Animal Models in Development Flashcards
What are the desirable characteristics of a model organism?
- Relevance/representative
- Accessibility/availability
- Experimental manipulation
- Genetics
- Cost/space
What main model systems are used in laboratories?
The most widely used models: • Caenorhabditis elegans (roundworm) • Drosophila melanogaster (fruit fly) • Danio rerio (zebrafish) • Xenopus laevis (frog) • Gallus gallus (chick) • Mus musculus (mouse)
What is C.elegans?
Why is this organism used in the lab to study embryo development?
- A very small worm that lives in the soil and can easily be maintained in laboratory conditions
- Its structure and organization is very simple and we can see through all the different cell types
- Genome has been fully sequenced and is compact
- Transparent
- Short embryogenesis of 15 hours
- We can manipulate the genetics well
- Invariant lineage – they are formed from exactly 558 cells at hatching, each worm has the same number of cells and the origin of each of these cells is known. This is shown in the slide below.
Here we can follow the cells fates from the original precursor cells. This allows us to study at good detail how specific cell characteristics and traits are established during development
Describe the Drosophila melanogaster (fruit fly) life cycle
- This insect has a particular way of developing
- It goes through 2 different forms throughout its life, a larvae form and an adult form.
- The adults lay the embryos and within 24 hours and they hatch to form a small larva
- This will then grow, the primordial epithelial structures grow.
- The larva will then form the pupa and undergo metamorphosis. The epithelial structures will re-organise to give rise to the adult tissues.
What are the advantages of using fruit flies to study development?
Advantages:
• The embryos are not transparent, we cannot see how the cell types through embryogenesis
• Short embryogenesis of around 24 hours
• Easily bred and maintained (14 days generation time)
• Genetics – very well understood and lots of mutants and ways of manipulating gene activity
• Mutants
Describe the zebrafish life cycle
- Females lay the eggs and the males fertilise them
- They then grow in the medium so they are accessible so we can collect them and grow in laboratory conditions in a simple way.
- The embryogeneis is very fast and hatch in two days
- A free swimming larva will hatch which will then grow over the next three months to form an adult.
What are the advantages of using zebrafish?
- It is a vertebrate so is close to humans and we can relate the physiology and organogenesis to humans
- They develop rapidly in two days – 2 days to hatch
- Short generation time of three months to give rise to adults
- The genome has been fully sequenced, we can see lots of mutants
- They are transparent so we can see every cells during embryogenesis, we can see how tissues are assembled.
Why are xenopus embryos used to study development?
• The embryos development in a simple medium and have a fast development in just two days to form a tadpole
Important features and advantages:
• They are large embryos and easy to manipulate
• They develop in simple salt solutions/ explants (little pieces of tissues that develop)
• They have a rapid devlopnent taking 30hours to hatch
• They can be injected with RNA and labels to manipulate gene activity
• The genetics is very poor and we don’t have many tools to manipulate the gene activity
How did grafting lead to the discovery of induction?
- They were trying to understand which tissues were deriving from different regions of the developing embryo to construct a fate map
- They were forming little graphs into a new species, this would give rise to different to different parts of the embryo
- However the dorsal blastopore lip was different. When this was grafted into the ventral p art of the embryo, the grafted tissue would instruct the host tissue surrounding it to generate a secondary axis giving rise to the image below.
- This led to the discovery of induction
What are the advantages of using a gallus chick to study embryology?
- Very big, accessible embryos meaning we can do a lot of experimental manipulation such as grafts, dye labelling and other types of manipulations
- They have poor genetics so we rely on experimental manipulation
- The embryogenesis is very short of 4 days and takes 21 days to hatch
What is DiI/DiO fate mapping in chick?
This aims to find out which structures derived from which cells in the early embryo
Here two groups of cells have been labeled with fluorescence dye in different colours and we can then see which structures they give rise to.
What are the advantage and disadvantages of using Mus musculus: mouse?
- They are mammals – so we can relate the morphology and reorganising during embryonic development to human development
- They are widely used, lots of protocols to maintain them during lab conditions
- They develop in utero so we cannot manipulate them as easily
- They have a gestation time of 21 days
- The genome has been fully sequenced, lots of tools have been developed that allow us to manipulate gene function to generate lesions in the genome
- They are expensive
- Ethical concerns – they are mammals, sacrifice the mother as they develop in utero. Assessed carefully and clearly justified
Describe Gastrulation in the fruit fly: invagination?
- The fruit fly blastoderm is an oblong balloon shape of epithelial cells, this is shown as a transverse section. They cells are mostly morphologically similar apart from the ones along the ventral portion will become the mesoderm, the one at the extremes will become the endoderm
- The cells become internalised inside this embryo by invagination. It happens via the folding of the epithelium of the ventral portion so epithelial cells will be internalised. This is driven by changes in the shape of the cells. They go from columnar shapes to bottle shaped.
- Once they have been internalised they transition to mesenchalial cells and spread inside the embryo to give rise to the endoderm and mesoderm
Describe Gastrulation in zebrafish and Xenopus: involution
- Here we have the blastoderm sitting on top of the yolk.
- The blastoderm cells spread around the yolk cells, this process is called epiboly, this cause sthe yolk to be engulfed by the blastoderm cells.
- Halfway through this process cells at the edge of the blastoderm start to become internalised in the embryo by involution, causing the cells to move in the opposite direction and will spread underneath the epiblast cells
- By the end of this process the embryo cells completely engulf the yolk cells and have formed 2 layers of cells which will give rise to the endoderm and mesoderm
- Here the animals pole cells spreads to engulf the vegetal pole cells to become endoderm and as they engulf them a blastopore will form
- In the blastopore there is folding of cells by involutioin, the cells will move in the opposite direction as shown in the diagram
- By the end the red will give rise to the mesoderm and the yellow will give rise to the endoderm will become engulfed
Describe Gastrulation in the chick embryo: ingression
- The embryo sits on top of a yolk, however the yolk is very big
- The embryo is developing as a flat structure
- The blastula embryo is a bilaminar disk of cells and the external groups of cells is called the epiblast which will give rise to the embryo proper and the internal cells are called the hypoblast which will give rise embryonic structure
- At the posterior end of the embryo is the primitive groove and is involved in the thickening of the epiblast and will form the midline of the embryo
- The cells in the primitive streak will start undergoing epithelial to mesenchymal transition and will ingress the embryo underneath the epiblast to form the mesoderm and endoderm
