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Fertilization
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Learning Objective
• Describe the four processes involved in fertilization
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Fertilization
1. Recognition and contact– between noncellular egg coverings and sperm
2. Sperm entry is regulated– prevents interspecific fertilization– prevents polyspermy (fertilization of egg by
more than one sperm)
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Fertilization
3. Sperm and egg pronuclei fuse
initiates DNA synthesis
4. Egg becomes activated and developmental changes begin
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Fig. 50-1, p. 1082
SF
SM
FC
SN
1 µm
FertilizationFertilization
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KEY CONCEPTS• Fertilization includes contact and
recognition between egg and sperm, regulated sperm entry, and fusion of egg and sperm pronuclei, Egg becomes activated and developmentalchanges begin.
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Steps of Fertilization
1. Recognition and contact
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Steps of Fertilization
2. Sperm Entry
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Acrosome Reaction• Facilitates penetration of egg coverings
– when sperm first contacts egg
• In mammals, acrosome reaction is preceded by capacitation – maturation process– results in ability of sperm to fertilize egg
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Effects of Capacitation on Sperm
• Increased rate of metabolism • Flagellum beats more rapidly;
– Result: Sperm are more motile (hyperactivated)
• Changes in sperm glycoproteins– Allow sperm-egg binding
• Pro-Acrosin (inactive) is converted to acrosin (active)– Able to digest zona pellucida proteins
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Capacitation• These are monitor screen images
from an instrument which records the movement paths of the sperm cells heads (white points) during a certain time span and displays them with a green line.
• UPPER PANEL: Before capacitation the majority of the lines are straight.
• LOWER PANEL: After capacitation almost all the sperm cells have now gone over to swinging their heads strongly as indicated by the jagged lines.
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http://biology.kenyon.edu/courses/biol114/Chap13/Chapter_13B.html
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Very good animation
• http://bcs.whfreeman.com/thelifewire/content/chp43/4301s.swf
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Acrosomal reaction in mammals
• http://www.youtube.com/watch?v=41qQTEhoNjYhttp://www.youtube.com/watch?v=41qQTEhoNjY
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Polyspermy
• Echinoderms– sea urchin fertilization is
followed by a fast block to polyspermy (depolarization of plasma membrane) and a slow block to polyspermy (cortical reaction)
• In mammals – changes in zona pellucida
prevent polyspermy
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Fast block polyspermy
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http://biology.kenyon.edu/courses/biol114/Chap13/Chapter_13B.html
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Cortical Reaction
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Steps of Fertilization (Continue)
3. Fusion of sperm and egg pronuclei
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Steps of Fertilization (continue)
4. Egg activation
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Learning Objective 4
• Describe fertilization in echinoderms
• Point out some ways in which mammalian fertilization differs
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Learning Objective 5
• Trace the generalized pattern of early development of the embryo from zygote through early cleavage and formation of the morula and blastula
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The Zygote
• Undergoes cleavage – a series of rapid cell divisions without a
growth phase – partitions zygote into many small blastomeres
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Cleavage
• Morula – a solid ball of cells
• Blastula – a hollow ball of cells
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KEY CONCEPTS• Cleavage, a series of rapid cell divisions
without growth, provides cellular building blocks for development
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Learning Objective 6
• Contrast early development, including cleavage in the echinoderm (or in amphioxus), the amphibian, and the bird, paying particular attention to the importance of the amount and distribution of yolk
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Invertebrates and Simple Chordates
• Have isolecithal eggs (evenly distributed yolk)– undergo holoblastic cleavage (division of
entire egg)
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Cleavage in Sea Stars
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Fig. 50-3 (a-c), p. 1084
Nucleus
(a) Unfertilized egg
100 µm(b) 2-cell stage
50 µm(c) 4-cell
stage
50 µm
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Fig. 50-3 (d-f), p. 1084
Blastocoel
Archenteron Blastopore
(d) 16-cell stage
50 µm(e) Blastula
50 µm(f) Early gastrula
50 µm
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Fig. 50-3 (g-i), p. 1084
ArchenteronMouth
Anus
StomachBlastopore
(g) Middle gastrula
50 µm (h)Sea star larva
50 µm (i)Young sea star
1 mm
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Cleavage in Amphioxus
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Fig. 50-4 (a-d), p. 1085
Polar body
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Fig. 50-4 (e-g), p. 1085
Blastocoel
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Fig. 50-4 (h-j), p. 1085
Ectoderm Archenteron
EndodermBlastopore
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Amphibians
• Have moderately telolecithal eggs– concentration of yolk at vegetal pole slows
cleavage (only a few large cells form)– large number of smaller cells form at the
animal pole
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Cleavage in FrogsAnimal pole
Vegetal pole
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Reptiles and Birds
• Have highly telolecithal eggs – large concentration of yolk at one end– undergo meroblastic cleavage (restricted to
the blastodisc)
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Cleavage in Birds
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Fig. 50-7a, p. 1086
Blastodisc
Yolk
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Fig. 50-7b, p. 1086
Epiblast
Hypoblast
Blastocoel
Yolk