Lecture 1: General principles Flashcards
How can we use cells to study development?
We can study development at the cellular level.
• Directly observe living embryos. Works best for C. elegans and zebrafish.
• Look at the anatomy of fixed and sectioned embryos.
• Mark cells to identify all descendants of a single embryonic cell. This can be done with dye injection of genetic tagging.
How can we use molecular biology to study development?
We can also biochemically phenotype cells. We can profile the RNA or protein in a single tissue or cell type. We can use techniques such as Western blots, RT-PCR or Northern blots.
How can we use physical manipulation to study development?
We can also move, add or remove cells to see what effect this has on the embryo.
• Grafting is the process of moving cells or tissues within or between embryos.
• We can isolate cells.
• We can ablate cells (remove them using a laser).
What is the difference between forward and reverse genetics?
Forward genetics is when you pick a phenotype and try and work out what is causing it. 1) Define a developmental process. 2) Mutagenize a population of animals. 3) Screen for the desired phenotype. 4) Characterise it. 5) Find the gene responsible. Reverse genetics is when you discover what happens when you manipulate a certain gene. 1) Choose a candidate gene. 2) Mutate it. 3) Examine the phenotype.
How can we use genetic methods to study development?
- We can also use genetic crosses to identify if a gene is dominant or recessive.
- Create genetic mosaics. Different genotypes in individuals from a single fertilised egg.
- Epistasis to examine double mutants.
- Find if a mutation is LOF or GOF.
- Find partial or conditional mutants (e.g. temperature sensitive).
- Control when or where genes are expressed.
- Over or under express genes and see what happens.
What are switch genes?
Switch genes are genes which are involved in controlling key developmental choices. Switch genes will have mutations which are both recessive and dominant.
Recessive mutations demonstrate that there is a LOF mutation and it is necessary. We can consider a mutant called Toe5, where a mouse has 4 toes instead of 5. If it is recessive, we know that wildtype toe5 is necessary for 5th toe development.
Dominant mutations demonstrate that there is a GOF mutation. For example, we can look at another mutation of toe5 called toe5D. Toe5D causes a phenotype of 5 5th toes on each foot. The gene is dominant, so this means that a gene is sufficient to direct development of a 5th toe.
If a gene is both necessary and sufficient for development, then it is a switch gene.
What is epigenesis?
Epigenesis is the process of development. Development of an animal depends on the execution of a generative program. There is no blueprint found in the genome.
• Development of an animal means modifying the development of its ancestors.
• Much of development has non-optimal design features and has to be understood partly as a consequence of evolutionary history.
• There are many redundant mechanisms as an evolutionary relic and also as a means to ensure normal development.
• There are many non-optimal design features which are a consequence of evolutionary history.
• Development may also involve metamorphosis.
How does cell division occur?
A single cell initiates development. It must then undergo divisions as its descendants differentiate.
• The spatial arrangement of cells is called pattern formation.
• Temporal patterning is also important.
• All cells have the same genome, but they must become progressive restricted in their potential.
• Most cells differentiate eventually.
• Some cells remain pluripotent or multipotent (e.g. haemopoietic stem cells).
• Sometimes cell death is required to ensure the correct pattern and shape of cells (e.g. forming fingers).
• Cell divisions are not always random. Sometimes they are oriented to form sheets or lines.
• Cell divisions can also be asymmetric. This depends on the control of spindle position and direction. This can be used to give daughter cells which have different cell fates.
• For example, the zygote (fertilised egg) in C. elegans gives unequal daughter cells. There is an AB daughter and a smaller posterior daughter known as P1. AB divides equally and P1 gives EMS (large) and P2 (small).
• 3D structure is often created by using sheets of cells which are folded.
• Most spatial patterning events happen in 1 or 2 dimensions.
• Induction events result from juxtaposition of different cell layers which can signal to each other.
• Fields of cells with different developmental potential can arise within sheets.
• Some developmental fate decisions result from random events. This can be necessary to break symmetry (e.g. sperm entering a symmetrical egg).
• Sometimes random patterns can be modified into precise patterns. For example, in fly eyes.
Why is cell signalling important?
Signalling pathways have distinct chemical/biological properties, which affect their use in different developmental contexts. ECM interactions can control extracellular diffusion.
There are different methods of signalling.
• Long range diffusion.
• Short range diffusion.
• Cell contact. Eph/ephrin
• Cytoplasmic connection. Gap junction.
Examples of longer range signals are RTKs and wingless.
Why is cell sorting and affinity important?
Cell affinity (stickiness) has an important role in many different processes of development.
• Cell sorting.
• Cell shape.
• Morphogenesis.
• Controlling cell migration. Sometimes embryonic cells are created a long way from their final destination.
What do slime moulds tell us about development?
Also known as slime moulds; these slugs have interesting developmental characteristics.
• The anterior cells become a stalk and the posterior cells become spores.
• If one cuts the slug in half, during development, both halves will develop a stalk and a spore.
• This implies that the cells know their location within the organism.
