Embryology Flashcards
Where does fertilisation usually occur?
Firstly fimbriae bring the ovulated oocyte into the oviduct
Fertilisation usually occurs in the ampulla region of the
oviduct
Ampulla: a cavity, or the dilated end of a duct
Describe a zygote
Fertilized zygote contains 2 haploid nuclei
Also has 2 polar bodies produced by last stage of
oogenesis (ovum production) when meiosis is done
Surrounding the zygote is the zona pellucida. This has a
non cellular structure like the shell of the egg
What happens straight after the zygote forms?
Zygote starts to divide exponentially by a process called cleavage
At this stage nuclei within the cell are diploid. The egg doesn’t get bigger, despite increase in cells
By day 3, the cell= 12-15 cells large and is a morula
The zona pellucida starts to dissolve in process called compaction
Shape of egg changes internally
Egg prepares to implant and is now called a blastocyst
Describe blastocysts
32-64 cells. Forms in 4-5 days
Blastocysts consist of 2 types of cells:
-Trophoblast cell layer on the outside (of internal environment of egg) to do with implanting procedure
-Inner cell mass (ICM): sticks in the middle, will form the embryo body
Rest of cell is called blastocoele (by day 6/7)
ICM will hatch out and egg will implant
Each blastocyst are totipotent (can become any type of cell)
Give applications of preimplantation
- Permits fertilisation and pre-implantation development in vitro so its the basis of ARTs (IVF, ICSI)
- 1 or 2 blastomeres can be removed for PGD to check gene defects, gender selection
- ICM cells can differentiate into ANY adult cell type. ICM cells can be removed and cultured as (ES) cells
- Animal ES cells can be genetically manipulated and introduced to blastocyst to form transgenic animals
- Human ES cells can be created from “spare” donated embryos, or from IVF
Give the significance of pre-implantation development with regards to IPSCs
- Somatic cell nuclei can be placed into an enucleated, ovum and give rise to an adult animal. This is how Dolly the sheep was created (SCNT)
- Demonstrates that epigenetic changes can be reversed to restore totipotency
- Somatic cells can be re-programmed to become ES cell-like by expressing transcription factors present in the ICM
- These IPSCs may supersede ES cells
Where does implantation occur and what issues arise if this doesent happen?
Implantation normally occurs in posterior uterine wall
Ectopic pregnancy is when the blastocyst implants into anywhere other than the uterus
eg. Oviduct, Ovary, Peritoneal surfaces, Lower part of uterine or cervix
As the ectopic embryo grows there could be blood vessel ruptures which is life threatening
How successful is implantation?
-Morning after pills and IUD’s interfere with implantation
-70% of blastocysts implant
-40% abort over week 2, 15% abort over week 3
50% of spontaneously aborted embryos are chromosomally abnormal or have other genetic defects
What happens at the beginning of week 2?
The outer layer of trophoblasts invade into the uterine wall similarly to metastasis- they secrete chemicals that break down the collagen matrix of the uterine wall, allowing implantation
Trophoblasts synthesise hCG. The corpus luteum secretes progesterone to help maintain pregnancy.
The amniotic cavity develops in the middle of the ICM. The embryoblasts in the ICM differentiate into epiblasts (on top) and hypoblasts (below) which juntos form the bilaminar plate.
The epiblasts will contribute to the human, hypoblasts will form the extraembryonic tissues. Below the hypoblasts is the chorionic cavity
The egg is now ready for gastrulation
What is the difference between monozygotic and dizygotic twins?
Dizygotic twins occur when 2 oocytes are separately fertilised in the oviducts
Monozygotic twins are where 1 fertilised oocyte splits, this division can occur at many different embryonic stages
Describe the different types of twins
How can you get conjoined twins?
- Conjoined twins occur in one of two ways
- Firstly, the ICM of one morula/blastocyst may partially divide
- Alternatively, one germ disc (the norm) can have a partially divided streak
What happens just after implantation? (week 3-4)
- Gastrulation: a primitive streak forms along the midline of the bilaminar disc
- From the primitive streak, the cells in the epiblast migrate outwards
- The migrating cells differentiate further to form a 3rd germ layer in the disc – it becomes a trilaminar disk
- The top germ layer is called the ectoderm, the middle layer is mesoderm and the lower layer is called endoderm. Each layer gives rise to diff tissues
What does the ectoderm germ layer form after gastrulation?
The ectoderm gives rise to 2 sub-types:
- Neuroectoderm gives rise to the neural tube (which eventually makes the CAN and spinal cord), along w the neural crest
- Surface ectoderm gives rise to the epidermis, hair, nails, internal ear, lens and teeth enamel
What does the mesoderm germ layer form after gastrulation?
The mesoderm gives rise to many subtypes too:
- Paraxial mesoderm forms the somites (muscle + bone)
- Intermediate mesoderm leads to the urogenital organs
- Lateral mesoderm forms the heart, spleen and blood
- Mesoderm also forms the skull and dentine of teeth
Describe neuralation (week 3-4)
Underneath where the primitive streak was, mesoderm cells differentiate to become a notochord
The notochord induces formation of the neural plate which is made of neuroectoderm. Neural plate cells move down to the mesoderm to form the neural tube
As this occurs, some cells migrate from the neural tube, forming the neural crests
The brain, spinal cord, cranial and spinal nerves form from the neural tube. The cranial and spinal ganglia form from the neural crest cells
