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explain the process of spermatogenesis

Production of spermatozoa

Occurs in the seminiferous tubules of the testes

Process takes about 72 days, continues after puberty

Tubules are lined with spermatogonia – immature mother diploid cells

At puberty spermatogonia divide my mitosis to provide a constant source of new cells for the production of spermatozoa

Some spermatogonia are pushed away from the tubule’s membrane toward the lumen where they begin a period of growth and maturation to become primary spermatocytes

Primary spermatocytes undergo first stage of meiosis to become secondary spermatocytes

Secondary spermatocytes undergo second meiotic division producing four haploid spermatids

Spermatids mature into spermatozoa – most of cytoplasm is lost and tail forms

Spermatozoa are nourished with sertoli cells


explain the process of oogenesis

Production of ova

Occurs in ovaries - 1 month

Coincides with ovarian cycle

Before birth a female has millions of oogonia (immature mother germ cells)

At birth each ovary contains thousands of oogonia that have grown and matured into primary oocytes

Primary oocytes are surrounded by a single layer of cells called the primary follicle

Primary oocytes begin prophase I but pause until puberty

At puberty follicle growth/maturation begins (one follicle grows out of thousands)

Primary oocyte completes first meiotic division

2 daughter cells aren’t equal in size:
- Larger one: secondary oocyte: receives most of
cytoplasm, goes on to become ovum
- Smaller one: first polar body: receives little cytoplasm,

Second oocyte completes 1 meiotic division, pauses at prophase II

Ovulation occurs: follicle ruptures, ovum and polar are expelled

Secondary oocyte enters uterine tube

If fertilised by sperm it completes meiosis 2:
- Produces 2 more uneven daughter cells
- Larger one: ootid – develops into ovum
- Smaller one: secondary polar body


explain the process of the ovarian cycle

Series of events that takes place in the ovaries during 1 cycle

20 – 40 days; 28 days

At puberty, the layer of cells of the primary follicle around the primary oocyte divides and enlarges – forming the secondary/developing follicle

Cells of the follicle secrete a fluid around the primary oocyte

As more fluid is secreted the follicle grows larger and moves toward the surface of the ovary

It bulges at the surface and is now a mature/grafian follicle

Maturation of a primary follicle takes 10 – 14 days

The mature follicle ruptures and releases the secondary oocyte – this is called ovulation

Egg is released in direction of uterine tube, fimbriae form a funnel over the ovary

Cilia and muscular contractions moves egg into the tube towards the uterus

Ovum is viable 12 – 24 hours after ovulation


describe what happens during fertilisation

Fusion of a sperm and egg

Occurs a third of the way down the uterine tube

Secondary oocyte is surrounded by a layer of follicular cells – corona radiata

Tips of sperm are capable of breaking down the acid, many sperm are needed to do this

The entrance of one sperm stimulates formation of fertilisation membrane, preventing entrance of any more sperm

Once the sperm enters the egg, it is stimulated to complete the second meiotic division

The sperms tail is absorbed and the head moves through the egg’s cytoplasm in a male pronucleus

Egg’s nucleus develops into the female pronucleus and fuses with male pronucleus to forma single nulceus

Fertilized egg is called a zygote


what happens during implantation and early embryonic development

Zygote begins to divide by mitosis in the uterine tubes – called cleavage (1 cell  2 cells  4 cells  8 cells etc.)

During this time there’s not cell growth

As the cells divide they get smaller and form a solid ball of cells – called morula

6 days after fertilisation the morula reaches the uterus and has become a blastocyst (hollow sphere with a layer of about 100 cells that surround a fluid filled cavity)

At one side of the cavity is a group of 30 cells called the inner cell mass

These cells are stem cells and will develop into the embryo

The blastocysts remains in the uterus for 2 – 3 days then sinks into the endometrium and becomes firmly attached to the wall of the uterus - implantation

This allows the blastocyst to gain nourishment for growth and development from the glans and blood vessels in the endometrium


explain the development of the placenta

Foetal blood and maternal blood are separated by a few layers of cells

Exchange of material takes place by diffusion and active transport through the cells

After implantation it develops from chorionic villi – contain blood vessels that grow into the endometrium
The large number of villi provides a large surface area to assist in diffusion

Fully formed by 3rd month of pregnancy
The placenta is attached to the foetus by the umbilical cord which contains:

2 umbilical arteries – towards mother
 Carries blood towards capillaries of chronic villi
 Wastes and carbon dioxide are removed from the

Umbilical vein – away from mother
 Carries blood from placenta to foetus
 Delivers oxygen, nutrients and hormones to the

Blood from the mother enters into the placenta through the uterine arteries

Blood flows through the spaces were exchange of substances takes place and leaves again through the uterine veins.


describe what the primary germ layers are

After implantation blastocyst develops into 3 primary germ layers

- Outermost germ layer
- Forms outer layers of the body e.g. skin, hair, mammary
glands and nervous system

- Middle germ layer
- Form the skeleton, muscles, connective and lymph
tissue, heart, blood, urogenital tract, stomach and

- Innermost germ layer
- Forms epithelium tissue and lining of digestive system,
lungs and thyroid


what happens during the first stage of labour


During the last 3 months of pregnancy the uterus undergoes irregular weak contractions that become more frequent and stronger during final weeks

Eventually the contractions become strong and occur every 30 minutes – marks onset of labour

Contractions occur in waves from upper part of the uterus downwards toward the cervix

Uterus muscle fibres pull the cervix to shorten it

cervix opens – dilation

Complete dilation is 10cm which allows the foetus to move deeper into the pelvis with its head pushed forcefully against the cervix

Birth canal: passage made up of uterus, cervix and vagina


what happens during the second stage of labour

EXPULSION OF FOETUS - 20mins-2 hours

Amniotic cavity bursts and fluid gushes from vagina

As the foetus moves through the cervix it stretches the vagina which stimulates contractions of the mothers’ abdominal muscles

These contractions with the uterus contractions push the foetus through the vagina

As the head is delivered it stretches the vaginal entrance and the tissues between the vagina and anus

Once the head has emerged the foetus turns sideways to face the hips

At birth the baby is covered in a waxy material called vernix that forms a protective layer


what happens during the third stage of labour


The umbilical cord is clamped, ties in 2 places and cute between the ties

Arteries and veins in the umbilical cord contract

The uterus continues to contract after the baby is born and the placenta, umbilical cord and other membranes are expelled after approximately 5 minutes – afterbirth

Little blood is lost during this due to:
- Placental blood vessels vasoconstricting
- Contractions of the uterus squeezing shut uterine
vessels that supply the placenta
- Blood clots forming that stop all leakage of blood


explain foetal circulation

Foetal blood is carries to and from placenta via blood vessels in the umbilical cord

Foetus’s liver isn’t fully developed and blood is treated from the mother’s liver

Most blood bypasses the liver via the ductus venosus into the inferior vena cava

Blood enters the right atrium, then:
- Little blood flows into right ventricle and then delivered
to lungs (collapsed)
- Most blood flows into right ventricle then into ductus
arteriosus allowing blood to travel from the pulmonary
artery directly into the aorta
- Some blood flows into left atrium through an opening
between the 2 sides of the heart – foramen ovale

These allow oxygenated blood from placenta to quickly be transported around the body


what changes occur to circulation after birth

Baby’s first breath is triggered by shock of birth or a slap on their bottom

If this fails the umbilical cord is clamped and CO2 rise in blood stimulates brain’s respiratory centre, causing lungs to function

Lungs expand and don’t resist blood flow – ductus arteriosus deteriorates

Blood returns to heart and increases pressure of let atrium, forcing shut the foramen ovale

Blood no longer flows through ductus venosus and blood flows through the liver


what is a totipotent stem cell

Have the potential to become any cell necessary for embryonic development e.g. embryo, placenta

Zygote is totipotent

Cells undergo several rounds of division and specialises to form a blastocyst after 6 days


what is a pluripotent stem cell

Can give rise to many but NOT ALL cell types in foetal development

A blastocyst’s inner cell mass is pluripotent

E.g. foetal tissue but not placental tissues


what is a multipotent stem cell

Pluripotent cells undergo specialisation to become multipotent stem cells

Can give rise to cells of 1 tissue type

E.g. muscle stem cells


what happens during sexual intercourse

Allows sperm being brought into contact with ova for fertilisation

The penis becomes enlarged as blood rushes into the spaces of erectile tissue – erection

When sexual stimulation is sufficient rhythmic contractions of the epididymis, vas deferens and the 3 glands occur

The contractions propel the contents into the urethra and out the body – ejaculation

This consists of semen (200 – 300 million sperm)

Accompanying this is rapid heartbeat, increase in blood pressure and breathing rate, and pleasurable sensations – orgasm

When the female is sexually stimulated erectile tissue of the vaginal opening fills with blood

Mucous secretions increase and lubricate epithelial lining of vagina

When sexual stimulation reaches sufficient intensity and orgasm/climax occurs

Insemination is the process of the sperm being released into the vagina

Sperm travels through the cervix, uterus and uterine tubes

Muscular contractions of uterus and uterine tubes help transport the sperm

Sperm mortality rate is high


what are the functions of the placenta

Endocrine gland: secretes a number of hormones to maintain pregnancy

Excretory: transports nitrogenous waste e.g. urea, uric acid, ammonia and creatine from foetal blood to maternal blood for excretion by the mothers kidneys

Immune: transport antibodies from mother to the foetus so it has some immunity to some diseases

Nutritional: transports glucose, amino acids, fatty acids, vitamins and minerals from mother to foetus. Also stores some nutrients in early pregnancy and releases them later when demand is high

Respiratory: Exchange of oxygen and carbon dioxide


what are hormones

• Chemical messengers
• Secreted by endocrine glands
• Transported to a target organ


list the 4 embryonic membranes



yolk sack



describe the amnion

- Develops first
- Surrounds embryo and encloses a cavity
- Secretes amniotic fluid into cavity – protects embryo
and shock absorber
- Maintains temperature
- Allows embryo to move freely
- Expands with growth of foetus
- Ruptures before birth


describe the chorion

- Formed from outer layers of blastocyst and a layer of
mesodermal cells
- Surrounds embryo and 3 other membranes
- Amnion fuses with it as it enlarges – forms main part of
foetal side of placenta


describe the yolk sack and allantois

- Becomes part of the umbilical cord
- More significant in animals
- Allantois is for waste exchange
- Yolk sack is an early circulation system