Question

• Explain the steps that lead to the fusion of a sperm with an egg

Answer 1

FERTILIZATION
Fertilization refers to the fusion of male and female gametes (i.e. spermatozoon and ovum). It takes place in the middle segment (ampulla) of the fallopian tube. It involves following events:
1. Transport of gametes
Before fertilization, the ovum and sperms reach the ampulla for fertilization.
Transport of ovum. At the time of ovulation, the ovum is
directly expelled into the peritoneal cavity and then enters
into the fallopian tube. When ovulation occurs, the fim-
briae of infundibulum encircle the surface of ovary, rub it
and pick up the ovum and then direct it towards the ostium
by continuous beating of cilia. The contractions of smooth
muscle fibres present in the wall of fallopian tube also help
in transport of the ovum.
Transport of sperms in the female genital tract. After ejac-
ulation, several million sperms (average–200 million
sperms per ejaculation) get deposited in the vagina. After
ejaculation, normal sperm shows flagellar movements in
the fluid medium at a rate of 1–4 mm/min. Therefore, in
30–60 min, they are able to reach the fallopian tube.

2. Sperm capacitation
Sperm capacitation refers to the process that makes a sperm to fertilize an ovum. It takes about 1–10 h (capacitation period). Sperm capacitation occurs due to removal of certain factors, which normally remain quiescent in male genital tract. These are:
Cholesterol contents of acrosomal membrane. In the
female genital tract, the cholesterol contents of acroso-
mal membrane decreases and it becomes weak leading
to easy release of enzymes from the head.
Calcium ions. The membrane of sperm becomes per-
meable to calcium ions. The influx of Ca2+ acts by two
ways: it makes the flagellar movements of the sperms
more strong and whipish (hyperactivation of sperms)
and secondly, it triggers the release of enzymes from the
acrosome.
3. Fusion of gametes
The fusion of ovum and sperm involves the following steps Chemoattraction. Chemoattraction of the sperms to ovum
occurs by substances produced by the ovum.
Penetration of sperm through ovum coverings. It is made
possible by the release of enzyme hyaluronidase and other proteolytic enzymes present on the acrosome of the sperm.
The binding of sperm to zona pellucida glycoprotein (ZP3)
triggers acrosomal reaction.
Acrosomal reaction.It involves release of acrosin (protease enzyme) from anterior membrane of
acrosome of the sperm. Acrosin opens the penetrating
pathway for passage of sperm head into the perivitelline
space (space between zona pellucida and oocyte mem-
brane) .
Fertilin is a protein present on acrosomal reacted sperm
which interacts with the protein present on vitelline
membrane and within 30 minutes the membranes of
sperm and oocyte fuse, and genetic material of sperm
enters into the oocyte.
Only one sperm can enter into the oocyte and further
entry of sperms is prevented by the activation of ovum.