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