1. Learning Objectives
By the end of the lesson you should be able to:
Label a sperm and ovum
Describe a zygote
Explain what happens
after fertilization

2. Gametes – e.g. sperm or egg (ovum)
Gametes is the name given
to sex cells
Men produce sperm
Women produce eggs, Ova
Sperm are produced in great
numbers in the testes
(millions every day)
Ova mature one at a time in
the ovaries (takes 28 days)

3. Sperm cell
The sperm cell has 23
unpaired chromosomes from
the father, these are
contained in the nucleus (3)
The head has an enzyme
filled acrosome (1), this
breaks through the zona
pellucida of the ovum
The mid-piece (4) contains
mitochondria to provide
energy for the tail.
The tail (flagellum) (5)
thrashes to propel the sperm
forward

4. Ovum
This large structure has
plenty of nutrient rich yolk in
its cytoplasm
It’s nucleus is bigger but still
only contains 23
chromosomes from the
mother
The zona pellucida is a jelly
layer through which the
sperm must break to reach
the nucleus of the egg
There may also be old follicle
cells stuck to the egg

5. The zygote
Once a sperm has fused with
an egg it is known as a
zygote
In a human the 23 unpaired
chromosomes from the
sperm join with the 23
unpaired chromosomes in
the ovum
This makes the full human
complement of 46
chromosomes forming a
unique zygote.

6. Fertilization
Once the zygote is formed
fertilization is complete, it is
still not guaranteed that
pregnancy will take place.
If a zygote is formed the
corpus luteum will not
degenerate (why is this
useful?)
The zygote finds its way into
the uterus and implants
(pregnancy has begun)

7. First cell divisions
As the zygote travels along it
begins to divide.
The yolk makes this possible
The one becomes two, the
two become four etc.
By the time it reaches the
uterus the yolk is used up and
the zygote must implant
If the dividing zygote
completely splits at this early
stage identical twins are
formed.

8. First 3 months
The Progesterone and Oestrogen inhibit Follicle
Stimulating Hormone (FSH) production (why?)
Progesterone and oestrogen also cause the breasts to
enlarge
The corpus luteum finally degenerates after 3 months
By this time the placenta is fully formed (it takes over
the role of the corpus luteum in producing progesterone
and oestrogen)