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Activation of Sperm and Egg During Fertilization

David Epel

10.1002/cphy.cp140123

Source: Supplement 31: Handbook of Physiology, Cell Physiology

Originally published: 1997

Published online: January 2011

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 Chemotaxis (or, the Dating Game)
1.1 Historical Background
1.2 Motility and Chemotactic Substances in Sea Urchin Gametes
1.3 Does Chemotaxis Exist in the Real World?
2 The Acrosome Reaction
2.1 Description of the Phenomenon
2.2 Signal Transduction Molecules
3 Sperm‐Egg Binding
4 Passage of the Sperm Through the Egg Coat
5 Sperm‐Egg Receptors and Speciation
6 A Description of Egg Activation
6.1 What Happens When Sperm Meets Egg?
7 The Role of Calcium in Egg Activation
7.1 Historical Background
7.2 The Natural History of Calcium Changes at Fertilization
7.3 Mechanisms of the Calcium Increase
7.4 How Does the Sperm Trigger the Calcium Increase?
8 What is Calcium Doing to Activate the Egg?
8.1 The Timetable of Fertilization Responses in the Sea Urchin Egg
8.2 Intracellular pH Changes
9 Entry into the Cell Cycle
9.1 Synthesis of Cyclin as a Prerequisite for Mitosis
9.2 Activation of Protein Kinase Activity As a Prerequisite for Mitosis
10 Structural Changes as an Important Concomitant of Fertilization
10.1 Microfilaments
10.2 Microtubules
10.3 Endoplasmic Reticulum
10.4 Introduction of the Centrosome as a Critical Event
10.5 Ooplasmic Segregation
11 Epilogue—Is Fertilization a Unique Event in the Life History of the Organism?
Figure 1. Figure 1.

Diagram of acrosome reaction. Upper part depicts reaction in invertebrate (sea urchin) and lower part comparable reaction in mammal (hamster). In both cases exocytosis of the acrosomal vesicle results in exposure of new surface which will later fuse with egg plasma membrane. In sea urchin case, surface exposed by the acrosome reaction is pushed in front of sperm by polymerization of actin. The new acrosomal process which is formed is also coated with bindin, a protein contained within acrosomal vesicle, which is externalized by acrosomal exocytosis and thence involved in sperm‐egg adhesion.

In mammal (lower part of figure), there is no comparable acrosomal process formed by actin polymerization. Rather, exocytosis exposes surfaces which later fuse with egg surface, although initial fusion is with subacrosomal sperm surfaces which are already exposed before acrosome reaction.
Figure 2. Figure 2.

Cartoon illustrating microscopically visible changes of fertilization in sea urchin. Female pronucleus of unfertilized egg (A) is placed eccentrically in cytoplasm, and prominent feature of egg cortex is presence of numerous cortical granules imbedded in cytoplasm. Sperm‐egg fusion (B) initiates wave of cortical granule exocytosis and elevation of fertilization membrane. Sperm enters through actin‐filled fertilization cone (C). Once inside cytoplasm, sperm centrosome is activated and organizes microtubles around it which are involved in moving sperm and egg pronucleus to egg center (D). Centrosome duplicates and surrounds zygote nucleus (E and F) and then organizes mitotic apparatus (G and H) for first mitosis.
Figure 3. Figure 3.

Illustration of experiment in which current changes and capacitance changes associated with fertilization are simultaneously measured. A depicts experimental setup, in which microelectrode measures voltage around entire egg and patch clamp measures capacitance and ion currents in region of egg where sperm‐egg fusion occurs. Sperm is introduced in pipette, which isolates membrane and allows capacitance measurements to be made in this small area of membrane. As seen in parts B and C, capacitance and ion changes occur simultaneously, indicating that fusion and current changes are temporally associated. Membrane potential change (not shown) also occurs at same time.

Adapted from study of McCulloh and Chambers 113
Figure 4. Figure 4.

Polyspermy as function of membrane potential in eggs of Urechis caupo. Membrane potential is varied by fertilizing eggs in different concentrations of extracellular sodium (concentration noted in parentheses). As seen, there is increase in number of sperm that can enter egg in relation to degree of depolarization

Adapted from study of Gould‐Somero et al. 59
Figure 5. Figure 5.

Program of events following fertilization in eggs of sea urchin S. purpuratus at 16° C. The timing is in seconds, presented in a logarithmic fashion on the left‐hand side of the figure. Period of calcium release is highlighted in the period between 30 and 90 s after fertilization.
Figure 6. Figure 6.

Experiment depicting rise in intracellular calcium following fertilization in sea urchin eggs in which various calcium release mechanisms have been prevented. In the IP3 case, IP3‐induced calcium release was prevented by injection of egg with IP3 inhibitor heparin. In cyclic ADP ribose (cADPR) case, cADPR‐induced calcium release was prevented by co‐injection of cADPR analog. As seen, in either case there was release of calcium following fertilization. However, if egg was injected with both inhibitors (lowest tracing), there was no calcium release in response to fertilization

From study of Lee et al. 100


Figure 1.

Diagram of acrosome reaction. Upper part depicts reaction in invertebrate (sea urchin) and lower part comparable reaction in mammal (hamster). In both cases exocytosis of the acrosomal vesicle results in exposure of new surface which will later fuse with egg plasma membrane. In sea urchin case, surface exposed by the acrosome reaction is pushed in front of sperm by polymerization of actin. The new acrosomal process which is formed is also coated with bindin, a protein contained within acrosomal vesicle, which is externalized by acrosomal exocytosis and thence involved in sperm‐egg adhesion.

In mammal (lower part of figure), there is no comparable acrosomal process formed by actin polymerization. Rather, exocytosis exposes surfaces which later fuse with egg surface, although initial fusion is with subacrosomal sperm surfaces which are already exposed before acrosome reaction.



Figure 2.

Cartoon illustrating microscopically visible changes of fertilization in sea urchin. Female pronucleus of unfertilized egg (A) is placed eccentrically in cytoplasm, and prominent feature of egg cortex is presence of numerous cortical granules imbedded in cytoplasm. Sperm‐egg fusion (B) initiates wave of cortical granule exocytosis and elevation of fertilization membrane. Sperm enters through actin‐filled fertilization cone (C). Once inside cytoplasm, sperm centrosome is activated and organizes microtubles around it which are involved in moving sperm and egg pronucleus to egg center (D). Centrosome duplicates and surrounds zygote nucleus (E and F) and then organizes mitotic apparatus (G and H) for first mitosis.



Figure 3.

Illustration of experiment in which current changes and capacitance changes associated with fertilization are simultaneously measured. A depicts experimental setup, in which microelectrode measures voltage around entire egg and patch clamp measures capacitance and ion currents in region of egg where sperm‐egg fusion occurs. Sperm is introduced in pipette, which isolates membrane and allows capacitance measurements to be made in this small area of membrane. As seen in parts B and C, capacitance and ion changes occur simultaneously, indicating that fusion and current changes are temporally associated. Membrane potential change (not shown) also occurs at same time.

Adapted from study of McCulloh and Chambers 113


Figure 4.

Polyspermy as function of membrane potential in eggs of Urechis caupo. Membrane potential is varied by fertilizing eggs in different concentrations of extracellular sodium (concentration noted in parentheses). As seen, there is increase in number of sperm that can enter egg in relation to degree of depolarization

Adapted from study of Gould‐Somero et al. 59


Figure 5.

Program of events following fertilization in eggs of sea urchin S. purpuratus at 16° C. The timing is in seconds, presented in a logarithmic fashion on the left‐hand side of the figure. Period of calcium release is highlighted in the period between 30 and 90 s after fertilization.



Figure 6.

Experiment depicting rise in intracellular calcium following fertilization in sea urchin eggs in which various calcium release mechanisms have been prevented. In the IP3 case, IP3‐induced calcium release was prevented by injection of egg with IP3 inhibitor heparin. In cyclic ADP ribose (cADPR) case, cADPR‐induced calcium release was prevented by co‐injection of cADPR analog. As seen, in either case there was release of calcium following fertilization. However, if egg was injected with both inhibitors (lowest tracing), there was no calcium release in response to fertilization

From study of Lee et al. 100
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Article Title: Activation of Sperm and Egg During Fertilization
 

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David Epel. Activation of Sperm and Egg During Fertilization. Compr Physiol 2011, Supplement 31: Handbook of Physiology, Cell Physiology: 859-884. First published in print 1997. doi: 10.1002/cphy.cp140123