Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 121

Egg cytoplasm

Protein receptors
Jelly coat
Acrosome

Egg plasma

membrane
Vitelline layer
Hydrolytic enzymes
Cortical
granule Fused plasma
membranes

Cortical granule
membrane

2

Perivitelline
space

Sperm
head

Contact

1

1

Acrosomal
process

Fertilization
membrane

2 Acrosomal

reaction

6 Cortical

reaction Sperm

nucleus

Sperm plasma
membrane

Growth of
acrosomal
process

3

5 Entry of

sperm nucleus

4 Fusion of plasma

membranes

Figure 3.1 Staged penetration and fusion of the sperm nucleus and the centriole in the egg
in the process of fertilization.
Source : From http://www.bio.davidson.edu/courses/molbio/molstudents/spring2005/dresser/
review%20paper.htm

the egg ( Guidobaldi et al., 2008 ). However, some biologists think that the sperm
guidance mechanism still remains elusive.
The sperm first penetrates the cumulus oophorus layer of cells. Then, when it
comes in contact with zona pellucida, the acrosome releases enzymes and begins an
acrosome reaction that brings the sperm head in contact with the sperm receptors on
the vitelline layer ( Figure 3.1 ). Reaching the plasma membrane, the acrosome mem-
brane fuses with it, thus releasing the acrosome content, which contains hydrolytic
and proteolytic enzymes that break the egg coat and allow penetration of the sperm
nucleus and the centrosome into the egg cytoplasm.

Egg Activation
The entry of sperm in the egg induces Ca 2+ release, which triggers egg activa-
tion and exocytosis of the cortical granules, secretory organelles containing
enzymes, and glycosylated components that fill the previtelline space, thus pre-
venting other sperm cells from entering the egg cytoplasm ( Gilbert, 2000; Liu,
2011 ). The sperm thus “lights the fuse” for a Ca 2+ fertilization wave, inducing
the egg nucleus to resume meiotic division, which results in the production of a
polar body that is extruded outside the egg, leaving the egg in a haploid state. The
Ca 2+ waves from internal calcium stores are induced by activation of two main
pathways, inositol 1,4,5-trisphosphate (IP3), and nicotinic acid adenine dinucleo-
tide phosphate (NAADP) ( Stricker, 1999; Whitaker, 2008 ) ( Figure 3.2 ). Although