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11.4 – Reproduction
11.4.1 - Annotate a light micrograph of testis tissue to show the location and function of
interstitial cell (Leydig cells), germinal epithelium cells, developing spermatozoa and
Sertoli cells
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11.4.2 - Outline the processes involved in spermatogenesis within the testis, including
mitosis, cell growth, the two divisions or meiosis and cell differentiation
In the testes of the male embryo, the primordial germ
cells will differentiate to form the spermatogonia. These
are still diploid cells.
Endless mitosis of the germinal epithelium cells produces
many more spermatogonia which are found in the outer
wall of the seminiferous tubule. This is a process that
continues into adulthood, allowing for the continuous
production of sperm.
These diploid cells grow larger to become primary
spermatocytes (2n)
They will then enter meiosis I, dividing to become
secondary spermatocytes (2n)
Following this, they enter meiosis II, during which they will
divide to become two spermatids (n), making a total of
four spermatids produced from each germinal epithelium
cell.
The spermatids are then nurtured by the Sertoli cells to
develop and differentiate into spermatozoa (n)
The sperm then detach from the Sertoli cells. The fluid of the seminiferous tubule carries
them out of the testis and into the epididymis.
The final product of spermatogenesis is four sperm cells from each germinal epithelium cell.
Differentiation of primordial germ cells
Repeated mitosis
Grow to become primary spermatocytes
Meiosis I to become secondary
spermatocytes
Meiosis II to produce two spermatids
The Sertoli cells nurture the spermatids to
become spermatozoa
Detach and leave the testis
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11.4.3 - State the role of LH, testosterone and FSH in spermatogenesis
Luteinising Hormone – Secreted from the pituitary gland, and stimulates the secretion of
testosterone by the testes.
Testosterone – Secreted from the interstitial cells in the testes. It stimulates the
development of secondary spermatocytes into mature sperm.
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Follicle Stimulating Hormone – This is secreted from the pituitary gland. It stimulates the
primary spermatocytes to undergo meiosis I, forming secondary spermatocytes.
11.4.4 - Annotate a diagram of the ovary to show the location and function of germinal
epithelium, primary follicles, mature follicle and secondary oocyte
The oogonia are formed by mitosis from the germinal epithelium and will then grow to
become primary oocytes. Each menstrual cycle, a few primary follicles are formed from an
oocyte and a layer of follicle cells. Meiosis I then occurs to produce the secondary oocyte
and the first polar body.
The secondary oocyte will continue to mature in the follicle, or Graafian follicle. The oocyte
will continue to mature, entering meiosis II, but stopping at prophase II. It will not develop
further until a sperm enters the ovum.
The follicle burst open to release the secondary oocyte, with follicle cells, into the fallopian
tube. This is ovulation. The remaining follicle becomes the corpus luteum, which
temporarily produces progesterone. If the ovum is not fertilised, then this will degenerate.
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11.4.5 - Outline the processes involved in oogenesis within the ovary, including mitosis,
cell growth, the two divisions of meiosis, the unequal division of cytoplasm and the
degeneration of polar body
In oogenesis, ova are produced in the ovaries.
When the female is still a foetus, diploid cells divide by
mitosis in the germinal epithelium, creating more
diploid cells (2n).
These cells then grow to become primary oocytes (2n)
The primary oocytes will then start meiosis I, but will
stop during prophase I. The primary follicle consists of
the primary oocyte and a layer of follicle cells around it.
In general, a female is born with about 400 000
primary follicles.
During each menstrual cycle, a primary follicle will
develop. In this time, the primary oocyte completes
meiosis I to form two haploid nuclei. However, due to
unequal cytokinesis, the cytoplasm in divided
unequally and the result is a large secondary oocyte
and a small polar cell (n)
The secondary oocyte will then enter meiosis II, stopping in prophase II. Simultaneously, the
surrounding follicle cells will proliferate and form follicle fluid.
At ovulation, the follicle will burst and release the secondary oocyte into the fallopian
tubes.
The remaining follicle then becomes the corpus luteum, which secretes progesterone.
Oogonia
Primordial Follicle
Primary Follicle
Secondary Follicle
Mature Follicle
Ovulation
Corpus Luteum
Corpus Albicans
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If the ovum is fertilised, then it will complete meiosis II with the sperm nucleus inside it and
a second polar body is formed due to unequal cytokinesis. The first and second polar bodies
will degenerate.
11.4.6 - Draw and label a diagram of a mature sperm and egg
11.4.7 - Outline the role of the epididymis, seminal vesicle and the prostate gland in the
production of semen
Epididymis – The sperm reach the epididymis after the leave the testes. At this stage, they
are unable to swim. The sperm are stored here to continue to mature and become able to
swim.
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Seminal Vesicles – Produce and store fluids that are released for ejaculation, mixed with the
sperm to increase the total volume of the ejaculate. This fluid contains nutrients such as
fructose, providing energy for the sperm, as well as mucus to protect the sperm when they
reach the vagina.
Prostate Gland - Produce and store fluids that are released for ejaculation, mixed with the
sperm to increase the total volume of the ejaculate. This fluid contains mineral ions and is
alkaline to protect the sperm from the acidic environment of the vagina.
11.4.8 - Compare the processes of spermatogenesis and oogenesis, including the number
of gametes and the timing of the formation and release of gametes
Spermatogenesis Oogenesis
Equal cytokinesis Unequal cytokinesis
Continuous process of diving by mitosis All of the primary oocytes are already
present
Uninterrupted sequence with millions
of sperm produced daily
Has long resting periods, with only
one ovum produced every 28 days
Sperm are released during ejaculation Reduced about day 14 of the
menstrual cycle
Formation begins at puberty The formation begins in the female foetus
Production continues through adult life Production becomes irregular and stops at menopause
Four sperm are produced in meiosis One egg is produced in meiosis – the remaining polar bodies degenerate
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11.4.9 - Describe the process of fertilisation, including the acrosome reaction, penetration
of the egg membrane by a sperm and the cortical reaction
Fertilisation takes place in the fallopian tubes. A secondary
oocyte is released from the ovaries at about day 14 of the
menstrual cycle and enters the oviducts. The sperm enter the
female body during sexual intercourse through the vagina,
and then travel up into the fallopian tubes. Only a few sperm
will reach this stage, as many will die due to the high acidity
of the female system, along with other factors.
When the sperm and the ovum meet, the sperm must pass
between the follicle cells that surround it. They will then
arrive at the jelly coat or the zona pellucida. In the acrosome
of the sperm, there are digestive enzymes that break down
the coat to form a path.
The process of changing the head of the sperm is called
capacitation.
The head of the sperm will then fuse with the plasma
membrane of the ovum, allowing the nucleus to enter.
Once a sperm has entered the ovum, the cortical granules are
then released across the membrane through exocytosis to
prevent any more sperm from entering. If another sperm
does enter, then the zygote would not survive.
At this point, the secondary oocyte will recommence meiosis
II, forming a second polar body. Both the first and second
polar bodies are then released and degenerate.
Fertilisation is complete once the nuclei of the ovum and the
sperm fuse together. The cytoplasm will divide to form diploid cells of the embryo.
Ovum is released from the ovary
Sperm enter the vagina and travel up into the fallopian tubes
Sperm meet the egg in the fallopian tube
Sperm pass between the follicle cells
Arrive at the jelly coat of the secondary oocyte
Enzymes change the sperm head and creat a path for the sperm
Head of the sperm engulfed in the plasma membrane
Cortical granules pass across the plasma membrane to prevent
the entry of other sperm
Secondary oocyte undergoes meiosis II, producing a second
polar body
First and second polar bodies are degenerated
Cytoplasm divides to form the first two diploid cells of the
embryo
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11.4.10 - Outline the role of HCG in early pregnancy
Human Chorionic Gonadotrophin (HCG) is secreted once the embryo is implanted in the
wall of the uterus. This prevents the corpus luteum from degenerating, but causes it to
grow to continue producing oestrogen and progesterone. This maintains the pregnancy.
Eventually, the placenta will take the place of the corpus luteum in secreting oestrogen and
progesterone.
11.4.11 - Outline early embryo development up to the implantation of the blastocyst
The fertilisation of the ovum happens in the fallopian tube. When the zygote (fertilised
ovum) is formed, it travels down the fallopian tube, dividing by mitosis as it does so.
When the zygote first begins to divide, it does so through cleavage of the cell. At this stage,
the mass and size of the embryo do not change. The result is that, by the time it reaches the
uterus, the embryo has undergone several mitotic divisions to become a hollow ball of small
cells called blastomeres, which organise themselves to form the blastocyst, which is also
filled with fluid.
The embryo will usually implant in the endometrium at about days 7-14, at which point the
blastocyst contains approximately 100 cells. This is called implantation. A few blastomeres
will group together to form the inner cell mass, which later become the foetus. The
endometrium provides nutrients to embryo.
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11.4.12 - Explain how the structure and functions of the placenta, including its hormonal
role in secretion of oestrogen and progesterone, maintain pregnancy
The placenta is made up of maternal and fetal membrane tissues, allowing the blood of the
mother and child to come close enough together to allow for exchange. The umbilical cord
is formed from an artery and a vein, and connects the foetus to the placenta. The placenta
also protects the baby from bacteria; however some viruses are still able to cross it.
Exchange takes place through both active transport and diffusion.
In addition to the exchange of materials, the placenta also has important functions in the
production of hormones. In early pregnancy, it produces HCG in order to maintain the
corpus luteum. The placenta will later replace the corpus luteum to produce progesterone
and oestrogen. This is important for ensuring that the pregnancy is maintained.
11.4.13 - State that the foetus is supported and protected by the amniotic sac and
amniotic fluid
During gestation, the embryo is protected by the amniotic sac and fluid. The embryo is a
very small, delicate structure; hence such protection from injury is essential. It is able to
float in the amniotic fluid to support it and protect it from shock.
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11.4.14 - State that materials are exchanged between the maternal and foetal blood in the
placenta
The placenta is attached to the foetus, allowing for the exchange of materials between the
mother and child. This occurs through diffusion and active transport. These materials
include:
Respiratory gases – Including oxygen diffuses across the membrane to the foetus,
with carbon dioxide diffusing back.
Water – Enters the placenta by osmosis
Glucose – Enters by facilitated diffusion
Ions and Amino Acids – Transported across the membrane using active transport
Excretory Products – This includes urea, which leave the foetus
Antibodies – These enter the foetus’ bloodstream from the mother to protect it
from the same diseases, called passive immunity.
The placenta acts as a barrier to bacteria, but not all viruses
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11.4.15 - Outline the process of birth and its hormonal control, including the changes in
progesterone and oxytocin levels and positive feedback
Just before birth, the concentration of progesterone
decreases dramatically. Since progesterone inhibits the
contraction of the muscles in the uterus, this effect is
stopped and the contractions can begin.
Oxytocin is released from the pituitary gland to relax the
fibres of the bones of the pelvic girdle. The cervix begins
to dilate. Oxytocin will also cause contractions.
A positive feedback loop is established: as the cervix
stretches further, more oxytocin is released to continue
the contractions. This stops once birth is complete and
the cervix stops stretching.
Contractions move down towards the cervix to push the
baby out, becoming more and more powerful and
frequent as time goes on.
The placenta and the remaining umbilicus are discharged
afterwards in a period called the afterbirth.
Contractions of the uterus
Stretching of the cervix
Stimulation of the receptors in the cervix
Messages sent to the brain and the pituitary gland releases oxytocin
Muscles of the uterus contract more forcefully
Cervix stretches further
Birth
Decreased stretching of the cervix
No more oxytocin released