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
do a woman's hormones control her fertility ?
How do a woman's
hormones control her fertility ?
Reproduction is like an orchestra - and the reproductive
organs need to be synchronised to perform at just the
right time for them to work properly. It is the fertility
hormones which play the conductor's role.
Hormones are chemicals the body makes
to carry messages from one part of the body to another
. There are two major female hormones - estrogen and
progesterone - which are produced by the ovaries.
The cycle of ovarian hormone production
has two phases. In the first half called the follicular
phase, estrogen plays a dominant role. During this phase
the egg matures inside the ovary in its follicle. The
egg; the surrounding cells (which nurture the egg and
are called granulosa cells and theca cells); and the
fluid (called follicular fluid) which accumulates in
progressively larger amounts during this phase, is called
a follicle. The follicle secretes a large amount of
estrogen (produced by the granulosa cells) into the
bloodstream, and the estrogen circulates to the uterus
where it stimulates the endometrium to thicken.
The second phase of hormone production
begins at ovulation, midway through the cycle, when
the follicle changes into the corpus luteum. This produces
estrogen ; and also large quantities of progesterone
throughout the second half of the cycle. Travelling
through the bloodstream to the uterus, progesterone
complements the work begun by estrogen by stimulating
the endometrium to mature and making it possible for
a fertilized egg to implant in it. In case pregnancy
does not occur, production of estrogen and progesterone
falls 10 to 14 days after ovulation as the corpus luteum
dies, and the endometrium is shed from the body as the
menstrual period.
How is the release of hormones regulated
by the body ? This is a complex self-regulating system,
which uses negative feedback control loops, much like
a thermostat for an oven does. As the temperature increases,
the thermostat shuts off the heater to reduce its heat
output. When the temperature falls below the thermostat's
setting, the thermostat signals the heater to turn up
the heat again, thus maintaining the desired temperature.
A similar signaling relationship exists between the
pituitary gland and the ovaries in women; and the testes
in men . For example, as the concentration of gonadotropins
in the blood rises, this signals the woman's ovaries
to increase hormonal output of estrogen. In turn, when
the blood levels of estrogen rise , the pituitary gland
slows its release of gonadotropins, thus maintaining
the desired equilibrium.
![]()
Fig 5. A schematic of
the hormonal changes during the menstrual cycle. The
interplay of the pituitary and ovarian hormones regulate
the changes which occur in the uterine lining.
The male reproductive system begins in the scrotum,
the sack behind the penis. This contains two testicles,
which make men's sex cells, called sperm; and the male
sex hormone, called testosterone. The testicles feel
solid, but a little spongy, like hard boiled eggs without
the shell. They hang from a cord called the spermatic
cord. It's normal for one testicle to hang lower than
the other; and for one testicle to feel slightly larger
than the other.
The testicles make sperm best at a temperature
a few degrees cooler than normal body temperature. This
is why nature designed a scrotum - so that the testes
can hang outside the body to keep them cool.
The testicles start making sperm when
a young man reaches puberty. This is in response to
the male sex hormone, testosterone , which starts being
produced at this time. The testes keep making sperm
for the rest of the man's life.
The testes have two components, the
seminiferous tubules, where sperms are produced, and
the "interstitium" or the tissue in between the tubules,
which contain the Leydig cells which produce the male
sex hormone, testosterone, which causes the male sexual
drive.
Most of the testis is composed of the
tightly coiled microscopic seminiferous tubule, which
if uncoiled would reach a length of 70 cm. The sperms
are produced inside the seminiferous tubule, and these
converge and collect into a delta (like the mouth of
a river) near the upper part of the testis called the
rete testis which then empties through a series of very
small ducts out of the testis towards the epididymis.
The epididymis is an amazing structure - it is a very
long tiny tubule ( about 5-6 meters long), which runs
back and forth in convolutions and loops to form a tiny
compact structure with a head , body and tail that sits
like a cap on the top of and behind the testis . The
tail of the epididymis then leads to the vas deferens
- a thin cord like muscular tube, which is part of the
spermatic cord and which ends at the ejaculatory duct
in the prostate. Here is joined by the seminal vesicle
ducts and they all open into the prostatic part of the
urethra - which in turn leads to the urethra in the
penis.
Mature sperm take about 75 days to develop
in a process called spermatogenesis which takes place
in the seminiferous tubules. The primordial germ cells
in the testis, called the spermatogonia , which are
"immortal" stem cells , divide repeatedly to form primary
spermatocytes. These undergo meiotic ( reduction) division
to form secondary spermatocytes, which differentiate
to form spermatids , which then ultimately mature to
form spermatozoa. Sperm production takes place as though
it were on an assembly line - with the more mature sperms
being passed along toward the center of the tubule from
where they swim towards the efferent ducts of the testis
towards the epididymis. The spermatogenic cells are
supported and nourished by large cells called the Sertoli
cell, which help to support sperm maturation. This can
be a very "temperamental" assembly line - things often
go wrong, causing low sperm counts.
When the sperm leave the testis, they
are not yet able to swim on their own. They acquire
the capacity to do so in their passage through the epididymis
- which is like a swimming school for the sperm. They
spend between 2 to 15 days here during which they attain
maturity and fertilising potential. Sperm are propelled
along this tunnel by frequent contractions of its thin
muscular wall. Most of the mature sperm are then stored
at the end of the epididymis - where they wait to be
rushed through the vas deferens and ejaculated at the
time of orgasm.
continued . .
.
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