Fertilization
ACTIVATION OF THE EGG
Sperm contact
The act of fertilization changes the egg from a
stage of slow structural and metabolic decline to
one of renewed activation. Morphologically egg
activation is a series of surface changes
immediately following sperm contact.
(a) Marine invertebrates and some aquatic
vertebrates elevate a fertilization membrane in
order to prevent polyspermy. More than one reaction
involved in the prevention of polyspermy. The first
reaction must be a very fast one. In oyster and
echinoderm eggs the cortical granules swell and
break down, lifting the vitelline membrane off the
egg surface and causing it to fuse with the plasma
membrane of the ovum. Structures similar to this
fertilization membrane occur in amphibians and
teleosts. Cortical granules formed in Golgi region
and occur throughout the egg cytoplasm in immature
eggs. They accumulate in the cortical regions as
maturation proceeds.
(b) Mammals - No phenomenon comparable to the
raising of the fertilization membrane is displayed.
Mammalian eggs are surrounded by the zona pellucida
which undergoes a structural change known as the
zonal reaction after sperm penetration. On sperm
contact with the egg plasma membrane, cortical
granules break down as in above forms, substances
liberated into the perivitelline space rapidly
modify the zona pellucida resulting in a block to
further sperm penetration.
The Acrosome Reaction
Penetration of egg by sperm is initiated by the
acrosome reaction which takes different forms in
different species.
The central part of the acrosome elongates into
a tube which extends form the head of the
spermatozoon. On contact with the egg the acrosomal
membrane fuses with the sperm plasma membrane thus
opening the acrosomal vesicle and liberating the
granules containing acrosomal lysins. The inner
portion of the acrosomal membrane everts and
lengthens to form the acrosomal tubule through
which the sperm nucleus enters the egg. The
mammalian sperm must remain for a time in the
female genital tract before being capable of
fertilization - Capacitation - which is essentially
a modification of the acrosomal reaction. Mammalian
acrosomal lysins contain proteinases which lyse the
glycoproteins of the zona pellucida.
Formation and Fusion of pro-nuclei
Require the activation of the egg. The acrosome
produces and delivers substances believed to be
instrumental in the activation process.
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