FERTILIZATION
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FERTILIZATION
Gametes: fuse to create a new organism sex: combining the genes of both
parents reproduction: creation of a new
organism
Steps.Contact and recognition between gametes. Regulation of sperm entry. Fusion of genetic material. Activation of egg
7.6 The sea urchin egg cell surface
• Enclosing the cytoplasm is the egg cell membrane which must be capable of fusing with the sperm cell membrane and must regulate the flow of certain ions during fertilization.
• Outside of the cell membrane, there is a fibrous mat involved in sperm-egg recognition. In nvertebrates: vitelline envelope.
7.7 Hamster eggs immediately before fertilization
• In mammals, the vitelline envelope is a separate and thick extracellular matrix called zona pelucida.
• The mammalian egg is also surrounded by a layer of cells called the cumulus.
• The inner most layer of cumulus cells is called corona radiata.
• Beneath cell membrane is the cortex (globular actin molecules and cortical granules).
7.9 Sperm chemotaxis in the sea urchin Arbacia punctulata (Part 1)
Resact is a 14 aa peptide that diffuses readily in seawater.
1 sec 20 sec 40 sec 90 sec
7.11 The acrosome reaction in sea urchin sperm
• The acrosome reaction has 2 components:
1) the fusion of the acrosomal vesicle with the sperm cell membrane: the contact causes the exocytosis of the sperm’s acrosomal vesicle and the release of proteolytic enzymes.
2) the extension of the acrosomal process: the actin molecules assemble to produce microfilaments extending the acrosomal process onwards.
7.17 Membrane potential of sea urchin eggs before and after fertilization
Fast block to polyspermy- change in the resting potential of the egg cell membrane.
Egg cell membrane increase the Na permeability.
Sperm cannot fuse with membranes having a positive resting potential.
7.18 Formation of the fertilization envelope and removal of excess sperm
Slow block of polyspermy- the cortical granules remove sperm
-Cortical granule serine protease: dissolves the protein posts that connect the vitelline envelope proteins to the cell membrane, and if clips off the bindin receptors and any
sperm attached to them.
- Fertilization envelope is formed (components of the cortical granules + vitelline envelope).
- Mucopolysaccharides: elevate the fertilization envelope.
- Peroxidase enzyme: hardens the fertilization envelope
- Hyalin: forms a coating around the egg providing support during cleavage
7.20 Wave of Ca2+ release across a sea urchin egg during fertilization
• Upon fertilization, the concentration of free Ca2+ in the egg cytoplasm increases greatly.
• The release of calcium comes from within the egg itself.
• The release of Ca2+ starts at one end of the cell and proceeds actively to the other end.
• Calcium ions are directly responsible for propagating the cortical granule reaction.
• The calcium ions are stored in the endoplasmatic reticulum .
7.30 Sperm-zona binding
Sperm - SED1- ZP complex
Galt activates G-protein that open Ca channels and initiate the acrosome reaction.
7.33 Entry of sperm into a golden hamster egg (Part 1)
A) Sperm fusing the egg.
B) Sperm head passing through the zona pelucida
C) Hamster sperm fusing parallel to the egg plasma membrane.