Concepts And Terminology Flashcards
1
Q
Remind yourself of the basic points
A
- Embryo implants at blastocyst stage (between 32 and 64 cells)
- Blastocyst has 2 layers = trophoectoderm and embryoblasts
- Trophoblasts consist of cytotrophoblasts which form syncytiotrophoblasts which invades the decidua to form placenta
- Embryoblast forms epiblast and hypoblast
- Epiblast becomes embryonic epiblast which gives rise to embryonic ectoderm and primitive streak (which involutes to form embryonic endoderm and embryonic mesoderm)
- Hypoblast becomes extraembryonic ectoderm then yolk sac
- Development is determined by restriction of cell fate.
- Once the blastocyst differentiates into trophoblasts and embryoblasts, the cells are committed. They cannot switch (this is a general rule but there are some exceptions)
2
Q
How is cell fate controlled? What are the 2 possibilities?
A
- Controlled by molecular changes
- There are 2 possibilities
- Mosaic development (cell autonomous specification):
- instructions/information are inherited from parent cell hence cell’s fate is pre-determined - Regulative development (conditional specification):
- cell isn’t committed to a particulate fate, instead it is influenced by its surrounding/positions within embryo.
- the cell initially has the potential to become any type of cell
3
Q
What did Weismann’s nuclear development?
A
- Proposed that nuclear determinants are unevenly segregated during cleavage
- E.g in the diagram, you can see the determinants go into different cells so each daughter cell has different nuclear determinants hence their cell fate will be different
- Represents mosaic development
(The one about nuclear/maternal determinants)
4
Q
What did Roux’s experiment on determination state?
A
-He tested Weismann’s theory and mosaic development using frogs
- He ablated one cell (but not remove) of a 2 cell embryo - this resulted in half an embryo which supports the mosaic development
- However, this was not the correct interpretation of the results
- When the experiment was repeated, but the 2 cells of the embryo were separated than being ablated, you get 2 perfect tadpole embryos
- This supports regulative development
- It was able to regulate and develop without any missing parts
5
Q
What is invariant cell fate in C.elegans?
A
- C.elegans = nematodes
- They are used to study embryonic development
- They are completely transparent so we can see every single cell within embryo
- Everytime you observe a C.elegans embryo, they follow the same developmental pattern
- We start off with one cell which divides into 2 which have known cell fates
- The cells divide into other cells with different cell fates but these cell types are always the same
- By the time the embryo is fully developed, there are 558 cells; each can be traced back to a specific origin, it never varies —> suggests mosaic development
6
Q
How did Driesch demonstrate regulative development?
A
- An intact 4-cell sea urchin embryo generates normal pluteus larva
- When the 4-cell embryo is removed and each cell is isolated, each cell can form a smaller but normal pluteus larva
-The 4 larvae derived this way were not identical despite their ability to generate all the necessary cell types
7
Q
What are development and decisions based on?
A
- Both mosaic model and regulative development
- Also more complex interactions exist and provide combinations of these 2 —> these allow robustness in embryonic development
A theoretical example:
- normally, the initial cell gives rise to 2 cells - one which forms skin and head tissue; the other would give rise to trunk and tail
- if we ablated a cell at 2-cell stage and mosaic development was the sole method of controlling cell fate, the embryo would develop either skin+head tissue or trunk+tail - not both
- if we ablated a cell at 2-cell stage and regulative development was in charge of controlling cell fate, the embryo would overcome the issues and all tissues would be present (as cells don’t have a predetermined fate)
An experiment in C.elegans:
- in the embryo, the pharyngeal tissue usually develops from the Aba cell, with its sister cell Abp giving rise to trunk and tail
- When the position of Aba and Abp are exchanged, the tissues they give rise to swap as well - Aba give rise to trunk and tail; Abp give rise to pharyngeal tissue
- This implies that the position of a cell within the embryo determines its fate - this is regulative development as the cells do not have a predetermined fate
(In the diagram, Aba and Abp are connected to a P cell in a different way, so it’s the interaction with the P cell which is responsible for regulation of development)
8
Q
What are the decisions of fate and commitment?
A
- Fate = what will normally happen to a cell during development
- Committment = specification (what tissues will develop in an autonomous environment). Determination (irreversible change in potential). When a cell is determined, it’s committed to a certain fate
9
Q
What are the definitions of potential/potency and differentiation?
A
- Potential/potency = range of tissue which a cell can give rise to
- Differentiation = restriction of potential with molecular and biochemical changes (describes the transition to a mature cell type)
10
Q
How do you test fate and committment?
A
- If you take a piece of tissue from an early embryo labelled with a dye, and then you graft it into an unlabelled host embryo at different times and locations, we see different effects
- If we graft it into the same place at the same time or to a different location, the embryo will develop as normal and the cell will have normal fate
- If we take a graft from a later embryo, normal development will not occur because determination would have already occurred
11
Q
What is induction?
A
- Restriction in potential depends on inductive interactions from neighbouring cells
- Competence = ability to respond to inductive signal
- Spermann and Mangold did an experiment that demonstrated cell-cell communication is important for regulating cell fate; one group of cells can act as an ‘organiser’ of embryonic pattern formation
- Hensen’s node plays this role in the primitive streak
12
Q
What are the 2 types of inductive interactions?
A
- Permissive induction
- Instrcutive induction
- Permissive induction
- where tissues create an environment where other factors can then act to induce changes - Instructive induction
- oppositional and morphogen gradient
- Appositional induction = instruction is passed by coming together of 2 tissue types
- Morphogen gradient = a localised signal is crested and diffuses outward. Different concentration along the morphogen gradient induce different responses
13
Q
What is a morphogen?
A
- Diffusible molecule that triggers different cell fates at different concentrations e.g transcription factors
- Different concentrations induce different cells
- Can provide positional information within embryo
14
Q
What are HOX genes?
A
- They confer positional identity
- Mutation of the annapedia gene (member of HOX family) causes Drosophila antennapedia mutants to have legs instead of antennae on their heads
15
Q
What are homeotic genes?
A
-Regulate development of anatomical structures
- In a theoretical animal, a morphogen gradient specifies 4 distinct segments
- Morphogen concentration is highest at caudal end compared to cranial end
- The morphogen induces the expression of all 3 genes
- At the highest levels (tail/caudal end), all 3 genes are expressed
- At lower concentrations of morphogen, genes 1 and 2 are expression
- At even lower concentrations, just gene 1 or no genes are expressed
-If we alter the morphogen gradient, we alter gene expression in different regions and alter the phenotypic identity of the segment
- Without knowing the identity of certain genes, we can begin to deduce their functions and how they interact
- E.g we can determine which gene is important for which segment
