessary for the sperm to be capable of fertilizing the egg

what is capacitation of the sperm and when is this (mammals)

1) recognize cues to be guided to egg

2) can undergo acrosome reaction

3) can fuse with egg cell membrane

what 3 factors are required for capacitation of sperm in mammals

muscular contractions

what is important in getting sperm in the oviduct

1) hyperactivation → flagellum can now beat with a ton of force, so sperm can move much faster

2) thermotaxis → sperm has sensors that can detect the heat the egg gives off to move to it

3) chemotaxis → egg releases progesterone that attracts the sperm to it

when the sperm is close to the egg in mammals what 3 things occur to allow for them to bind

it allows a rapid influx of Ca2+ which generates hyperactivated sperm motility

what does progesterone binding to CatSper channel on sperm flagella do

Zona Pellucida (similar to vitelline envelope in sea urchins)

after acrosome reaction, where does sperm recognize egg in mammals (what part of egg)

recognition between proteins on sperm head and receptors on egg

how do mammalian sperm and egg bind to each other

the side of the sperm head fuses with egg, then the sperm gets pulled in from the side

in what orientation does sperm fuse with egg in mammals

only slow ones are known of

what type(s) of blocks to polyspermy do mammals have (fast/slow)

1) cortical granules release their material which modifies the zona pellucida by removing the ZP2 protein so that it can no longer bind sperm

2) when sperm enter, zinc ions are released (zinc spark) which inhibit acrosomal enzymes and therefore any acrosome reactions

what are the slow blocks to polyspermy in mammals