fertilization

what is fertilization?

sperm and eggs fuse to create a new individual whose genome is derived from both parents

major events of fertilization

1. contact and recognition between sperm and egg

2. regulation of sperm entry (no polyspermy)

3. fusion of genetic material

4. activation of egg metabolism

sperm anatomy

• nucleus with haploid cells and tightly compressed DNA

• propulsion

• acrosomal process with fingerlike projections that help in fertilization and aid in species recognition

• enzymes (acrosomal vesicle » acrosome)

sperm flagellum

• microtubules emanating from centriole at base of nucleus

• core: 2 microtubules, 9 almost doublets surround it (circle 13 pro filaments, c-shape 11 profilaments)

• profilaments made of dimeric tubulin

dynein

ATPase that propels sperm; sliding of outer doublet microtubules, causing flagellum to bend

egg components

• nutritive proteins: yolk proteins

• ribosomes and tRNA

• mRNA

• morphogenic factors » direct early differentiation

• protective chemicals

cell membrane + extracellular envelope of egg

• vitelline envelope: ECM, glycoproteins

• egg jelly (attract or activate sperm)

• cortex (just within cell membrane)

what are cortical granules?

• membrane bound, Golgi derived structures with proteolytic enzymes

• also contain mucopolysaccharides (prevent polyspermy) and hyaline (support for cleavage)

mammalian egg

extracellular envelope:

• zona pellucida (separate, thick matrix)

• cumulus (layer of cells that surround egg; made of ovarian follicular cells)

• corona radiata (innermost layer of cumulus cells)

how do sperm find the egg in external fertilization?

chemotaxis

• sperm-activating peptides (SAPs)

  • ex. resact, from egg jelly, diffuses, has strong effect at low concentrations » receptors in sperm cell membrane

what are the steps that resact goes through?

1. activates guanylyl cyclase

2. cyclic GMP production

3. calcium channel in cell membrane of sperm tail

4. increase in mitochondrial respiration

5. increase in dynein activity

6. attractant and activator

acrosome reaction: external fertilization

1. fusion of the acrosomal vesicle with the sperm cell membrane

   1. exocytosis that results in release of vesicle contents

   2. initiated by contact of sperm jelly polysaccharides which bind specific receptors

2. extension of acrosomal process

bindin

• found on acrosomal process, receptors on egg surface

• species specific (extremely specific)

• helps ensure species specific fertilization

how do you prevent polyspermy?

fast block: changing the electric potential of the egg cell membrane immediately upon the entry of a sperm from -70mv to +20mvv » influx of Na

slow block: cortical granule reaction reaction (mammals)

• release of cortical granule contents outside the egg; clips off bindin receptors and sperm with it and disconnects vitelline envelope from cell membrane

• cortical granules plus vitelline envelope = fertilization envelope

• starts 20 seconds after sperm entry and complete within one minute

cortical granule

mucopolysaccharide: viscous compounds that absorb water to expand the space between the cell membrane and the fertilization envelope, radial expansion

what is the initiator of the cortical granule reaction?

calcium:

fertilization » increase Ca from within egg (intracellular storage) » cortical granule membranes fuse with the egg cell membrane

activation of egg metabolism in sea urchins

• release of Ca

  • release the inhibitors of mRNA

  • release inhibition of cell division

• how is calcium released?

  • a soluble factor from sperm changes ionic composition of egg cytoplasm

  • sperm binds receptors on egg cell surface and signals activation