Biology 122 – CHS (Chapter 39)
Mr. Gallop
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Upon completion of this unit, you should be able to:
Differentiate between asexual and sexual reproduction.
Summarize the process of male and female sex cell development.
Analyze and describe in detail the structure and function of the male
human reproductive system.
Analyze and describe in detail the structure and function of the female
human reproductive system.
Explain the human reproductive cycles, identifying the main hormones
involved.
Understand the process of negative feedback mechanisms as they
pertain to the hormones associated with reproduction.
Discuss the process of fertilization and describe the events of the three
trimesters of human pregnancy.
Consider the conditions necessary for the development of fraternal and
maternal twins.
Describe and explain the onset of labour and birth, in general terms.
Research and explore reproductive technologies (such as in vitro
fertilization) and consider the ethical implications of these technologies.
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Reproduction occurs in animals in two distinct ways.
When one individual organism donates all of its DNA to the
new offspring. This method is called asexual reproduction
and is typically reserved for simpler lifeforms where diversity
is less of a concern.
These creatures reproduce through mitosis and in essence
make exact clones of themselves. These organisms are not
able to adapt as quickly to changing environments since they
are all genetically identical.
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Sexual reproduction - meiosis and the creation of
sex cells. Such unions lead to a genetic blending of
the two parent organisms.
Genetic diversity follows such a union as the
offspring are not simply identical clones of either
parent. There tends to be a commonality in sexual
reproduction in that female sex cells are large and
immobile, while male sex cells are both small and
mobile.
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Humans invest a great deal of time and energy into rearing
offspring.
A typical bacterium, with unlimited resources for living, would
require only 36 hours to produce enough offspring to cover
the entire earth's surface 30 cm deep in bacteria!
Elephants, allowed to mate at will and with unlimited
resources, would require 750 years to produce a population
of 19 million beasts.
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Most primates - only one or two offspring in their lifetime;
great deal of effort and time into each and every one of them.
In the human species, males and females produce sex cells
inside sexual glands known as gonads. In males they are
called testes, and in females they are the ovaries. Testes
produce sperm, and ovaries produce eggs. Human males can
produce 100 to 650 million sperm per day throughout their
lifetime. Human males become fertile during puberty and
remain fertile until death. So throughout their lives, they are
capable of producing tremendous numbers of haploid sex
cells.
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Females are born with all of their sex cells already
produced, all 400,000 or so. Of those 400,000, only
400 of them will actually reach maturity. Once the
human female reaches puberty and is sexually mature,
she will usually ovulate one egg per month that is
capable of being fertilized.
Ovulation generally occurs for the first time during the
early teen years. Ovulation stops after 400 months
(about 33 years later).
When the two haploid sex cells, one from each parent,
unite in the process of fertilization, a diploid zygote is
produced. This zygote undergoes rapid mitosis and
passes into an embryonic stage, followed by a fetal
stage. Birth occurs approximately 266 days after
fertilization.
Male and female sex organs are
responsible not only for sexual
reproduction but also for liquid
waste excretion.
In the male, the testes are carried
externally on the body in a pouch
of skin called the scrotum
(descending into the scrotum
during the seventh month of
pregnancy).
By carrying the testicles outside of
the body, they are a few critical
degrees cooler than the rest of the
body. These slightly cooler
temperatures allow the testicle to
develop healthy, viable sperm.
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Sperm do not fully mature inside the testes but in a
tube just outside of the testes called the epididymus.
Mature sperm remain in this area until they are
ejaculated.
During sexual activity, the Cowper's gland secretes a
clear, alkaline substance designed to neutralize any
urine in the urethra.
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During ejaculation, sperm are forced out of the epididymus and into
the vas deferens, which connects to a duct emerging from the
seminal vesicles. This joining of ducts forms a short yet wider
ejaculatory duct. The seminal vesicles contribute seminal fluid to the
sperm, creating semen. Seminal fluids make up about 60% of the
volume of the total ejaculate.
This fluid consists of:
Fructose: a sugar that feeds the sperm
Prostoglandins: believed to cause small contractions within the
female to aid in fertilization
Alkaline fluids: to protect the sperm from the acidic environment
found in the vagina
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The semen continues on through the prostate gland. This gland
adds more nutrients to the semen, as well as an anticoagulant. From
here the semen passes through the Cowper's gland (which has
already lubricated and neutralized any acidity in the urethra). Once
the ejaculate has passed through the Cowper's gland, it travels
through the urethra and finally exits the body of the male.
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Inside the testes there are small tightly-coiled tubes known as
seminiferous tubules. (1/2 km of these)
Inside the tubules, sperm are created—a process known as
spermatogenesis. Cells in these tubules are constantly undergoing
mitosis, creating vast numbers of sperm.
Final preparations occur in the epididymus, a small holding area
consisting of tubes lining an area outside each testis. The process of
creating sperm takes 65 to 75 days (from spermatogonia to sperm).
At puberty, males begin secreting a hormone
called gonadotropin releasing hormone, or
GNRH. This hormone is released by the
hypothalamus in the brain. The GNRH trickles
down a small tube into the anterior pituitary.
The anterior pituitary responds by
releasing two more hormones: leutenizing
hormone (LH) and follicle stimulating
hormone (FSH). Both hormones are destined
for the testes.
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LH affects the interstitial cells to begin to produce
testosterone. Testosterone has many functions—increase sex
drive, promote hair growth in pubic regions, as well as
promote maturation in the penis and testes.
High levels of testosterone in the blood will cause the
hypothalamus to slow down its release of GNRH, setting up
an internal negative feedback loop that ultimately controls
how much more testosterone is released.
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FSH causes an increase in sperm production. Sperm
production is also increased by the presence of testosterone.
•Throughout a female's fertile years, the
ovaries produce a single egg each month.
• Eggs are released during ovulation.
Ovulated eggs travel down the fallopian tube,
also known as the oviduct. This is where
fertilization usually occurs.
• After fertilization, the zygote continues
down the fallopian tube until it reaches the
uterus. At the time of ovulation, there is an
abundance of blood lining the uterus. The
inner lining of the uterus is engorged with
blood in anticipation of the arrival of a
fertilized egg.
•The swollen inner area of the uterus is called
the endometrium. If the egg is fertile, it will
become embedded in the endometrium and
implant itself.
If an unfertilized egg arrives, the
endometrium will be shed within a
matter of days. This is the "bleeding"
of menstruation. The outer lining of
the uterus, the myometrium, is very
muscular and will contract, causing
cramping. This results in the inner
lining breaking free and being shed.
The contraction of the myometrium is
also instrumental during the process
of childbirth.
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At the lower end of the uterus is a small opening known as the
cervical canal, which is surrounded by the muscular cervix. This
muscular ring separates the uterus from the vagina and holds the
fetus inside the womb until birth. When the baby's head begins to
pass the cervix, the cervix is said to be dilated and can be measured
to determine how soon the baby is likely to be born. The urethra is
located between the vagina and the clitoris, and is responsible for
urinary waste excretion. The clitoris is located at the apex of the
labium minor and its primary function is for sexual stimulation. The
labium major are folds of skin which enclose the labium minor and
the vestibule. The labium minor are smaller folds of skin which also
enclose the vestibule. The vestibule in turn encloses the vaginal
opening and the urethra.
•The creation of an egg is referred to as oogenesis (remember
that the creation of sperm is spermatogenesis).
•In oogenesis, cell division is uneven. Spermatogenesis aims
to produce millions of sperm, while oogenesis aims to
produce just one egg.
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Less than 30% of all women have a menstrual
cycle that runs the so-called classic 28 days.
(most between 20 and 40 days.) This cycle has
four phases: the flow phase, the follicular phase,
ovulation and the luteal phase.
The flow phase marks the beginning of the
menstrual cycle. This phase is the only outwardly
visible sign of menstruation. During the flow
phase the egg is flushed out of the body due to
the muscular contractions of the outer wall of the
uterus. This phase lasts from four to six days,
and the intensity of the flow can vary greatly
from one month to the next.
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As soon as one egg has been flushed from the
uterus, preparations immediately begin for next
month's egg. This phase is known as the
follicular phase. Within the ovary, several follicles
begin to develop.
While several follicles may begin to develop, only
one will fully mature and ovulate. As the primary
follicle matures, the support cells produce
estrogen. This causes blood to gather in the
uterus (the creation of another endometrium).
The endometrium continues to grow for a week
or so.
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The follicle continues to develop and moves
toward the wall of the ovary. The follicle
becomes very large and eventually ruptures
through the wall of the ovary. This eruption
releases the oocyte into the fallopian tube.
This is ovulation. Ovulation generally occurs
on the day 14, midway through the menstrual
cycle.
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The remaining cells develop into a new structure called the corpus
luteum. This development marks the beginning of the luteal phase.
The corpus luteum manufactures two hormones—estrogen and
progesterone. The estrogen causes even more blood and nutrients
to gather in preparation for receiving the recently ovulated egg. The
second hormone, progesterone, causes several changes in the
woman's body. Initially it prohibits further ovulation so that no other
eggs can be released from the ovaries.
The corpus luteum maintains itself for about 10 days. After that
time, uterine contractions do begin—the endometrium, which is now
quite substantial in size, is flushed from the body. This marks the
end of the current menstrual cycle and the beginning of the next
one.