from
the book How to Have a Baby: Overcoming Infertility
by Dr. Aniruddha Malpani, MD and Dr. Anjali Malpani,
MD.
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How Babies are Made - The Basics (Page 3)
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Out What’s Wrong -- The Basic Medical Tests
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What
happens to the sperm once they enter the woman's vagina
?
What
happens to the egg when conception occurs ?
Testosterone does more than just allow
men to make sperm. It also triggers the growth of facial
hair, the deepening of men's voices, and the development
of a male physique - all the changes which make boys
into men. Testosterone is also important in creating
desire for sex - it increases libido.
What
happens to the sperm once they enter the woman's vagina
?
The sperm's odyssey in the female
reproductive tract
A million million spermatozoa,
All of them alive;
Out of their cataclysm but one poor Noah
Dare hope to survive.
-- Aldous Huxley
When a man and woman have sexual intercourse,
the man places his erect penis inside the woman's vagina.
Here it releases millions of sperm when ejaculation
occurs. Once the sperm have been deposited here they
have a long and arduous journey ahead of them, like
salmon entering the mouth of a river to swim upstream
to spawn.
Some of the sperm swim straight up into
the fallopian tubes through the cervix and uterus -
and some of them are so fast, that sperms have been
found in the tubes in as little as a few minutes after
ejaculation. Some sperms die in the acidic vaginal fluid;
and some enter the cervical mucus and cervical crypts.
They are stored here and can remain alive here for as
long as 48 to 72 hours.
During this time, the sperms are released
in small numbers and these continue to swim towards
the fallopian tubes. This is why you don't need to have
sex every day to get pregnant even though the egg remains
alive for only 24 hours.
Sperms in the female reproductive tract
swim under their own steam - as a result of the whip-
like activity of their tail which propels them on. Of
the millions of sperms released in an ejaculate, only
a few hundred will make the arduous trip upto the egg
successfully. Perhaps this is why so many millions of
sperms are produced in the first place even though only
one is needed to fertilize the egg - because the wastage
is so prodigal.
What happens
to the egg when conception occurs ?
What about the other partner in this
mating dance, the egg ? Remember that a mature egg is
released from the ovary ( this process is called ovulation)
only once during the menstrual cycle. This is the "fertile
time", during which a pregnancy can occur.
How does the egg reach the tube ? When
ovulation occurs, the mature egg is released from the
follicle in the ovary. This process of follicular rupture
looks a bit like a small volcano erupting on the ovarian
surface. At this time, the tubal fimbria, like tentacles,
sweep over the surface of the ovary, and actually "swallow"
the egg.
The egg has a shell, called the zona
pellucida, which looks like the ring around Saturn.
It is surrounded by a cluster of nest cells called the
corona cells which serve to nurture the egg. They form
the cumulus oophorus which is a sticky gel which protects
the egg and also helps the beating of the hair-like
cilia of the fallopian tube to propel the egg towards
the uterus - like a conveyor-belt. The egg must now
wait in the protective confines of the fallopian tube,
for a sperm to swim up and reach it. An egg remains
alive for about 24 hours, and if fertilization does
not occur, it dies.
Of the few hundred sperm which reach the egg, only one
will successfully fertilize it. The process of fertilization
is truly the primeval mating dance - the fertilization
tango - when the mother's chromosomes (in the egg) and
the father's chromosomes (in the sperm) fuse together
to create a new life - one which is totally different
from all others, because of its unique genetic composition.
We have now learnt quite a lot about fertilization thanks
to in vitro fertilization (IVF) - and it is truly one
of Nature's miracles.
During the time the sperm spend in the
female reproductive tract, while swimming towards the
egg, they acquire the capacity to fertilize it - a process
called capacitation. When the sperms reach the corona
cells (only a few hundred successfully make the trip,
guided by chemicals produced by the egg which serve
as guiding beacons to the sperms) they become hyperactivated
- they start beating their tails in a frenzy. This is
useful because it provides the mechanical energy the
sperm head needs to burrow its way through the outer
shell of the egg called the zona.
The sperms disperse the cumulus oophorus
(and so far it's a team effort ) and when they reach
the egg, they first bind to the zona. A chemical is
released here by the sperms in a process called the
acrosomal reaction in which the acrosome (which sits
like a cap on the head of the sperm and behaves much
like a battering ram) is removed. The acrosomal enzymes
dissolve the zona pellucida by making a tiny hole in
it, so that one sperm can swim through and reach the
surface of the egg. At this time, the egg transforms
the zona to an impenetrable barrier, thus preventing
other sperm from entering it.
The genetic material of the sperm (the
male pronucleus) and the genetic material of the egg
(the female pronucleus) then fuse - to form an embryo,
which then divides into 2 cells. These cells in turn
then continue to divide rapidly, producing a ball of
cells - the embryo. The embryo then travels through
the fallopian tube (which nurtures it and propels it
) into the uterus - a journey which takes about 3 to
5 days. The embryo must then break through its zona
( this is called embryo hatching); and then attach itself
to the lining of the uterus in a process called implantation
- and in 9 months , if all goes well, a baby is born.
Fig 6. How an egg is fertilised
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