fertilization. learning objective describe the four processes involved in fertilization
TRANSCRIPT
Fertilization
Learning Objective
• Describe the four processes involved in fertilization
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)
Fertilization
3. Sperm and egg pronuclei fuse
initiates DNA synthesis
4. Egg becomes activated and developmental changes begin
Fig. 50-1, p. 1082
SF
SM
FC
SN
1 µm
FertilizationFertilization
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.
Steps of Fertilization
1. Recognition and contact
Steps of Fertilization
2. Sperm Entry
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
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
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.
http://biology.kenyon.edu/courses/biol114/Chap13/Chapter_13B.html
Very good animation
• http://bcs.whfreeman.com/thelifewire/content/chp43/4301s.swf
Acrosomal reaction in mammals
• http://www.youtube.com/watch?v=41qQTEhoNjYhttp://www.youtube.com/watch?v=41qQTEhoNjY
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
Fast block polyspermy
http://biology.kenyon.edu/courses/biol114/Chap13/Chapter_13B.html
Cortical Reaction
Steps of Fertilization (Continue)
3. Fusion of sperm and egg pronuclei
Steps of Fertilization (continue)
4. Egg activation
Learning Objective 4
• Describe fertilization in echinoderms
• Point out some ways in which mammalian fertilization differs
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
The Zygote
• Undergoes cleavage – a series of rapid cell divisions without a
growth phase – partitions zygote into many small blastomeres
Cleavage
• Morula – a solid ball of cells
• Blastula – a hollow ball of cells
KEY CONCEPTS• Cleavage, a series of rapid cell divisions
without growth, provides cellular building blocks for development
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
Invertebrates and Simple Chordates
• Have isolecithal eggs (evenly distributed yolk)– undergo holoblastic cleavage (division of
entire egg)
Cleavage in Sea Stars
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
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
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
Cleavage in Amphioxus
Fig. 50-4 (a-d), p. 1085
Polar body
Fig. 50-4 (e-g), p. 1085
Blastocoel
Fig. 50-4 (h-j), p. 1085
Ectoderm Archenteron
EndodermBlastopore
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
Cleavage in FrogsAnimal pole
Vegetal pole
Reptiles and Birds
• Have highly telolecithal eggs – large concentration of yolk at one end– undergo meroblastic cleavage (restricted to
the blastodisc)
Cleavage in Birds
Fig. 50-7a, p. 1086
Blastodisc
Yolk
Fig. 50-7b, p. 1086
Epiblast
Hypoblast
Blastocoel
Yolk