Introduction Flashcards
What question does developmental biology try to answer?
How do we get from a single cell to a multicellular organism?
Does development stop with birth?
NO
It is a lifelong process
What challenges does aging put on our society?
Strain on resources and money
What changes cell behvaiour?
Change in the genetic programme (gene expression)
Why are animal models used in developmental biology?
Most of the concepts that drive developmental processes are evolutionarily conserved
Understand what goes on in animal model - can help to understand what goes on in human
What are the 2 opposing theories of developmental biology?
1) Epigenesis
2) Preformation
What is the epigenesis model of developmental biology?
- Embryo develops in a step-wise process
- PROGRESSIVELY acquires new structures and function
Who proposed the theory of epigenesis?
Aristotle
Why was the theory of epigenesis not widely accepted?
Scientists struggled to understand how the generation of new structures and tissues were formed
What is the preformation theory of developmental biology?
- Start from a mass of cells which are a predetermination of something that will exist later on
- Organism develops from a miniature version of themselves (homunculus)
- Homunculus sits inside the sperm head
- Just needs to be allowed to grow
What discovery lead back to the epigenesis theory?
The discovery of the cell by Robert Hooke (1655)
How did Robert Hooke discover the cell?
- Placed thin sections of cork underneath the microscope
- Saw compartments, which he called cells - stated that these were the basic structure of life
What did the discovery of the cell produce?
- A tool to understand how you could get progressive development
- Have a basic unit with can be studied and manipulated
What did the discovery of the cell lead to?
The END of the preformation theory
From the theories of developmental biology, what 3 conclusions were made?
1) All organisms are composed of one or more cells
2) The cell is the most basic unit of structure, function and organisation in all organisms
3) All cells arise from pre-existing living cells
What are the 2 theories that describe how cells become different to each other, in order to generate an embryo?
1) ‘Germ plasm determinants’ theory
2) ‘Induction theory’
Who proposed the ‘germ plasm determinants’ theory?
Weismann
What did Weismann suggest?
- Germ cells have a set of determinants
- All germ cells have the SAME SET of determinants
- Somatic cells only have a SUBSET of these determinants: this is what determines the structure and behaviour of the cell
Who performed an experimental demonstration of Weismann’s theory?
What did he do?
William Roux:
- Took a fertilised xenopus egg at the 2-cell stage and killed one cell using a burning needle
- Saw that only HALF the embryo developed
- This supports Weismann’s theory
Who proposed the ‘induction theory’?
Hans Driesch
What experiment did Hans Driesch perform?
- Sea urchin eggs at the 2 or 4 cell stage
- Separated the cells from each other and allowed them to develop
- Lead to the formation of a FULL embryo
What was the difference between Roux and Drieschs experiment?
What does this show?
Driesch disrupted cell-cell communication
Shows that NORMAL development is dependant on cell-cell communication
What did Driesch prove about Weismanns theory?
How?
Proved it not to be entirely true:
- Each somatic cell does have the full set of determinants
- BUT, only choose to execute a subset of the programme
How does the cell choose what subset of determinants it is going to express? How?
Who proved this? How?
Cell-cell communication
- Cells INSTRUCT neighbouring cells to carry out a certain genetic programme or PREVENT from carrying out a certain genetic programme
Proved by Hans Driesch - when the cells were separate, there was no positive or negative signals - allowing to form the whole embryo
What drives the differences between cells?
Differenced in genes that are transcribed and translated
Describe the protein content in cells
- Some proteins are ubiquitous (the same in every cell)
- Some proteins are SPECIFIC to different cells
What does the REPERTOIRE of proteins in a cell determine?
The IDENTITY and FUNCTION of the cell
What 2 things determine the repertoire of proteins in a cell?
1) Intrinsic cues
2) Extrinsic cues
What are intrinsic cues?
Cell directly differentiates into another cell
Driven by transcription factors becoming active
What are extrinsic cues?
External cells in the environment which instruct another cell when there is NO RELATIONSHIP between these 2 cells (no lineage)
How do cells acquire new properties that make them different to each other?
1) Cell division - symmetric, asymmetric
2) Cell adhesion - dispersal, condensation
3) Cell shape - Epithelial or mesenchymal
4) Cell migration
5) Cell death
What is symmetric cell division?
Formation of IDENTICAL daughter cells
What does symmetrical division allow?
The growth of a tissue
What is asymmetric cell division?
Formation of 2 daughter cells that are different to each other
What gives rise to asymmetrical cell division?
Differential segregation of cell determinants
What does asymmetric cell division lead to?
Diversity
Which type of cell division is widely used in stem cell biology?
Why?
Asymmetrical cell division:
- Leads to the generation of a new cell type from the mother cell
- Whilst continuing to propagate another cell type (maintain the stem cell)
What 3 ways does cell-cell communication occur?
1) Paracrine
2) Autocrine
3) Juxtacrine
What is paracrine signalling?
Secreted ligand which recognises a receptor on a NEIGHBOURING cell
What is autocrine signalling?
Secreted ligand which recognises a receptor on the SAME cell
What is juxtacrine signalling?
Ligand and receptor are both MEMBRANE bound
What does juxtacrine signalling require?
Very close interactions between different cells
What is an example of juxtacrine signalling?
Notch signalling
What does signal reception require cells to be?
What does this mean?
COMPETENT:
- Permissive environment, receptor and transduction components must be present
What does signal reception require signals to be?
INSTRUCTIVE or PERMISSIVE
What do instructive signals do?
Initiate a new programme
What do permissive signals do?
Provide a favourable environment for a specific programme
What can signals act as?
What do these do?
Morphogens
Instruct distinct cell fates according to their concentration
What does transmission of a signal to the nucleus involve?
What does this lead to?
A signalling transduction cascade - cell surface, cytoplasmic and nuclear proteins that relay the signal
Leads to changes in gene expression
What are 3 common features in signal transduction cascades?
1) Reception
2) Transduction
3) Response
What modulates receptor activity?
Series of proteins:
- Co-receptors
- Inhibitors
How is a signal transduced from the receptor to the nucleus?
Signalling cascade of secondary messengers
What is the response of signal transduction pathway?
- Transcription factor activated
- Induction of the expression of specific target genes
- Change in the gene expression of a cell (mRNA)
What are the levels at which a cell can operate to control a gene programme?
(As transcription/translation determines the protein content of a cell, which determines the structure/function)
1) Production of mRNA
2) Processing/stability of mRNA
3) Production RATE of proteins
4) Activity of proteins (eg. phosphorylation)
Does the presence of a protein/expression of a gene imply the function of the gene is present?
Why?
NO
Gene expression/translation/transcription can be modulated at many different stages
What might the product of the gene that is newly transcribed do?
Activate or repress another gene from being transcribed (which may drive the identity of another cell)
How can the control of a genetic programme occur at the level of gene transcription?
- Enhancer-mediated control of gene expression
- miRNAs
- Control at the level of chromatin
