15. Embryonic Development Flashcards
(213 cards)
Give some reasons for studying embryology?
- Model system for cell/molecular biology
- Understanding congential defects
- Model for regenerative medicine
What are some model systems used in embryology?
- Xenopus (frog)
- Chick
- Mouse
- C elegans
- Zebrafish
Give an example of a gene that is conserved between species.
- Pax6 is conserved in humans, mice, zebrafish and drosophilia
- A mutation in the Pax6 gene results in eyeball abnormalities in all of these species
Name some advantages of the fly as a model system for embryology.
- Cheap
- Short lifespan
- Easy to manipulate
Name some advantages of the mouse as a model system for embryology.
- Foremost mammalian model
- Short gestation period
- Highly similar development to humans
Name some advantages of C elegans as a model system for embryology.
- Cheap
- Have fixed no. of cells -> Exact development + position of each cell has been mapped
Name some advantages of the chick as a model system for embryology.
- Accessible -> Grows in eggs
Name some of the important aspects of development.
- Cell differentiation
- Patterning
- Induction
- EMT and MET (epithelial to mesenchymal transitions and mesenchymal to epithelial transitions)
- Morphogenesis
- Cell death
Are signalling pathways ever reused in the development?
Yes, the same signalling pathways appear in many places in the human body.
What are the main important stages in embryonic development?
- Fertilisation (Day 0)
- Pre-embyronic stage (Day 0)
- Implantation
- Bilaminar germ disc
- Embryonic period (3rd week)
- Gastrulation + Axis formation
- Neurulation
- Embryonic folding
- Somitogenesis
- Limb bud formation
- Organogenesis
- Fetal period (8th week)
- Growth
- Maturation
- Birth
What is the product of fertilisation?
Diploid zygote
Define fertilisation.
The joining of an egg and sperm to produce a diploid zygote.
Describe the process of fertilisation.
- Occurs in ampullary region of oviduct (the far end, near the ovary) – Note: Oviduct = Fallopian tube
- In order to fertilise oocyte, spermatozoa need:
- Removal of glycoprotein coat (capacitation) -> This is calcium dependent
- Binding to zona pellucida -> Activates acrosome reaction, where acrosome (a Golgi-derived organelle in the head of the spermatozoon) releases enzymes to break through the zona pellucida
- As spermatozoon enters the zona pellucida, the membrane of the spermatozoon fuses with the membrane of the oocyte, releasing the spermatozoon nucleus into the oocyte
- At the same time, the cortical granules of the oocyte release their contents, making the zona pellucida impenetrable to other spermatozoa
- Entry of the spermatozoon nucleus stimulates the oocyte to complete the second meiotic division (meiosis has not yet been completed) -> It is now called a definitive oocyte and the two nuclei are called the female pronucleus and male pronucleus (the name for the nucleus during fertilisation)
- Since it has been fertilised, the definitive oocyte can also be called the zygote
- The two pronuclei approach each other and duplicate their DNA, ready for the first mitotic division
- Pronuclear membranes break down and the chromosomes line up for metaphase
- First cell division takes place
What percentage of sperm reach the cervix and what happens to them there?
- About 1%
- Ovulation induces them to move further into the oviduct
What are the technical names for the sperm and egg?
- Sperm = Spermatozoon (pl. spermatozoa)
- Egg = Oocyte
Where does fertilisation occur?
Ampullary region of oviduct (a.k.a. fallopian tube)
(far end, near the ovary)
Draw out the path of spermatozoa to the oocyte.

What two things must happen in order for the spermatozoon to enter the oocyte?
- Removal of glycoprotein coat (capacitation) -> This is calcium dependent
- Binding to zona pellucida -> Activates acrosome reaction, where acrosome (a Golgi-derived organelle in the head of the spermatozoon) releases enzymes to break through the zona pellucida
What prevents multiple spermatozoa fertilising the oocyte?
Cortical granules (just inside the plasma membrane) release their contents, making the zona pellucida impenetrable.
Describe the process of meiosis involved in generating the female pronucleus.
- The process of meiosis is halted just before the second meiotic division
- When the male pronucleus enters the oocyte, this triggers the second meiotic division -> This creates the female pronucleus
How is the first mitotic division different to normal mitotic divisions?
The DNA duplication occurs before the male and female pronuclei even fuse.
What are pronuclei?
The nuclei of the gametes during the process of fertilisation.
Draw a diagram of an oocyte and spermatozoon.

What is a zygote?
The union of the sperm cell and the egg cell. Also known as a fertilized ovum.















