As in all other organisms reproduction is the formation of new individuals of
the same species. Sexual
reproduction is the only method of reproduction in our species.
Sexual reproduction involves the fusion
of specialised haploid sex cells. The fusion of
sperm and egg cell is called fertilisation. Fertilisation results in the formation of a diploid zygote from which a new
individual develops.
Reproductive Structures
Both the male and female reproductive
structures have 3 levels of organisation:
1.
Production
of sex cells.
2.
Transport
tubes.
3.
Glands
to secrete hormones.
The Male
Reproductive System
The gonad is the name for the
organ that produces sex cells in organs. The male gonads are called the testes.
The testes are contained in
the scrotum.
The testes
produce the sperm cells by meiosis.
The temperature must be lower than body temperature for this to occur. There
are tubules that are lines with sperm producing cells. Testosterone, the male sex hormone, is also produced in the testes.
Once the sperm are produced they mature in the epidymis. This structure is located outside of the testis. If they
are not released within about 6 weeks they are broken down and released to the
bloodstream by a process called resorption.
The sperm are carried to the urethra
by the sperm duct. The urethra carries both sperm and urine.
The sperm cells
are carried within a liquid called semen.
The semen is produced by the seminal
vescicles, the
prostate gland, and Cowper’s glands. The semen also contains nourishment for the sperm
cells.
Sperm cells are
released by ejaculation. About
50-300 million sperm cells are released at one time.
Sperm cells,
also called spermatozoa, are haploid containing 23 chromosomes. Their production begins at puberty.
The penis is adapted to place sperm cells
into the female. The tip is called the glans.
Erection occurs when blood rushes
into the penis.
THE STRUCTURE OF THE SPERM
CELL
Watch an animation about
sperm production.
Take some online quizzes on the male reproductive
parts.
MALE HORMONES
Male hormones
are produced by the pituitary gland during puberty. They are:
- FSH- Follicle Stimulating Hormone: This causes the production of
sperm by meiosis.
- LH- Leuteinising Hormone: Stimulates the testes to produce testosterone.
TESTOSTERONE
During the
period of pregnancy testosterone
causes the development of primary male sex characteristics. These include the development of the penis and the other male
reproductive parts.
Later in life, at
puberty, testosterone causes the enlargement of the reproductive parts as well
as the development of secondary sexual
characteristics. These are characteristics that distinguish males from
females.
Male secondary
sexual characteristics included:
- hair growth on the face, underarm, chest and
pubic region
2. enlarged larynx
producing a deeper voice
3. wider shoulders
4. greater skeletal
muscular development
5. growth in height and
weight
MALE INFERTILITY
The most common cause of male
infertility is the low production of sperm. There are many causes of low sperm
production. Stress, alcohol and drug abuse, high temperature of the testes, and
low testosterone production are all causes.
THE FEMALE REPRODUCTIVE SYSTEM
OVARIES
Ovaries produce
eggs and female hormones. At puberty there are about 40,000 diploid eggs. Each egg is enclosed in a
group of cells called a follicle. About
20 haploid eggs are produced each
month. Usually all but one die. The haploid egg cell is called the ovum and is surrounded by the Graafian follicle which produces the
female hormone called oestrogen. Ovulation is the release of the egg from the
follicle. This occurs when the follicle
bursts.
FALLOPIAN TUBES
The
fallopian tubes are about 12 cm long and have ends that are funnel shaped.
These ends collect the egg after ovulation. Cilia and peristalsis move the egg
along the tube. The egg will die in the tube if it is not fertilised.
UTERUS
The uterus,
also known as the womb, is made of
involuntary muscle. It is lines with the endometrium.
This lining thickens with cells and blood every month. This happens in order to
nourish the embryo (if present). The opening of the uterus is called the cervix.
VAGINA
The vagina is a muscular tube which
allows the sperm to enter the female as well as the baby to exit. It is lined
with mucous secreting cells. The uretrhra opens near the vagina. The
vagina is protected by folds of skin called the vulva. The hymen
partially blocks the entrance of the vagina. It is broken by sexual intercourse or with the use of tampons.
Take some short online quizzes on the female reproductive parts:
THE MENSTRUAL CYCLE
The menstrual cycle occurs every 28 days from puberty to menopause (the end of the female’s
reproductive life). It occurs only if fertilisation of the egg has not taken
place.
The typical events of the menstrual cycle are:
Day 1 to day 5-
a.
The
endometrium breaks down and is shed
from the body. This is called menstruation.
b.
Meiosis occus in the ovary to produce a new
egg surrounded by the Graafian follicle.
Day 6 to day 13-
a.
Oestrogen is produced by the Graafian follicle. Oestrogen also
stimulates the endometrium to thicken again.
One Graafian follicle with one egg develops.
b.
Oestrogen
stimulates the production of LH (leuteinising
hormone)
Day 14-
a.
The
surge of LH stimulates ovulation.
b.
The
egg enters the funnel of the Fallopian tube. It can be fertilised for the next
48 hours.
Day 15 to day 26-
a.
The
corpus luteum (yellow body) develops
from the remains of the Graafian follicle. This produced progesterone and some oestrogen. The progesterone causes the
endometrium to continue to thicken. It also prevents new eggs from forming.
b.
The
egg that was released at day 14 will die if it is not fertilised.
c.
If
fertilisation did not take place the corpus luteum begins to degenerate.
Day 26 to day 28-
a.
Oestrogen
and progesterone levels decline.
b.
The
endometrium begins to break down.
c.
Day one of the cycle begins.
Watch
a Disney Film about menstruation.
Click here to see an
animation of the menstrual cycle.
Click here to
see another animation of the menstrual cycle.
Click
here to see another animation of the menstrual cycle
Watch an animation about
egg production.
Take
an online quiz on the menstrual cycle.
FEMALE HORMONES
In summary:
Endometrium thickened by oestrogen in days
1-14 and by progesterone in days
15-28.
Both prevent egg development.
At puberty, oestrogen causes the primary
female sexual characteristics of the growth of the sex organs.
At puberty both oestrogen and
progesterone cause the secondary
female characteristics. They include:
a.
The
enlargement of the breasts
b.
Widening
of the hips
c.
Increased
body fat
d.
Growth
of public and underarm hair
e.
General
growth spurt in height
|
HORMONE |
SITE OF PRODUCTION |
TIME OF PRODUCTION |
FUNCTIONS |
|
FSH-follicle stimulating hormone |
Pituitary Gland |
Days 1-5 of menstrual cycle |
Stimulates egg production within
Graafian follicles. Sometimes used in fertility
treatment to stimulate egg production. Graafian follicles secrete
oestrogen. |
|
Oestrogen |
Graafian follicle |
Days 5-14 of menstrual cycle |
Development of endometrium. Inhibits FSH so no new eggs
develop. Stimulates the release of LH
(luteinising hormone). |
|
LH- leuteinising hormone |
Pituitary Gland |
Day 14 of menstrual cycle |
Causes ovulation. Causes Graafian follicle to develop
into the corpus luteum. The corpus luteum makes progesterone. |
|
Progesterone |
Corpus luteum |
Days 14-28 of menstrual cycle |
Maintains endometrium. Inhibits FSH so no new eggs
develop. Inhibits LH so no new ovulations
occur. Prevents contractions of the uterus. |
FEMALE INFERTILITY
Female infertility is the inability to conceive either by
fertilisation failure or implantation failure. Egg cell formation or ovulation
may not occur due to a hormone imbalance.
The egg cell may not be able pass to the uterus due to blockage of the Fallopian tubes. Treatment with hormones may be successful. In-vitro fertilisation and implantation is often used to treat
female infertility.
FIBROIDS
Fibroids are benign tumours of the
uterus. They are slow growing and range in size. Small fibroids produce no
symptoms while large ones can cause heavy and prolonged menstrual bleeding.
They can also cause pain, miscarriage, or infertility. Some science shows that
they may be caused as an abnormal response to oestrogen. Large fibroids are
removed by surgery. In severe cases where there are many large fibroids the
uterus may have to be removed. This is called a hysterectomy.
COPULATION
Copulation is also called coitus or sexual intercourse.
During this process the penis moves into the vagina in order to deposit semen
which contains sperm cells. The
depositing of the semen is called insemination.
FERTILISATION
a. After insemination the sperm will
move up the Fallopian tubes.
b. If ovulation has occurred and an egg is present the egg will release a
chemical that attracts the sperm. This is called chemotaxix.
c. The sperm that reaches the egg will
use an enzyme in its acrosomes to make an opening in the
membrane of the egg.
d. Once one sperm enters the egg
(only the head enters) the egg forms a membrane
that prevents other sperm from entering.
e. The nucleus of the egg fuses with the
nucleus of the egg. A diploid zygote forms.
f. Fertilisation may take place during
days 11-16 of the menstrual cycle.
EVENTS OF FERTILISATION
Watch a video about
conception.
Watch a video about
fertilisation.
IMPLANTATION
About 6-9 days after fertilisation
the fertilised egg becomes embedded into
the lining of the uterus. The zygote
has now become an embryo. A membrane
called the amnion develops around
the ebbryo. This membrane will secrete amnion
fluid which surrounds and protects the embryo.
Watch an animation about
amniotic fluid.
PLACENTA FORMATION
a.
After
implantation the embryo forms another membrane called the chorion. This surrounds the embryo.
b.
Projections
of the chorion called villi join with blood
vessels in the endometrium to form
the placenta.
c.
The
placenta become fully functional in about 3 months.
d.
The
umbilical cord connects the embryo
(at the navel) with the placenta.
Watch
a video about placenta formation.
FUNCTIONS OF THE PLACENTA
1. Protection of the Embryo:
A. It hinders the entry
of pathogens from the mother.
B. It allows the entry
of antibodies from the mother (passive induced immunity).
C. It keeps the embryo
separated from the mother’s higher blood pressure.
D. It prevents exchange
of red blood cells avoiding the deadly possibility of agglutination
2. Gas Exchange:
a.
It supplies O2 from the mother.
B. It excretes CO2 from the embryo to the mother’s blood.
3. Nutrient Supply:
Glucose,
amino acids, lipids, vitamins and minerals pass to the embryo from the mother’s
blood.
4. Endocrine:
It secretes
a variety of hormones including oestrogen and progesterone. The hormones
maintain the pregnancy and prepare the mother’s body for birth and lactation.
e.
Excretion:
Metabolic
wastes, CO2 and urea, pass from embryo into the mother’s blood.
EARLY DEVELOPMET OF THE ZYGOTE
(higher level)
1. The zygote divides many times by cleavage (increase in the number of
cells by division but no overall increase in size) to double its cell
number. A solid clump of about 100 cells
called the morula is formed.
2. About 5 days after fertilisation the morula
develops into a hollow ball called a blastocyst.
The outer cells of the blastocyst form the trophoplast.
This will become the membranes around the embryo. The inner cells, called
the inner mass will become the embryo.
3. The blastocyst is pushed down the fallopian
tube and into the uterus for implantation.
|
|
In normal development
the sperm fertilizes the egg. The cortical reaction occurs raising the
fertilization membrane and cell divisions occur until the blastula stage.
When the embryo reaches the blastula stage the embryo releases an enzyme that
dissolves the fertilization membrane and the young embryo swims free to
continue development. |
DEVELOPMET
OF THE EMBRYO (higher level)
The
mesoderm cells further develop into the muscles and blood, the endoderm
develops into the digestive tract and lungs, and the ectoderm develops into the
skin, nerves and brain.
WEEK TO WEEK DEVELOPMENT
|
4 WEEKS AFTER FERTILISATION |
1. HEART FORMSAND STARTS TO BEAT. 2. BRAIN DEVELOPS 3. UMBILICAL CORD FORMS. |
|
5 WEEKS AFTER FERLILISATION |
1. INTERNAL ORGANS START TO FORM. 2. LIMBS START TO FORM. (EMBRYO
HIGHLY VULNERABLE TO ALCOHOL AND DRUGS) |
|
6 WEEKS AFTER FERLILISATION |
1. EYES BECOME VISIBLE. 2. MOUTH, NOSE AND EARS BEGIN TO
FORM. |
|
8 WEEKS AFTER FERLILISATION |
1. TAIL
IS GONE. 2. FACE
BECOMES HUMAN LOOKING. 3. MAJOR
ORGANS ARE FORMED. 4. OVARIES
OR TESTES ARE SEEN. 5. BONE REPLACES CARTILAGE. 6. THE EMBRYO IS NOW CALLED A
FOETUS. |
|
12 WEEKS AFTERFERTILISATION |
1. BONE GROWTH CONTINUES TO REPLACE
CARTILAGE. 2. NERVES AND MUSCLES COORDINATE
ARM AND LEG MOVEMENT. 3. THUMB SUCKING AND KICKING BEGIN. 4. MILK (BABY) TEETH FORM. 5. FOETUS TAKES AMNIOTIC FLUID INTO
MOUTH AND RELEASES URINE AND
FAECES INTO AMNIOTIC FLUID. 6. EXTERNAL SEX ORGANS CLEARLY
SEEN. GENDER CAN BE DETERMINED WITH A SCAN. |
The Gestation
period is the length of time from fertilisation to birth. In humans it is
generally 266 days (38 weeks/9months).
Watch an animation of
embryo development.
Watch
an animation of RISKS associated with embryo development.
Watch an animation about
the formation of twins.
Watch an animation
showing natural birth.
Watch an animation about
Cesarean section
Watch
a video about the stages of pregnancy.
Watch
a video about the determination of the sex of a baby.
Watch a
You Tube Video about embryo development.
BIRTH
1. The placenta
stops producing progesterone. This
causes the walls of the uterus to
contract.
2. Oxytocin is produced by
the pituitary gland. This hormone
causes contractions of the uterine
muscle. This is the beginning of labour.
3. There are 3
stages of labour:
a. Stage 1: The
contractions push the foetus down toward the cervix. The membrane around the foetus (amnion) breaks. The amniotic fluid is released through the vagina.
b. Stage 2: The cervix dilates (widens) and the foetus
is pushed out through the cervix and vagina. At this time the umbilical cord is
cut.
c. Stage 3: The placenta and foetal
membrane (afterbirth) are released
through the vagina.
LACTATION
Lactation is the secretion of milk by the mammary glands of the mother. Colostrum is a thick yellow fluid
produced during the first few days. It is low in fat and sugar but rich in minerals, protein, and antibodies.
Prolactin is a hormone produced by the
pituitary gland. This hormone stimulates
milk production. The suckling of a baby at the breast stimulates the
mother’s pituitary to release prolactin. When breast feeding stops the mother stops
secreting prolactin and therefore stops producing milk. Suckling also
stimulates the pituitary to secrete oxytocin. Oxytocin causes the milk ducts to
contract ejecting the milk from the breast.
BENEFITS OF BREAST FEEDING
Human milk
has a lot of advantages for the baby’s growth and development. Human milk is nutritionally balanced for a developing
human baby. It also contains a wide variety of beneficial chemicals that include mother’s antibodies. These antibodies protect the child against common
pathogens. Human milk also contains chemicals favourable for brain growth and development. Human milk also
encourages the growth of mutualistic
bacteria in the large intestine.
CONTRACEPTION
Contraception is the deliberate prevention of fertilisation.
There are 4 methods:
1. Natural
contraception:
` a. Abstinence: No sexual intercourse. 0%
failure rate.
b. Temporary
abstinence: No sexual intercourse during the most fertile period of the
menstrual cycle. 24% failure rate.
c. Withdrawal: The penis is removed from
the vagina before ejaculation. 25% failure rate.
2. Mechanical contraception:
a. Condom: This is a thin
impermeable sheath covering the penis. Semen cannot be deposited in the vagina.
10% failure rate.
b. Vasectomy: The cutting, sealing or tying off the sperm ducts. Sperm
will not be in the semen. 0.4% failure.
c. Cervix Barriers: diaphragm or cap – These
prevent sperm entering the uterus. 15% failure rate.
d. Intrauterine Devices (IUD): These are plastic or metal loops or
coils that prevent implantation. 5% failure rate.
3. Chemical contraception:
a. Spermicides: These chemicals are placed in the vagina to kill
sperm. 20% failure rate.
b. Oral contraceptive pill: Contain hormones such as
progesterone and oestrogen that prevent ovulation as the follicles do not
mature. 6% failure rate.
4. Surgical
contraception:
a. Tubal ligation: The Fallopian tubes are cut and sealed preventing
sperm and egg cells meeting. 0.4% failure rate.
b. Vasectomy:
The vas deferens (sperm duct) is cut. Sperm is not present
in the semen.
The % failure rate is the number of sexually active females out of a hundred who will become pregnant within a year.