Sexual reproduction
Sexual reproduction involves the joining of two sex cells (gametes) – the sperm (male gamete) and the egg/ovum (female gamete) to make a baby.
The male reproductive system
The male The organs and tissues involved in producing offspring. makes The male sex cell or gamete. and delivers it into the female reproductive system.
Sperm are specialised cells highly adapted for their function.
Their adaptations include:
A tail – called a flagellum – allowing it to swim through the female reproductive system.
A A cell that contains half the normal number of chromosomes. nucleus – containing half the normal number of The structure made of DNA that codes for all the characteristics of an organism..
Many The site of cell respiration. to produce energy.
The female reproductive system
The female reproductive system makes and releases eggs/ova.
It also protects and nourishes the foetus until birth.
Fertilisation and pregnancy
Fertilisation occurs in the A tube in the female reproductive organ through which an egg passes from an ovary to the uterus..
Fertilisation is the fusion of a haploid sperm nucleus and a haploid egg nucleus to form a diploid (normal chromosome number) zygote.
A zygote is the first cell of a new individual.
After fertilisation:
The zygote travels down the oviduct dividing by A type of cell division that produces cells that are genetically identical to the parent cell and to each other. to form a ball of cells (An organism in the early stages of development.).
Implantation occurs when the embryo reaches the Also known as a womb. This is where the fertilised egg (ovum) develops. and attaches to its thick lining to receive nourishment.
The The organ in the uterus of pregnant mammals that allows the transfer of nutrients and waste products between the mother and the foetus through the umbilical cord., The cord that connects the foetus to the placenta. It contains blood vessels., Bag surrounding the foetus in the uterus and containing the amniotic fluid. and The fluid surrounding a foetus that cushions it. form.
The embryo differentiates to produce a variety of tissues and organs.
The embryo is referred to as a foetus when all its organs start to form.
Placenta and umbilical cord
The placenta allows substances to diffuse from the mother’s blood to the foetus (eg oxygen and glucose).
Waste substances diffuse from the foetus to the mother’s blood (eg carbon dioxide and A nitrogenous waste product resulting from the breakdown of proteins. It is excreted in urine.).
The placenta is adapted for diffusion by having:
a large surface area for exchange between it and the uterus wall.
villi (finger like projections that extend into the uterus wall), which further increase the surface area of the placenta.
a good blood supply.
The umbilical cord attaches the placenta to the foetus. It contains the umbilical artery and the umbilical vein.
The umbilical artery carries urea and carbon dioxide from the foetus to the mother’s blood.
The umbilical vein carries oxygen and nutrients from the mother’s blood to the foetus.
Belonging to the mother. blood and Belonging to the foetus. blood systems are close together but not joined. They are separated by thin membranes that allow diffusion to occur.
Amnion and amniotic fluid
The amnion is a membrane that forms around the embryo.
Amniotic fluid surrounds the embryo inside the amnion, cushioning and protecting the developing embryo.
Sex hormones
The The plural form of testis, the male organ that produces sperm. produce Male sex hormone produced in the testes, which is responsible for puberty in boys..
The A pair of organs in the female reproductive system where ova (eggs) and hormones are produced. produce oestrogen.
These hormones cause secondary sexual characteristics to develop at The physical changes a child’s body goes through when developing into an adult..
| Male | Female | Both |
|---|---|---|
| Testes enlarge | Breasts develop | Body and pubic hair grows |
| Muscles develop | Hips widen | Growth spurt |
| Voice deepens | Menstruation begins | Sexual awareness and sex drive increase |
The menstrual cycle
Watch: Explaining the menstrual cycle
The menstrual cycle happens every month when the female body gets ready for a potential pregnancy.
It starts with menstruation, or a monthly period. The uterus, or womb, sheds its lining through the vagina as blood. This means that no fertilisation has taken place, so a new cycle begins.
Now, immature eggs, or ova, in the ovaries develop into mature eggs. The hormone oestrogen is made and released by the ovaries. It thickens the uterus lining ready for a possible fertilised egg and pregnancy.
On day 14, ovulation occurs. A mature egg is released from the ovary. It travels down the fallopian tube, or oviduct. This is the most fertile period of the cycle. If sperm is present in the oviduct, fertilisation can occur up to two to three days after ovulation.
From day 15, the hormone progesterone is released from the ovary. It maintains the thickness of the womb lining. This means that, if fertilisation takes place, the uterus is ready for the fertilised egg to embed itself and grow into an embryo.
If fertilisation doesn’t happen, the egg breaks down. This leads to a drop in oestrogen and progesterone levels. The body sheds the lining of the uterus and the cycle begins again.
Oestrogen and progesterone control the menstrual cycle. When levels are high, the womb lining thickens ready for fertilisation. When the levels drop, the uterus lining thins, ready to start a new cycle.
The menstrual cycle prepares the female reproductive system for pregnancy and it occurs from The physical changes a child’s body goes through when developing into an adult. until approximately age 50.
It is controlled by the hormones A female sex hormone produced in the ovaries, which is responsible for puberty in girls and the regulation of the menstrual cycle. and Sex hormone produced by the ovaries and placenta..
A 28 day cycle:
Days 1-5
Menstruation occurs – theAlso known as a womb. This is where the fertilised egg (ovum) develops. lining breaks down and is released through the A muscular tube that leads from the cervix to the outside of a woman's body..
Oestrogen levels are low.
Progesterone levels are low.
Days 6-13
- Oestrogen increases causing the initial repair of the uterus lining and its buildup.
Day 14
- The egg is released and this is called ovulation. It happens after oestrogen levels peak.
Days 15-28
Progesterone levels peak after ovulation to maintain the thick uterus lining.
If no pregnancy occurs, hormone levels decrease and the cycle restarts.
A woman can get pregnant if The male sex cell or gamete. are present in the oviduct a few days before or after ovulation (day 14), if it fertilises an egg.
Image caption, Days 1 - 5
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Infertility
What are the causes of female infertility?
A pair of organs in the female reproductive system where ova (eggs) and hormones are produced. don’t produce eggs.
Blocked/twisted A tube in the female reproductive organ through which an egg passes from an ovary to the uterus. (maybe due to infection).
Complication from a previous A sexually transmitted infection. (for example, chlamydia)
Also known as a womb. This is where the fertilised egg (ovum) develops. lining not developing enough to allow embryo In reproduction, when the fertilised egg attaches to, and embeds itself in the wall of the uterus..
A muscular tube that leads from the cervix to the outside of a woman's body. too hostile to allow The male sex cell or gamete. to enter (eg too acidic).
What are the causes of male infertility?
Not enough sperm produced.
Non-healthy sperm produced (can be due to smoking or drinking alcohol to excess).
Impotence (no erection).
What are the treatments for infertility?
Fertility drugs
Hormones are given to a woman to increase the production of eggs.
This may solve the problem but only if low egg production was the issue.
IVF – in vitro fertilisation
Process:
A woman is given fertility drugs to increase egg production.
Eggs are surgically collected from the A pair of organs in the female reproductive system where ova (eggs) and hormones are produced..
The male sex cell or gamete. is collected.
Sperm and egg cells are mixed to allow fertilisation (in vitro fertilisation).
The woman is given hormones to ensure the Also known as a womb. This is where the fertilised egg (ovum) develops. lining is thick.
An organism in the early stages of development. are placed into the uterus.
If successful an embryo/embryos will implant into the uterus lining – the woman is pregnant.
Usually only one or two embryos are placed into the uterus to give a balance between ensuring success and avoiding multiple births.
Before placing the embryos into the uterus, it is possible to genetically screen them for abnormalities or check their gender.
Explaining the IVF process
Infertility can affect both men and women.
It means they cannot have children naturally.
It can be caused by a variety of factors.
These include hormone imbalances, women who don't produce eggs or have blocked fallopian tubes, and men who have low sperm counts.
IVF stands for in vitro fertilisation.
IVF starts with hormone injections which stimulate a woman's ovaries to produce multiple eggs.
These eggs are collected from the ovaries and taken to a lab.
Here, the eggs are mixed with sperm to fertilise them.
This happens in vitro, which means outside the body in a test tube rather than inside the woman's fallopian tube.
The next step is for the fertilised eggs, or zygotes, to divide by mitosis into a ball of cells that forms embryos.
Once that's happened, two or three healthy embryos are transferred into the uterus to give the greatest chance of pregnancy.
Once an embryo implants, the pregnancy continues naturally.
The development of fertility treatments like IVF has made a difference to many people's lives, helping them to overcome infertility and have children.
Contraception
There are mechanical, chemical, surgical and natural contraceptive methods used to prevent a pregnancy.
Mechanical
| Example | Method | Advantage | Disadvantage |
|---|---|---|---|
| Male condom | A barrier that prevents sperm entering the vagina | Easily obtained. Protects against STIs (eg HIV) | Unreliable if not used properly. |
| Female condom | A barrier that prevents sperm passing up the female reproductive system | Protects against STIs (eg HIV) | Unreliable if not used properly |
Chemical
| Example | Method | Advantage | Disadvantage |
|---|---|---|---|
| Contraceptive pill | Tablet taken every day by the female. Stops development of egg by changing hormone levels so prevents ovulation. | Very reliable. Easy to reverse. | Female needs to remember to take the pill. Side effects include weight gain, mood swings or an increased risk of blood clots. Does not protect against STIs. |
| Contraceptive implant | A small tube placed under the skin of the upper arm releases hormones slowly, preventing ovulation by changing hormone levels. | Very reliable. Easy to reverse. Can work for up to three years. | Does not protect against STIs. Can prevent menstruation. |
Surgical
| Example | Method | Advantage | Disadvantage |
|---|---|---|---|
| Vasectomy | Sperm tubes are cut preventing sperm from leaving the testes. | Virtually 100% reliable. | Difficult or impossible to reverse. |
| Female sterilisation | Oviducts are cut preventing the sperm reaching the egg. | Virtually 100% reliable. | Difficult or impossible to reverse. |
Using contraception can raise ethical issues for some people.
| For contraception | Against contraception |
|---|---|
| Prevents unwanted pregnancies | Interferes with natural processes |
| Aids family planning | Religious or moral beliefs may oppose its use |
Watch a video
Find out more about sexual reproduction in animals
Sexual reproduction in animals involves the fusing together of male and female sex cells, called gametes. This is fertilisation.
Gametes in females are called eggs and are produced in the ovaries.
In human females an egg is usually released from an ovary every 28 days. This is called ovulation.
The egg will travel down the oviduct towards the uterus.
Male gametes are called sperm. They're produced in the testes continuously.
Sperm leave the body by travelling through the sperm duct into the urethra and out through the end of the penis.
The fertilisation process begins with sexual intercourse.
During sex, sperm is ejaculated into the vagina and travel through the cervix and urethra until they reach the oviduct.
This is no easy journey for the sperm and only a few will survive.
If an egg is present in the oviduct, fertilisation may occur.
This happens when the nuclei of the egg and sperm fuse. Human eggs and sperm each contain 23 chromosomes.
When they come together they form a single cell called a zygote, which contains a full set of 46 chromosomes - 23 from each parent.
The zygote then divides over and over, developing into a baby in the uterus.
From that single act of sexual intercourse at the start of the fertilisation process, the male and female gametes fuse, creating a zygote, that should become a human baby in around 40 weeks time.
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Human reproduction interactive
Play this game to explore human reproduction and the stages of pregnancy.
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