Fertilization And Cleavage Flashcards
What are the functions of the polar bodies produced during oogenesis?
Removes defective or extra chromosomes
Ensure that the ovum will have most of the cytoplasm
Where and when does meiosis begin in humans?
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
What are gynogenetic embryos and what’s wrong with them?
Contain maternal chromosomes only - poorly developed trophoblast
What are androgenetic embryos and what is wrong with them?
Contain paternal chromosomes only - over developed trophoblast, poorly developed embryo
Why are both maternal and paternal chromosomes required for normal growth?
Genomic imprinting shows that a group of ~80 genes are inactivated during either oogenesis or spermatogenesis which are necessary for human development
What 4 things must a fertilised egg do?
- 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)
How do monozygotic twins come about?
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)
What can happen if identical twins share both a placenta? (<1%)
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
What is a tetragametic chimera?
2 ova and 2 sperm - fused two embryos together
What is a chimera?
A single organism composed of cells from different zygotes
How do powerful genetic tests work?
- 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
What does the trophoblast form?
Forms the chorion
What does the inner cell mass form?
Divides into the epiblast and hypoblast layers
Hypoblast discarded at birth as extraembryonic endoderm
What does the epiblast form?
Extraembryonic mesoderm
Amniotic membrane
All tissues of the fetus: ectoderm, mesoderm, endoderm
Pluripotent
A cell which can replicate and differentiate into all cell-types of the fetus but not extraembryonic tissues
How do stem cells differentiate into many cell-types?
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
What are the functions of the trophoblast?
Implantation
Differentiate chorion
Immunosuppression
Endocrine Gland
What is trophoblast disease?
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
How does the yolk sac come about?
- 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
What protrudes into the lacunae after 14 days?
The syncytiotrophoblast, cytotrophoblast, extraembryonic mesoderm protrude into the lacunae, forming the stem villi
What does the mesoderm form at 14 days?
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
What allows the transfer of oxygen and nutritional elements from the mother to the developing embryo?
Stem villi
What is transferred from mother to foetus?
O2 water nutrients hormones antibodies vitamins drugs alcohol viruses
What is transferred from foetus to mother?
Co2 Urea Water Uric acid Creatinine Bilirubin (breaks down heme in vertebrates) Hormones RBC antigens
