Fertilization And Cleavage

Question 1

What are the functions of the polar bodies produced during oogenesis?

Answer 1

Removes defective or extra chromosomes

Ensure that the ovum will have most of the cytoplasm

Question 2

Where and when does meiosis begin in humans?

Answer 2

In the fetal ovary but eggs are halted at Prophase I until activated in the menstrual cycle

• a few primary oocytes then reactivate meiosis at each menstrual cycle, producing a secondary oocyte and a polar body
• after ovulation, the oocyte and first polar body are surrounded by a membrane called the zona pellucida

Question 3

What are gynogenetic embryos and what’s wrong with them?

Answer 3

Contain maternal chromosomes only - poorly developed trophoblast

Question 4

What are androgenetic embryos and what is wrong with them?

Answer 4

Contain paternal chromosomes only - over developed trophoblast, poorly developed embryo

Question 5

Why are both maternal and paternal chromosomes required for normal growth?

Answer 5

Genomic imprinting shows that a group of ~80 genes are inactivated during either oogenesis or spermatogenesis which are necessary for human development

Question 6

What 4 things must a fertilised egg do?

Answer 6

• Produce trillions of cells through the process of mitotic cell division
• Instruct these cells on what cell-type they must become
• Organise them into tissues and organs
• Allow different cell types to acquire special characteristics required for their function (differentiation)

Question 7

How do monozygotic twins come about?

Answer 7

30% - Division of the human embryo occurring at cleavage stages (separate placentas and amnion)

70% - Division of the human embryo occurring in the inner cell mass (shared placenta, separate amnion)

Question 8

What can happen if identical twins share both a placenta? (<1%)

Answer 8

Risk of being conjoined - 50% are stillborn and further 25% die soon after birth of severe abnormalities

Female conjoined twins 3x more common than males as the embryo divides first into two which partially fuse back together again

Question 9

What is a tetragametic chimera?

Answer 9

2 ova and 2 sperm - fused two embryos together

Question 10

What is a chimera?

Answer 10

A single organism composed of cells from different zygotes

Question 11

How do powerful genetic tests work?

Answer 11

• Embryos made
• Single cleavage stage cell isolated while the remaining cells are allowed to grow to the blastocyst stage
• Embryos containing defective genes are discarded but normal embryos are transferred to mother womb

Question 12

What does the trophoblast form?

Answer 12

Forms the chorion

Question 13

What does the inner cell mass form?

Answer 13

Divides into the epiblast and hypoblast layers

Hypoblast discarded at birth as extraembryonic endoderm

Question 14

What does the epiblast form?

Answer 14

Extraembryonic mesoderm
Amniotic membrane
All tissues of the fetus: ectoderm, mesoderm, endoderm

Question 15

Pluripotent

Answer 15

A cell which can replicate and differentiate into all cell-types of the fetus but not extraembryonic tissues

Question 16

How do stem cells differentiate into many cell-types?

Answer 16

Inner cell mass of the blastocyst is isolated and cultured in vitro as embryonic stem cells.

By adding the right combination of signalling molecules it is possible to make them differentiate into many cell types -> grow into tissues and organs for transplantation

Question 17

What are the functions of the trophoblast?

Answer 17

Implantation
Differentiate chorion
Immunosuppression
Endocrine Gland

Question 18

What is trophoblast disease?

Answer 18

Relate to pregnancy related rumours eg

Hydatidiform moles are formed when the trophoblast grows uncontrollably - if left untreated most will be miscarried but some will go to form a tumour

Question 19

How does the yolk sac come about?

Answer 19

• Trophoblast proliferates rapidly, forming a syncytium called the syncytiotrophoblast
• syncytiotrophoblast invades the uterine wall, dragging blastocyst behind it
• ICM divides into epiblast and hypoblast
• Lancunae form in the syncytiotrophoblast, which fills with blood from uterine capillaries
• amnion forms in the epiblast and the hypoblast spreads along the cytotrophoblast, lining the yolk sac

Question 20

What protrudes into the lacunae after 14 days?

Answer 20

The syncytiotrophoblast, cytotrophoblast, extraembryonic mesoderm protrude into the lacunae, forming the stem villi

Question 21

What does the mesoderm form at 14 days?

Answer 21

The blood vessels of the placenta, increasing the efficiency with which oxygen, co2, nutrients and waste products are transferred between maternal and embryonic blood supplies

Question 22

What allows the transfer of oxygen and nutritional elements from the mother to the developing embryo?

Answer 22

Stem villi

Question 23

What is transferred from mother to foetus?

Answer 23

O2
water
nutrients
hormones
antibodies
vitamins
drugs
alcohol
viruses

Question 24

What is transferred from foetus to mother?

Answer 24

Co2
Urea
Water
Uric acid
Creatinine
Bilirubin (breaks down heme in vertebrates)
Hormones
RBC antigens

Question 25

What is the foetus connected to the chorion via?

Answer 25

The umbilical cord and it’s blood vessels

Question 26

Where is the mammalian egg normally fertilised?

Answer 26

In the ampulla of the oviduct (Fallopian tube)

As it divides it moves down oviduct to uterus where is hatched from zona pellucida + implants into uterine wall

Question 27

When does implantation happen in humans?

Answer 27

~ 6 days after ovulation

Question 28

What is an ectopic pregnancy?

Answer 28

When a fertilised egg implants itself outside of the womb eg in a Fallopian tube -> egg will get stuck and not develop