The document outlines the human reproductive system, explaining the formation and function of male and female gametes, the process of fertilization, and the subsequent development of the embryo and placenta. It describes the anatomical structures involved in reproduction, including ovaries, testes, and the uterus, as well as the physiological processes from ovulation to implantation. Additionally, it highlights the significance of the placenta and amniotic sac in supporting fetal development during pregnancy.

1. REPRODUCTION IN HUMANS

2. VIDEO LINKS
• https://www.youtube.com/watch?v=-ekRRuSa_UQ (introduction)
• https://www.youtube.com/watch?v=5EvnCzOpy2o (fertilization till 1:34)

3. THE HUMAN REPRODUCTIVE SYSTEM
• Humans like all mammals, reproduce sexually
• A new life begins when the nucleus of a male gamete fuses
with a female one, forming a zygote
• This is how every human being is formed

4. THE FEMALE REPRODUCTIVE ORGANS

5. THE FEMALE REPRODUCTIVE ORGANS
• The female gametes called eggs or egg cells, are made in the
two ovaries
• Leading away from the ovaries are the oviducts (also called
Fallopian tubes)
• The tubes do not connect directly to the ovaries but have a
funnel shaped opening just a short distance away
• The two oviducts lead to the womb or the uterus
• Uterus has very thick walls, made of muscle

6. THE FEMALE REPRODUCTIVE ORGANS
• Uterus is very small – only about the size of a clenched fist, but
it can stretch a great deal when a woman is pregnant
• At the base of the uterus is a narrow opening, guarded by
muscles
• This is the neck of the uterus, or the cervix
• The narrow opening connects to the vagina, which then leads
to the outside of the body

7. THE FEMALE REPRODUCTIVE ORGANS
• The opening from the bladder, called the urethra, is a tube
that is in front of the vagina
• While the rectum is just behind the vagina
• These three tubes open quite separately to the outside

8. THE MALE REPRODUCTIVE ORGANS
• The male gametes called spermatozoa or sperm are made in two
testes
• Testes are outside the body in two sacs of skin called the scrotum
• The sperm are carried away from each testes in a tube called the
sperm duct (vas deferens)
• The sperm ducts from the testes join up with the urethra just
below the bladder
• The urethra continues downwards and opens at the tip of the
penis
*vas deferens = sperm duct

9. THE MALE REPRODUCTIVE ORGANS
• The urethra can carry both urine and sperm at different times
• When the sperm ducts join the urethra, there is a gland called the
prostate gland
• Prostate gland makes a fluid called semen, which the sperm swim
in
*vas deferens = sperm duct

10. THE MALE REPRODUCTIVE ORGANS

11. HUMAN GAMETES
• Eggs begin to be formed inside a girl’s ovaries before she is born
• At birth, she will already have thousands of partly developed eggs inside her ovaries
• When she reaches puberty, some of these eggs begin to mature
• Usually, only one develops at a time

12. HUMAN GAMETES
• An egg is a single cell
• When it is fully developed, it bursts out of the ovary and into the funnel
at the end of the oviduct
• This is called ovulation
• In humans, it happens once a month throughout a woman’s reproductive
cycle

13. HUMAN GAMETES
• Figure shows a section through a testis
• It contains thousands of very narrow, coiled tubes or tubules
• These are where the sperm are made
• Sperm are then stored in the epididymis
• Sperm develop from cells in the walls of the tubules
• Sperm are continually made from puberty onwards

14. HUMAN GAMETES
• Sperm production is very sensitive to heat
• If they get too hot, the cells in the tubules will not develop into sperm
• This is why the testes are positioned outside the body, where they are
cooler then they would be inside

15. Figure shows the structure of a sperm and how it is adapted for its function of fertilizing an egg

16. HUMAN GAMETES
• Eggs and sperm are haploid cells
• Each human egg and each human sperm has a single set of 23 chromosomes in its nucleus
• Eggs and sperm are made when cells in the ovaries and testes divide by meiosis

17. COMPARING MALE AND FEMALE GAMETES
• An egg cell usually millions of times larger than a sperm cell
• Egg cell and sperm cells differ greatly in their structures (as already seen)
• Usually one egg cell is released during one reproductive cycle of a woman, while millions of sperms are
released at once in a man
• Egg cells are non-motile while sperm cells are motile

18. FERTILISATION
• After ovulation, the egg is caught in the funnel of the oviduct
• Very slowly, the egg travels towards the uterus
• Cilia lining the oviduct help it to sweep along
• Muscles in the walls of the oviduct also help to move it, producing gentle
rippling movements

19. Movement of egg from the oviduct into the uterus

20. FERTILISATION
• If the egg is not fertilized by a sperm within 8 to 24 hours after ovulation, it
dies
• By this time it has only travelled a short way along the oviduct
• This means that a sperm must reach an egg while it is quite near the top of
the oviduct if fertilization is to be successful

21. FERTILISATION
• To bring the sperm as close as possible to the egg, the man’s penis is
placed inside the vagina of the woman
• Sperm travel out of the penis into the vagina
• This happens when muscles in the walls of the tubes containing the sperm
contract rhythmically
• The wave of contraction begins in the testes, travels along the sperm ducts
and into the penis
• The sperm are squeezed along the tubes, out of the man’s urethra and into
the woman’s vagina

22. FERTILISATION
• The fluid containing the sperm is called semen
• The liquid part of the semen is produced by the prostate gland, and it
contains sugars and other nutrients to provide the sperm with energy
• The semen is deposited at the top of the vagina, near the cervix
• The sperm are still quite a long way from the egg
• They swim (using their flagella and energy released in their mitochondria)
up through the cervix, through the uterus and into the oviduct

24. FERTILISATION
• Sperm can only swim at a rate of about 4 mm per minute, so it takes
quite a while for them to get as far as the oviducts
• Many will never get there at all
• But about a million sperm are deposited in the vagina at once,
so there is a good chance that some of them will reach the egg

25. FERTILISATION
• When a sperm cell contacts the jelly coat surrounding an egg cell, the
acrosome is activated
• It releases its digestive enzymes, and these enzymes digest the jelly coat
• This allows the head of the sperm to push through and get into the cytoplasm
of the egg
• Only the head of the sperm goes in; the flagellum is left outside
• The nucleus of the sperm fuses with the nucleus of the egg
• This is fertilisation

26. FERTILISATION
• As soon as the successful sperm enters the egg, the egg membrane and
jelly coat become impermeable, so that no other sperm can get in
• The unsuccessful sperm will all die

27. A sperm as it makes its way into and egg cell

29. IMPLANTATION
• When the sperm nucleus and the egg nucleus have
fused together, they form a zygote
• The zygote continues to move down the oviduct
• As it goes, it divides repeatedly
• After several hours, it has formed a ball of cells called
the embryo
• The embryo obtains nutrients from the yolk of the
egg

30. IMPLANTATION
• It takes several hours for the embryo to reach the uterus, and by this time it is a ball of 16 or 32 cells
• The uterus has a thick, spongy lining, and the embryo sinks into it
• This is called implantation

31. THE PLACENTA
• The cells in the embryo, now buried in the soft wall of the uterus, continue
to divide
• As the embryo grows, a placenta also grows, which connects it to the wall of
the uterus
• The placenta is soft, dark red, and has finger-like projections called
villi
• The villi fit closely into the uterus wall
• The placenta is where substances are exchanged between the mother’s
blood and the embryo’s blood
• It is the embryo’s life support system

32. THE PLACENTA
• After 11 weeks, the embryo has developed most of its organs, and is
now called a fetus
• The placenta is joined to the fetus by the umbilical cord
• Inside the cord are two arteries and a vein
• The arteries take blood from the fetus into the placenta, and
the vein returns the blood to the fetus

33. THE PLACENTA
• The placenta contains capillaries filled with the fetus’s blood
• The lining of the uterus contains large spaces filled with the
mother’s blood
• The fetus’s blood and the mother’s blood do not mix
• They are separated by the wall of the placenta
• But they are brought very close together, because the wall of the
placenta is very thin

34. THE PLACENTA
• Oxygen and dissolved nutrients in the mother’s blood diffuse
across the placenta into the fetus’s blood and are then carried
along the umbilical cord to the fetus
• Carbon dioxide and other excretory products diffuse in the
other direction and are carried away in the mother’s blood
• Some unwanted substances can also cross the placenta, these
include toxins (alcohol, carbon monoxide)
• Some pathogens such as the virus that causes rubella, can also
pass across the placenta

35. THE PLACENTA
• The arteries in the uterus lining, dissolve and release their blood, forming a
pool of blood
• Hence, the mother’s blood found in the uterus lining is no longer enclosed in
blood vessels
• This reduces one membrane that the materials from the mother’s blood have
to pass through, to the reach the fetus’s blood

36. THE PLACENTA
• As the fetus grows, the placenta grows too
• By the time the baby is born, the placenta is a
flat disc
• It is, at that time, about 12 cm in diameter and 3
cm thick

37. THE AMNIOTIC SAC
• The fetus is surrounded by a strong membrane, called the
amniotic sac
• This makes a liquid called the amniotic fluid
• Amniotic fluid helps to support the embryo, and to
protect it from mechanical injury
• The embryo floats in the fluid and is able to freely move its
arms and legs, which helps the muscle and skeleton to
develop correctly

38. THE AMNIOTIC SAC
• After nine months, when the fetus is ready to be born,
contractions of muscles in the uterus wall often cause the
amniotic sac to break
• This releases the amniotic fluid which flows into the vagina
• This is often the first sign that a woman is about to give
birth

40. • Mother’s blood to the embryo’s blood: oxygen, glucose, amino acids, fatty
acids, glycerol, minerals and water
• Embryo’s blood to the mother’s blood: carbon dioxide and urea

41. • https://www.youtube.com/watch?v=pX55X9au6zQ (video shows a very rare condition in which a baby is shown
born through a C-section with the amniotic sac still intact)
• https://www.youtube.com/watch?v=izOa3-AX8zQ (how a wonder is born) A very detailed video with amazing
animations! You must watch this till the end and focus on the details that you have covered in class.