Acrosomal vs Cortical After Fertilization Fertilization Fertilization brings
Acrosomal vs. Cortical After Fertilization
Fertilization • Fertilization brings the haploid nuclei of sperm and egg together, forming a diploid zygote • The sperm’s contact with the egg’s surface initiates metabolic reactions in the egg that trigger the onset of embryonic development
The Acrosomal Reaction • The acrosomal reaction is triggered when the sperm meets the egg • This reaction releases hydrolytic enzymes that digest material surrounding the egg
LE 47 -3 Contact and fusion of sperm and egg membranes Acrosomal reaction Sperm plasma membrane Contact Basal body (centriole) Sperm nucleus Entry of sperm nucleus Cortical reaction Acrosomal process Sperm head Actin Acrosome Jelly coat Sperm-binding receptors Fused plasma Cortical membranes granule Hydrolytic enzymes Perivitelline space Vitelline layer Egg plasma membrane EGG CYTOPLASM Fertilization envelope Cortical granule membrane
• Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy
The Cortical Reaction • Fusion of egg and sperm also initiates the cortical reaction • This reaction induces a rise in Ca 2+ that stimulates cortical granules to release their contents outside the egg • These changes cause formation of a fertilization envelope that functions as a slow block to polyspermy
LE 47 -4 500 µm 1 sec before fertilization 10 sec after fertilization Point of sperm entry 20 sec Spreading wave of calcium ions 30 sec
Activation of the Egg • The sharp rise in Ca 2+ in the egg’s cytosol increases the rates of cellular respiration and protein synthesis by the egg cell • With these rapid changes in metabolism, the egg is said to be activated • In a sea urchin, a model organism, many events occur in the activated egg
Minutes Seconds LE 47 -5 1 Binding of sperm to egg 2 3 4 Acrosomal reaction: plasma membrane depolarization (fast block to polyspermy) 6 8 10 Increased intracellular calcium level 20 Cortical reaction begins (slow block to polyspermy) 30 40 50 1 Formation of fertilization envelope complete 2 Increased intracellular p. H 3 4 5 Increased protein synthesis 10 20 30 40 60 90 Fusion of egg and sperm nuclei complete Onset of DNA synthesis First cell division
Fertilization in Mammals • In mammalian fertilization, the cortical reaction modifies the zona pellucida as a slow block to polyspermy
LE 47 -6 Follicle cell Zona pellucida Egg plasma membrane Acrosomal vesicle Sperm Cortical basal ganules body Sperm nucleus EGG CYTOPLASM
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