Contact stage of fertilization includes: acrosomal reaction, denudation, penetration, cortical reaction and zona reaction.

Contact stage of fertilization includes: acrosomal
reaction, denudation, penetration, cortical reaction and zona reaction.

Fusion of oocyte and spermatozoon cell membranes

Spermato-zoon nucleus

Cell mem-brane of spermato-zoon

Acrosoma

Stage 1

Corona radiata cells

Dividing polar
body

Penetration

Oocyte meiosis

Stage 3

Stage 2

Sheme of fertilization. Stage 1 – adhesion, stage2 – penetration through zona
pellucida, stage 3 – penetration of spermatozoon head into oocyte.

When
spermatozoa get in contact with oocyte (contact
stage of fertilization), they are binding
to it. Corona radiata and zona pellucida of oocyte have an important role in
the recognition of homologous sperms and blocking polyspermy.
Glucosyltransferase receptors of the sperm cell membrane bind to zona pellucida
receptors, ZP-3 molecules.

The last
ones have two regions:

1) the sperm receptors that recognise
intergral proteins of their plasmalemma;

2) the other region of ZP-3 molecule binds
to receptor proteins located in the head of the sperm, triggering the acrosomal
reaction.

Acrosome
membrane fuses with corona radiata of oocyte and acrosomal enzymes are released
from acrosome. The effusion of enzymes results in digestion of intercellular
junctions. 200 – 500 spermatozoa reach the site of fertilization and introduce
in the tunica granulosa. First of all denudation
– dispersion of the corona radiata cells – occurs. During 12 hours due to the
contraction of spermatozoa tails the oocyte rolls to the uterus with the speed
4 times in a minute and loss follicular cells

Contact stage of fertilization includes: acrosomal reaction, denudation, penetration, cortical reaction and zona reaction. Clip_image001

Then penetration of zona pellucida
begins, most important enzymes are hyaluronidase and trypsin-like protease,
acrosin, which lyses zona pellucida, permitting the flagella movement of the
sperm to propel the sperm toward the oocyte. Penetration is accomplished by
limited proteolysis of the zona pellucida in front of the advancing sperm.
Additional sperms may be found attached to zona pellucida. Several sperms may
penetrate zona pellucida, but only one sperm completes the fertilization
process (monospermy).

At last,
the oolemma (the proper cell membrane of oocyte) is solved and spermatozoon
enters ooplasm. The sperm binds to receptors of oocyte plasma membrane. Cell
membranes of gametes fuse and break down at the area of fusion. The sperm
nucleus with 23 chromosomes, neck with centriol and mitochondria of middle
piece enter cytoplasm of oocyte. The sperm’s plasma membrane remains behind.

The
fusion of germ cells is responsible for several postfusion reactions: fast block of polyspermy, cortical reaction and zona reaction.

The fast
block of polyspermy includes changes of membrane resting potential of oolemma
that prevent contact between oocyte and another sperms. It consists of large
and long-lasting (few minutes) depolarization of the oolemma.

Cortical reaction is
slow component, after oolemma polarity changes Ca⁺⁺is released from
ooplasmic stores and promotes effusion of the oocyte cortical granules content
outside and its reaction with remnant of zona pellucida and oolemma. As a
result of this cell size decreases thus forming perivitelline
space.
Enzymes within the cortical granules (proteases) act to hydrolyze ZP-3
molecules in zona pellucida, the sperm receptors, thus preventing additional
sperms from reaching the oocyte. These enzymes degrade the glycoprotein oocyte
receptors for sperm binding and make it impermeable to other sperms
(monospermy).The content of cortical granules, which are released into perivitelline space, also causes changes if the
plasma membrane.

These
enzymes form the perivitelline barrier by cross-linking proteins on the surface
of zona pellucida. This event promotes final and permanent block to polyspermy.
So, tunica of fertilization appears.
There are such mechanisms of cortical reaction:

1. Modification of cell membrane
potential (minus to plus).

2. Releasing of the Ca++ from the depot
to hyaloplasm.

3. Cortical granules exocytosis.

4. Storage of the water between the
ovolemma and zona pellucida in perivitellin space.

5. Hardening of zona pellucida with
formation of fertilization tunica.

Sperm
nucleus enters the secondary oocyte and the ovum completes its second meiotic
division. This result in two haploid cells formation: the ovum and polar body.
Polar body with 23 s-chromosomes is extruded into the perivitelline space
between the oolemma and tunica of fertilization. Chromosomes, which remain in
ovum reconstitute into the female pronucleus.

Within the
cytoplasm of the oocyte, the nucleus of the sperm enlarges to form the male pronucleus, it turns around; at that time 2
nuclei may be seen in the cell: so called male and female pronuclei.
Since that time till the moment of their fusion such unicellular
organism is named “synkarion”. Two pronuclei soon meet
approximately the center of the ovum. They fuse, forming a zygote with the
diploid chromosomes number. After conjugation mitosis begins. Zygote is
genetically unique because it contains new combination of chromosomes that is
different from that in the cells of parents.

Chromosomal
sex of embryo and child is determined at the time of fertilization. It is clear
that one chromosome of each of the 23 pairs is derived from the mother and the
other from the father. Fertilization by the X-(gynecospermium) or Y-bearing (
androspermium) sperms
will result in origin of female embryo in the first case and male in the second
one.

» Biologic significance of spermatozoon in the fertilization.