human bio reproduction test Flashcards
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,
disintegrates
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
foetus
Umbilical vein – away from mother
Carries blood from placenta to foetus
Delivers oxygen, nutrients and hormones to the
foetus
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
Ectoderm:
- Outermost germ layer
- Forms outer layers of the body e.g. skin, hair, mammary
glands and nervous system
Mesoderm:
- Middle germ layer
- Form the skeleton, muscles, connective and lymph
tissue, heart, blood, urogenital tract, stomach and
intestines
Endoderm:
- Innermost germ layer
- Forms epithelium tissue and lining of digestive system,
lungs and thyroid
what happens during the first stage of labour
DILATION OF THE CERVIX - 8-9 hours
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
DELIVERY OF PLACENTA
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
