FERTILITY DRUGS
THE PROCESS OF FERTILIZATION
F
Fertilization occurs when the head of
a sperm penetrates a mature ovum.
After penetration, the nuclei (which
contain the genetic material) of the
sperm and ovum fuse and the body
and tail of the sperm drop off. The
newly fertilized ovum, called a zygote,
then forms an outer layer that is
impenetrable to other sperm. The
zygote undergoes repeated cell
divisions as it passes down the
fallopian tube. By the time it reaches
the uterus, it has grown into a solid
ball of cells called a morula. It then
develops an inner cavity with a small
cluster of cells to one side; this is
called a blastocyst.
FERTILE PERIOD
Ovulation occurs halfway through the menstrual cycle
(13
to
1 6
days before the
start of a period), after which the released ovum is available for fertilization for
about two days. Sperm can also live for about two days, so the peak fertile period
is about four days (two pre- and two post-ovulation).
M e n s t r u a t io n
P e a k f e r t i l e p e r i o d
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P o s s i b l e f e r t i l e p e r i o d
@
?
U n fe r t ili z e d o v u m d i e s a n d is
F o lli c le m a t u r e s in o v a r y
O v u la t io n
s h e d d u r i n g m e n s t r u a t i o n
Peak and possible fertile periods
Although the peak fertile period is about four days,
sperm can survive and the timing of ovulation.
the possible fertile period may last seven to
12
The illustration shows the peak and maximum
days, due to variations in how long the ovum and
possible fertile periods in a
28
-day cycle.
M o r u la
T h e f e r t i l i z e d o v u m ( z y g o t e )
d i v i d e s r e p e a t e d l y t o f o r m a
F a l l o p i a n t u b e
b a l l o f c e l l s ( m o r u la ) .
B l a s t o c y s t
T h e m o r u l a d e v e l o p s a
c a v it y , t h e r e b y b e c o m i n g
a b l a s t o c y s t .
Sperm and ovum
A single sperm penetrates the ovum, thereby
fertilizing it. To achieve this, the sperm releases
enzymes that dissolve a path through the
ovum’s outer layers.
JOURNEY OFTHE SPERM
When semen is ejaculated into
the vagina, as many as
5 0 0
million
sperm are released, most of which
are capable of fertilizing an ovum.
But, as they travel upwards
(propelled by their whiplike tails),
more than half are killed by acidic
vaginal secretions; many more die
during the journey up through the
cervix and uterus and into the
fallopian tubes. The journey can take
from less than half-an-hour to five
hours and, in the end, only a few
thousand sperm have survived.
O v u m
A m a t u r e o v u m is
r e l e a s e d f r o m t h e f o l l i c l e
in t o t h e f a l l o p i a n t u b e .
O v u m
U t e r u s
B l a s t o c y s t
U t e r in e
w a ll
Lifespan of sperm
A sperm can live in a fallopian tube for up to
48
hours, during which time it is capable of
fertilizing an ovum.
Blastocyst
The blastocyst embeds in the
uterine wall (implantation),
where it develops into an embryo
and also forms the placenta.
304
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