11.4 - Reproduction

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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


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


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.


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.


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


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.


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


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


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.


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.


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


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

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11.4 - Reproduction