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The Reproductive Systems
By the end of this class you should
understand:
• The key organs and functions of both male
and female reproductive systems
• The nature and behavior of human gametes
• The key sex hormones and their effects on the
body
• The major biological reasons why teenagers
are idiots
Reproduction
• Life is defined by its ability to
reproduce
• The cells of our body can
reproduce to make two
identical copies (mitosis)
– This was the only means of
reproduction for a long time
• To increase the spread of
beneficial genes, a new type
of reproduction was evolved
called sexual reproduction
Meiosis
• Sexual reproduction requires a
new type of cell division: meiosis
• Normal human mitosis takes one
cell with 46 chromosomes and
produces two cells with 46
chromosomes
• Meiosis instead produces four
cells with 23 chromosomes
– 1 of each instead of 2 of each
• These cells are called haploid,
and two haploid cells will make a
new whole cell
Fertilization
• Haploid sex cells are called gametes
• Human gametes are sperm cells
(male) and egg cells (female), which
are produced by the gonads
• The human reproductive systems
have two purposes:
– Initiate fertilization (combine a sperm
cell with an egg cell)
– Keep this newly-created life form in a
safe place until it is able to live on its
own (pregnancy)
Gamete Formation
• In men, gamete formation is called
spermatogenesis and takes places in the testes
– Spermatogenesis requires a precise temperature
which is less than normal body temperature, so the
testes are placed in a sac of skin called the scrotum
– Men produce millions of sperm a day after puberty
• In women, gamete formation is called oogenesis
and takes place in the ovaries
– In women, all gametes have begun their formation by
the time the baby is born, and will only release a few
hundred during her lifetime
Sex Hormones
• The primary mediator of the
reproductive system is sex
hormones, also produced by the
gonads
• Male gonads: testes (singular:
testis)
• Female gonads: ovaries
• The gonads produce sex
hormones, primarily
testosterone (men) and estrogen
and progesterone (women)
– Actually a broad array of hormones
in both
Hormonal Activities
• Sex hormones have major effects on the body
– Development of reproductive organs and secondary sexual
characteristics
– Change brain behavior (e.g. testosterone supposedly
increases aggression, though it’s not clear that it does)
– Make sexual activity appealing
• Sex hormones have strong effects on the limbic system,
and the frontal lobe must spend its teenage years
learning to cope with hormone-originated impulses
– This is why teenagers are idiots
Reproductive Organs
• The gonads produce the
gametes, but other organs are
involved in the meeting of the
gametes
• Male organs are devoted to
producing, processing, and
delivering sperm cells
• Female organs are for receiving
semen and growing and
nourishing the fetus once it is
formed
Male Reproductive Organs
• The primary male reproductive
organs are the testes that
produce sperm and testosterone
• The other major organ is the
penis, used to deliver sperm to
inside the woman
– The tubes running from the testes
to the penis are called the vas
deferens, and these also meet with
the urethra which carries urine
Accessory Male Organs
• The other male reproductive
organs help create semen, which is
a mixture of sperm cells with
sugars, mucus, and alkaline
chemicals to enable them to
survive inside the woman
– The vagina is acidic and generally
hostile to life, so that it does not
become infected
• The most well-known of these is
the prostate, which produces the
watery alkaline fluid of semen
– Enlargement can cause discomfort,
also prone to cancer
Female Reproductive Organs
• The female ovaries are
attached via the oviducts to
the uterus
– The oviducts were formerly
known as the Fallopian tubes
• The uterus’s connection to the
exterior of the body is a
muscular canal called the
vagina
– The vagina has a small penislike
organ called the clitoris at the
anterior portion
Why Do Women Have A Clitoris?
Answer: The same reason men have nipples!
Pregnancy
• The actual fertilization event occurs in
the oviduct, which is where the egg
cell is initially placed at the peak of
the ovarian cycle
• If fertilized, the egg is migrated into
the uterus where it becomes
implanted
• The uterus then develops an organ
called the placenta which nourishes
the fetus
• The fetus develops for 9 months
before being pushed out of the vagina
by uterine contractions, a painful and
occasionally lethal process
Ovarian Cycle
• The ovaries do not secrete
estrogen and progesterone at a
fixed level
• During the ovarian cycle,
estrogen and progesterone
levels rise, triggering a release
of an egg cell, which is called
ovulation
• If no sperm is present to fertilize
the egg, the estrogen and
progesterone levels will drop
– This causes menstruation
Menstruation
• The increasing hormones from the ovarian
cycle trigger growth of the inner lining of
the uterus, which is called the
endometrium
• Once the estrogen and progesterone levels
fall, the endometrium disintegrates and
leaks out of the vagina, a process called
menstruation
• When fertilization occurs, hormonal levels
remain high so there is no menstruation
– This is how the woman’s body is aware it is
pregnant
– Hormonal contraceptives trick the body into
thinking it’s pregnant when it’s not (“The
Pill,” “The Shot,” etc.)
Sexual Response
• In both men and women, the sexual
response is the brain and hormones
working together to mate and is
stimulated by sexual interest
– Requires a combination of sympathetic
and parasympathetic activity
• In both sexes, blood is flushed to the
genitals (using a molecular mechanism
that is duplicated by viagra)
– In men this produces an erection, or
enlargement of the penis
– In women this stimulates glands to
secrete mucus to lubricate the vagina
Sexual Response
• The brain is programmed to focus
on the sexual response through
controlled positive feedback loops
• As with other systems, positive
feedback loops build up to a climax,
and in the sexual response the
climax is the orgasm
• In men, orgasm triggers the release
of semen
– Recall the orgasm also triggers the
release of oxytocin
• If coitus has been occurring, the
semen will be released into the
vagina
Two ways to fail to avoid being pregnant:
• Withdrawal method (pull-out): Attempting to
ejaculate outside the vagina while being in it
for the run up
– Sperm cells may also be found in the male
lubricants secreted before orgasm
• Periodic abstinence (rhythm method):
Attempting to not mate during peak fertility
– Fertility can be difficult to estimate based only on
menstruation, also sperm may be long-lived in the
vagina given the right conditions
Contraception
• As indicated previously, many female
contraceptive technologies are
hormonal, in that they deceive the body
by creating artificial hormone levels
– These have an advantage of not
interrupting the parasympathetic nervous
system’s attempt to relax you into having
sex
• Barrier methods such as condoms also
prevent pregnancy and also prevent
excessive mucousa-to-mucousa contact
– Mucosal layers of the genitals may carry
bacterial and/or viral sexually transmitted
diseases (STDs)
That’s our show!
• There is no new content (aside from labs) for
the semester!
• Next week is exam #3
• The following week is the last week with the
cumulative final exam!