Lecture 10 - Invertebrate models - sea urchin & C. elegans Flashcards
Why is a sea urchin a good genetic model?
- large number of embryos
- experimental manipulation
Why are genetic models good?
- bred easily in a lab
- genes within the genome can be altered to study their effect on development
What are sea urchins?
echinoderms
What are echinoderms?
deuterostomes
In protostomes, where does the blastopore form from?
the mouth
In deuterostomes, where does the blastopore form from?
the anus
Why are sea urchin’s embryos useful?
transparent embryo, which is large & easily accessible for manipulation
How long does it take for a blastula to develop in a sea urchin?
4 hours after cell division - meaning they have a strict order & orientation
When is the primitive gut formed in a sea urchin?
during gastrulation movements
How does the extensions on the plateaus larva form?
the larva has internal calcareous skeleton that is responsible for the extensions on the plateaus larva
What were the 2 types of development discussed surrounding sea urchins?
mosaic model and regulative development
What is the mosaic model (Weissman)?
the nucleus of the egg contain determinants that specify different fates to different cells by specific segregation to these cells.
What is the regulative development model?
cells are communicating and differences can be generated DE NOVO by cell-to-cell communication
What divisions do sea urchins have in their early stages?
- 2 divisions along the animal-vegetal axis
- 1 perpendicular to these divisions, separating the animal from the vegetal half
In what way are the first 2 cleavages in the development of a sea urchin made?
First 2 cleavages are perpendicular
In what way is the 3rd cleavage cut?
Perpendicular - and separates the 4 animal cells from 4 vegetal cells
Where does gastrulation occur?
at the vegetal pole
How was the sea urchin’s development suggested to be regulative?
if determinants were present, as suggested in the mosaic model, then you wouldn’t get a normal embryo, from a single blastomere
- separated 2 blastomeres at the 2 cell stages
- didn’t get 2 half embryos, but rather got 2 smaller complete embryos
How was the sea urchin development suggested to be mosaic?
- splitting the embryo at the 4 cell stage indicated that development was regulative.
- splitting the 8 cell embryo in a ventral & dorsal half showed that there is at least this axis a degree of mosaicism is present.
- normal embryo not found - instead there was an animalised and vegetalised incomplete larve - meaning there appears to be some form of mosaicism.
- now known that the localised determinists for this are in the cytoplasm rather than the nucleus.
Why do we use genetic model organisms?
by looking at what ‘goes wrong’, you can infer the normal role of that gene on development
What are the ideal characteristics for ease of genetic analysis?
- small organism (because they need to keep large numbers)
- large batches of embryos
- short generation time
- easy to breed
- easy scoring of phenotypes of +/-
- sequenced genome
What are the main anatomical parts of C elegans?
mainly consists of a gut & reproductive organs. Also has muscles that allow it to move & neurons and sensory cells that can convey information to the animal about the environment is navigating
Why are C. elegans good gene model organisms?
- C. elegans develop rapidly
- hatches from egg within 24 hours at normal temperatures
- develops as a hermaphrodite (first male, then female) - use its own sperm to fertilise eggs
- occasionally male only individuals occur - these can mate with females.
- thus prevents continuous inbreeding, which would be detrimental for their fitness
Describe the cleavages of a C. Elegans
1st cell division create: AB & P1 cell
- P1 cell - P2 & EMS
- AB cell - ABa & ABp
ABa (anterior)
ABp (posterior)
What does the AB cell do?
make hypodermis & neurones
What does the EMS cell do?
make mesoderm & endoderm
What do P2 & P3 do?
make C & D cells, which somatic tissues
What does P4 do?
form the germline - it also receives cytoplasmic granules named P-granules
What causes the first division to be asymmetrical?
Par proteins (par from partitioning) - par genes also found in Drosophila
Is cell fate deterministic?
lineage is invariant, but cell fate isn’t necessarily absolutely determined
- this can be seen by changing the cell position.
- e.g. ABa & ABp swapped and lineage produced will be according to their NEW position - this would be impossible if they would contain determinants.
- EMS cell is formed by what originally .would’ve been the P2 cell, so there is no fixed determinant for this either
What has C. elegans development study contributed to biomedical science?
- apoptosis = cell death
- RNA interference –> switching off genes
How many cells are in C. elegans & what do they do?
C. elegans is made up of 1090 cells but precisely 131 cells are programmed to die - apoptosis
When does apoptosis occur?
this happens during development but is also essential in a variety of biological processes in adult
Why is apoptosis essential for proper development?
- formation of reproductive organs: male/female
- skin between digits
- immune system maturation
Why is apoptosis essential for homeostasis?
- mitosis/apoptosis to maintain constant number of cells
- removal of damaged cells (DNA damage, viral infections)
What diseases can improper regulation of apoptosis can to?
- autoimmune disease
- cancer
What are the 4 factors involved in programmed cell death?
- BID
- Bcl2
- APAF1
- Caspase
What is RNA interference (RNAi)?
a powerful mechanism to control gene activity:
- double-stranded RNA triggers a biochemical process that degrades identical mRNAs, thus it blocks gene activity AFTER transcription has happened.
How was RNA interference (RNAi) found?
discovered while studying muscle development in C. elegans
How does RNA interference (RNAi) work?
- Double-stranded RNA is taken up by cells & recognised and cut into smaller pieces by an enzyme named Dicer creating short interfering (si) RNAs
- they are loaded into a protein complex named RISC, which uses the siRNA sequence to find mRNAs that are complementary to the siRNA and causes their degradation
Why is it important that short RNAs are used in RNA interference?
such oligo can be synthesised chemically
How has RNA interference been useful?
this has allowed the creation of siRNA libraries that can target every gene in the human and of course also C. elegans genomes.
this is a great tool to find genes that are involved in the process of choice
What is the application of RNA interference?
3 drugs on the market that are directly based on RNAi.
- aim to downregulate genetic diseases caused by overactivity of a gene
What are the 3 drugs based on RNAi?
- hereditary transthyretin-mediated amyloidosis
- acute hepatic porphyria
- hyperoxaluria type 1
