Oogenesis

Oogenesis is the production of female gametes (ova / egg cells) in the ovaries of a
female. It occurs when the diploid germinal epithelium of the ovaries starts to
produce follicles by mitosis.
Oogenesis takes place as follows:
 The diploid germinal epithelium cells (2n) of the ovaries go through the
process of mitosis to form many follicles
 Every 28 days, the follicle stimulating hormone (FSH) stimulates one follicle.
Only one cell inside of that follicle enlarges and goes through the process of
meiosis
 Out of the 4 (four) haploid cells produced through meiosis, only one cell will
survive to form a mature ovum
 The other three cells from meiosis will degenerate

Figure 6 shows the structure of a human egg (ovum). Each ovum is made up of
follicle cells, a layer of jelly, cytoplasm and a haploid nucleus.

follicle cells

nucleus

cytoplasm

layer of jelly

Figure 6: The structure of an ovum

 The nucleus contains 22 autosomes and one sex chromosome (X)

 The cytoplasm nourishes the egg
 The jelly layer provides protection for the early developmental stages of the
fertilised egg

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The structure of sperm and egg cells:
https://www.youtube.com/watch?v=CuxaXghfyeE&list=PLW0gavSzhMlQYSpKryVcE
r3ERup5SxHl0&index=43

Activity 2: Gametogenesis
1. Name the organ where meiosis takes place in the male and female
reproductive systems respectively. (2)
2. Define gametogenesis. (1)
3. Name the type of gametogenesis that takes place in the male and female
reproductive systems respectively. (2)
4. Draw a fully labelled diagram of an ovum. (5)
5. Discuss the functions of the four main parts of a sperm cell. (8)
(18)

The menstrual cycle

Key terminology
mature follicle inside the ovary filled with fluid in which the
Graafian follicle
ovum grows
ovulation the release of an ovum from the Graafian follicle of the ovaries
endometrium the inner lining of the uterus wall
the monthly loss of blood and tissue as a result of changes
menstruation
that occur in the lining of the uterus
stage in the life of a woman when she stops ovulating and
menopause
menstruating; usually occurs between the ages of 45 and 55
the fusion of the haploid sperm cell nucleus and the haploid
fertilisation
egg cell nucleus to form a diploid nucleus of the zygote
the attachment of the embryo to the endometrium lining the
implantation
uterus

The menstrual cycle refers to changes that occur in the ovaries and uterus of a
female over a period of 28 days. This cycle begins at puberty and ends at
menopause.

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The menstrual cycle is made up of two separate cycles that happen at the same
time:
1. Ovarian cycle
2. Uterine cycle

1. Ovarian cycle

The ovarian cycle refers to the development and release of an ovum (or egg cell).
This takes place inside the ovary. The ovarian cycle begins when FSH (Follicle
Stimulating Hormone) is secreted by the pituitary gland. FSH is transported to the
ovary by the blood. The following diagram (Figure 7) illustrates the ovarian cycle.

growing ovarian follicle ovum corpus luteum

Graafian follicle ovulation

Figure 7: The ovarian cycle

1. FSH stimulates a primary follicle to become a Graafian follicle which

contains a mature ovum (or egg cell).
2. As the Graafian follicle develops, it produces the hormone oestrogen,
increasing the oestrogen levels in the blood.
3. Around Day 14, the Graafian follicle ruptures and releases an ovum in a
process called ovulation. Ovulation is stimulated by the Luteinising Hormone
(LH) which is released by the pituitary gland.
4. LH causes the ruptured Graafian follicle to change into a structure called the
corpus luteum. The corpus luteum secretes the hormone progesterone
increasing the levels of progesterone in the blood.

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 5. If fertilisation does not take place, the corpus luteum shrinks and stops
producing progesterone. The ovum passes down the fallopian tube, enters the
uterus and leaves the body through menstruation.

NOTE:
 If the ovum is fertilised, the corpus luteum remains active and continues
secreting progesterone
 Oestrogen and progesterone produced by the ovaries during the ovarian
cycle influence the uterine cycle

2. Uterine cycle

The Uterine cycle shows the changes that occur in the uterus wall as it gradually
thickens and becomes more vascular (richly supplied with blood vessels) over a
period of 28 days.

3
4
1
2
endometrium

Day 1 Day 14 Day 28

Figure 8: Illustrating the changes to the endometrium

1. The endometrium breaks down and is released (menstruation). This lasts for
approximately 4 to 7 days.
2. The endometrium is stimulated by oestrogen to become thicker and develop
more blood vessels and glands.
3. Progesterone stimulates the endometrium to become even thicker and
develop more blood vessels and glands. This happens in preparation for
possible implantation of the fertilised ovum.
4. If fertilisation does not take place, the endometrium tears away resulting in
menstruation.

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Figure 9 and Table 4 below summarise the changes that occur in the ovarian and
uterine cycles over a period of 28 days (the menstrual cycle)

Figure 9: The menstrual cycle

Table 4: A summary of changes in the ovarian and uterine cycles

Days Ovaries Uterus
new follicles develop and lining breaks down and is released
1–7
secrete oestrogen (menstruation)
mature Graafian follicle oestrogen stimulates the
8 – 13 develops and secretes endometrium to become thicker,
oestrogen more glandular and vascular
Graafian follicle bursts to
14
release an ovum (ovulation)
progesterone stimulates the
Graafian follicle becomes the
endometrium to become even
15 – 22 corpus luteum which secretes
thicker, more glandular and more
progesterone
vascular to receive a fertilised ovum
no fertilisation: the corpus luteum shrinks and stops producing
progesterone

23 – 28 with fertilisation: corpus luteum remains active and continues

producing progesterone
no more follicles develop
no menstruation takes place

The menstrual cycle:

https://www.youtube.com/watch?v=Vl2wRbO8LZU&list=PLW0gavSzhMlQYSpKryVc
Er3ERup5SxHl0&index=11

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 Hormonal control of the menstrual cycle

The menstrual cycle is controlled by hormones. The ovarian cycle is influenced by

follicle stimulating hormone and luteinizing hormone while the uterine cycle is
influenced by oestrogen and progesterone. The levels of the hormones change
during the different stages of the menstrual cycle and have an influence on each
other.
The graph in Figure 10 below shows the changes in the levels of the individual
hormones during the menstrual cycle.

ovarian follicle ovum corpus luteum

ovulation

FSH
LH

oestrogen
progesterone
endometrium

Day 1 Day 7 Day 14 Day 28

menstruation

Figure 10: Hormonal control of the menstrual cycle

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 NOTE: This graph often appears in examinations. Make sure that you can
interpret and understand what is happening to the levels of the different
hormones and how they influence each other.

The hormonal control of the menstrual cycle takes place as follows:

 Follicle stimulating hormone (FSH) released by the pituitary gland stimulates
the development and maturation of a primary follicle in one of the ovaries
 As the follicle develops into a mature Graafian follicle it releases oestrogen
 The increasing oestrogen levels stimulate the pituitary gland to release
luteinising hormone (LH)
 The increase in luteinising hormone (LH) causes ovulation to occur
 After ovulation occurs the Graafian follicle is changed into the corpus luteum
which secretes progesterone
 The increased amount of progesterone prevents the release of follicle
stimulating hormone and luteinising hormone (it inhibits them)
 As the corpus luteum breaks down the level of progesterone decreases,
causing the endometrium to break down
 The endometrium and unfertilised ovum are released through the vagina as
blood during menstruation
 Due to the decreased level of progesterone, the follicle stimulating hormone
and the luteinising hormone are no longer inhibited. They are produced by the
pituitary gland and the cycle begins again

Negative feedback mechanism between progesterone and FSH

A negative feedback system occurs in the menstrual cycle. A negative feedback

mechanism is an interaction between two hormones, where an increase in one
hormone stimulates an increase in the other hormone, which inhibits the first
hormone, thus restoring balance. The negative feedback system can be seen in the
hormonal control of the menstrual cycle where progesterone influences the secretion
of follicle stimulating hormone. If the ovum is fertilised, the corpus luteum remains
active and continues secreting progesterone. Increased levels of progesterone in
the blood inhibit the secretion of the follicle stimulating hormone. As a result, no
further development of the follicle occurs. Ovulation does not take place.

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Activity 3: Hormones
Study the graphs below, then answer the questions that follow.

Levels of female hormones during the menstrual cycle

Levels of female hormones
(arbitrary units)

1 2 3 4 5 6 7 8 9 10 .. 12 .. 14 .. 16 ... 18 .. 20 .. 22 ... 24 .. 26 .. 28

Days
Key:
Oestrogen
Progesterone
Luteinising hormone

1. On which day did ovulation occur? (1)

2. Give one reason for your answer to question 1 that can be seen on the
graph. (1)
3. Which structure in the ovary produces the following hormones?
a) Oestrogen (1)
b) Progesterone (1)
4. Explain why there is a sharp increase in the production of …
a) oestrogen from day 9 to 13. (2)
b) luteinising hormone from day 13 to 14. (2)
5. What conclusion can be drawn if the level of progesterone …
a) Remains high from day 20 to 28? (1)
b) Drops as shown in the graph above? (1)
(10)

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Activity 4: Menstrual cycle
The diagram shows some of the changes that may take place during the menstrual
cycle.

Growth of
follicle

Thickness of
uterine lining

1 7 14 21 28
Days

1. The menstrual cycle is controlled by hormones. Name one hormone that will
increase in level between days 2 and 10. (1)
2. Give one observable reason for your answer to question 1. (2)
3. Explain what evidence there is in the diagram to indicate that no fertilisation
took place? (3)
(6)

Fertilisation & development of zygote to blastocyst

During copulation (sexual intercourse) the penis is inserted into the vagina and
sperm cells are released through ejaculation close to the cervix. The sperm cells
swim through the cervix up into the uterus and through the fallopian tubes.
The haploid ovum released during ovulation enters the fallopian tubes. If an ovum
(haploid) is present in the fallopian tubes, one sperm cell (haploid) may penetrate
through the jelly layer and fertilise the ovum resulting in a diploid zygote. The
nucleus of the ovum and the nucleus of the sperm cell fuse resulting in fertilisation.
The zygote divides by mitosis as it moves down the fallopian tube towards the
uterus. Mitosis continues and a solid ball of cells known as the morula is formed. The
morula develops into a hollow fluid-filled ball of cells called the blastocyst. Once the
ovum is fertilised it takes approximately 5 days to form the blastocyst.

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 8 cell
2 cell stage 4 cell blastocyst
stage
(48 hours) stage morula (4 days)
implanted
(72 hours) blastocyst

zygote

ovum
fertilisation

Figure 11: Fertilisation and development of the blastocyst

Development of the blastocyst to implantation:

https://www.youtube.com/watch?v=bHYAMjwgeV8

Implantation of the blastocyst and gestation

The blastocyst moves from the fallopian tube into the uterus where it embeds itself
into the endometrium. This is known as implantation and takes place as follows:
 The outer cells of the blastocyst secrete enzymes which break down a small
portion of the thickened uterine wall causing it to become softer
 The blastocyst sinks into this softer area and the outer layers develop into two
extra-embryonic membranes called the amnion and the chorion
 The chorion extends finger-like outgrowths called the chorionic villi into the
endometrium and form part of the placenta which secretes progesterone.
 The blastocyst is now called the embryo

The gestation period, also known as pregnancy, is the time in which the embryo
develops inside the uterus. Gestation and the development of the embryo lasts for
about 40 weeks or 280 days. After 12 weeks the embryo is known as a foetus.

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 uterus

umbilical vein O2
CO2
umbilical cord
area enlarged
umbilical
artery

placenta umbilical cord

maternal vessels

Figure 13: The umbilical cord

The placenta has the following functions:

 It is the point of attachment of the foetus to the mother
 It allows for diffusion of nutrients from the mother to the foetus
 It allows for the diffusion of oxygen from the mother to the foetus and for the
diffusion of carbon dioxide from the foetus to the mother (gas exchange)
 It allows for the diffusion of waste products from the foetus to the mother
 After 12 weeks, the placenta secretes progesterone to maintain the
pregnancy

The formation of a foetus: https://www.youtube.com/watch?v=-

ekRRuSa_UQ&list=PLW0gavSzhMlQYSpKryVcEr3ERup5SxHl0&index=126

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Activity 5: Fertilisation
The diagram below shows a human ovum about to be fertilised

A
B
F
C

1. Identify the parts labelled A – G. (7)

2. Give the letter of the part that …
a) contains the mitochondria. (1)
b) contains enzymes required to penetrate the ovum. (1)
c) will enter the ovum during fertilization. (1)
3. Describe the developmental changes in the fertilised ovum until implantation
occurs in the uterus. (5)
4. Define gestation. (1)
(16)

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