In order to understand the infertility
investigation and the rationale behind infertility
treatment, a sound understanding of the principles
of human reproduction is mandatory.
Male Reproduction 
The male
reproductive system includes the testes where
sperm production occurs every 72 days. The sperm are
delivered by the epididymis and vas deferens into
the urethra where ejaculation occurs through the penis.
At the time of ejaculation, the valve into the bladder
is closed, thus inhibiting retrograde ejaculation
into the bladder.
The seminal vesicles and prostate gland
contribute nourishing fluid for the sperm to live
in while in the ejaculate. It is important that the
testes be somewhat cooler than the rest of the body
so that effective spermatogenesis can occur.
The
Female Reproductive System
This includes the vagina where sperm
are deposited, the cervix
where mucus is produced which provides a reservoir
for the ejaculated sperm, the uterus where the pregnancy
develops for nine months, the fallopian tubes which
provide a conduit for sperm and eggs, and the ovaries
in which ovulation occurs as a monthly phenomenon.
The hypothalamus and pituitary glands are extremely important for the development of eggs in the female and sperm in the testicles. In both sexes, the gonadotropin-releasing hormone is released in pulses from the hypothalamus at the base of the brain sending a message to the pituitary gland to produce the hormones FSH and LH.
In the ovary, the FSH and LH provide
a signal for the development of eggs. As egg development
occurs, the hormones estrogen and inhibin, produced
by the ovary, provide negative feedback to the hypothalamus
and pituitary. This feedback causes the production
of FSH and LH to decrease. In the failing ovary, (premenopausal)
estrogen and inhibin levels are low; the negative
feedback is inhibited; and FSH and LH levels may be
high, signifying imminent ovarian
failure.
In the male, the FSH and LH act on the testicles to facilitate sperm production. Again, there is a negative feedback loop through the pituitary and hypothalamus via the hormone testosterone.
The hypothalamus is an extremely sensitive part of the human brain, especially in females. Changes in diet, excessive weight gain or loss, excessive stress, exercise, emotional upheaval, and even alterations in the light/dark cycles may alter its function. These alterations affect the GnRH pulses and hence pituitary function with alteration in the secretion of the hormones FSH and LH.
These changes affect ovarian function with alterations in the production of estrogen, which prepares the uterus to accept an embryo and progesterone, the hormone of the luteal phase, which is important in maintaining a pregnancy. Thyroid hormone balance is also extremely important to the overall functioning of this system.
Because of the hormonal changes discussed
above, a female undergoes a monthly menstrual cycle.
In the first half of the cycle, (proliferate phase),
the lining of the uterus is developed so that an embryo
can implant. This is the estrogenic phase. The second
half of the cycle is characterized by the secretion
of
progesterone (secretory phase) to maintain the
lining of the uterus.
Ovulation is the event that occurs between the proliferative and secretory phases, and is usually mid cycle. At the end of the secretory phase (which usually lasts fourteen days), menstruation (shedding of the lining of the uterus) occurs if there is no pregnancy.
The hormonal events in a normal menstrual
cycle are complex. FSH is secreted from the pituitary
gland causing the ovaries to produce estrogen. Just
before ovulation, the hormone LH is released from
the pituitary gland, triggering and ovulatory response
in the ovary. (This is a hormone measured in ovulation
prediction kits and is in the blood stream for
a short time).
After ovulation, progesterone is secreted by the corpus luteum of the ovary and functions to maintain a pregnancy.
If sperm have been placed in the uterus at the time of ovulation, they swim into the fallopian tubes and encounter an egg that has been released from the ovary. The sperm enters the egg (fertilization) and such a union of a sperm and egg forms a zygote. This is the beginning of life.
Having an understanding of the physiology
of male and female reproduction, we are now in a position
to consider the causes of male and female infertility,
which may be both varied and complex.
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