Developmental biology 3 (Prof. Dale) Flashcards
What were the 2 theories accounting for tissue formation have proposed by Aristotle (384-322 BCE) ?
Preformation Vs epigenesis
What does preformation theory say ?
Preformation: organs and tissues are preformed and correctly positioned in the fertilised egg. They simply enlarge during embryonic development.
What does epigenesis theory say ?
Epigenesis: organs and tissues are formed gradually, complexity of the embryo increasing with time. Aristotle preferred this explanation.
How did Aristotle provide us with one of the earliest descriptions of embryonic development ?
He cracked open chicken eggs on every day of their 3 week incubation and described the embryos with his naked eye (+subsequently provided 2 explanations for appearance of organs).
What did Antonie van Leeuwenhoek (1632-1723) mean when he said “[…] all manner of great and small vessels, so various and so numerous that I do not doubt that they be nerves, arteries and veins… And when I saw them, I felt convinced that, in no full grown body, are there any vessels which may not be found likewise in semen.” in 1677 ?
Van Leeuwenhoek believed that organs were preformed
in the spermatozoa i.e. he thought he could see a fully formed human (homunculus) in the head of each sperm.
What is the problem of preformation (having organs preformed in either the egg or the spermatozoa) ?
Children inherit characteristics from BOTH parents.
How does the development of the chick embryo provide evidence against preformation (and in favour of epigenesis) ?
Was this evidence widely accepted by the early 18th C. ?
- examining developing chick embryos –> complexity increases with time –> embryo not preformed
- Aristotle could see this without the aid of microscopes
- remarkably, this was not evident to many late 17th/early 18th C. scientists using dyes to stain the embryos and microscopes to see them!
What are the different levels of potency (in decreasing order) that a cell can have ?
Totipotent (all cell types) > pluripotent (cells from 3 germs layers but not extra embryonic tissues) > multipotent > oligopotent > unipotent
What pathway must a totipotent cell follow to become a sk muscle cell ?
Fertilized egg –> ICM –> mesoderm –> somite –> myotome –> sk muscle
What are the 3 different phases of cell commitment ?
What happens during these phases ?
(1) Specification = cells receive instructions on what they are to become but they do not become fully committed –> can be changed experimentally by moving them to a new environment
(2) Determination = cells become fully committed to their fate –> cannot be changed by moving them to a new environment
(3) Differentiation = cells acquire those characteristics that distinguish them from all other cell types –> often involves transcription of tissue-specific genes
What are the 3 major factors that contribute to cell commitment ?
- Localised Determinants
- Embryonic Induction
- Morphogen Gradients
What did Wilhelm Roux (1850-1924) suggest in 1888 concerning localised determinants ?
What theory does this seem to echo ?
- cell fates may be specified by maternal localised determinants, laid down in the cytoplasm of the egg during oogenesis
- each major cell-type having its own determinant
- only cells that inherit the determinant adopt the specified fate
This could look like preformation at the molecular level.
Edwin Conklin (1963-1952) found in 1905 that eggs of the ascidian Styela partita contained a crescent of yellow cytoplasm that was subsequently found in larval muscle cells. What did he suggest concerning this yellow cytoplasm ?
Conklin suggested that this yellow cytoplasm contained a muscle determinant.
Hiroki Nishida and Kaichiro Sawada identified in 2001
macho-1, a maternal mRNA that is localised to blastomeres containing yellow cytoplasm.
What did they find concerning macho-1 ?
What can we therefore say about macho-1 ?
- deleting macho-1 = loss of muscle formation
- injecting macho-1 into a different blastomere = ectopic muscle formation
Macho-1 = a transcription factor that regulates muscle-specific genes (e.g. m-actin, m-myosin, myf, tbx6, snail).
How did Hans Driesch (1867-1941) show in 1893 that sea urchins did not develop using local determinants ?
Driesch dissociated cleavage stage sea urchin embryos –> individual blastomeres formed small but otherwise nearly normal larvae ==> Regulation means that sea urchin eggs cannot be a mosaic of localised determinants
What did Driesch’s observe in his see urchin experiment ?
What did this imply about regulation ?
Each cell of the 4-cell sea urchin (and mammalian) embryo has greater potential than they normally exhibit –> they can form more tissues when isolated from their neighbours than when left in contact with them
Hyp: cells communicate with each other to restrict their potency
What is embryonic induction ?
What does it require ?
What are examplar signalling proteins ?
Embryonic induction = the process whereby a cells fate is changed by signals from an adjacent group of cells
It requires two types of cell = signalling cell + responding cell
Signalling proteins: Activin, BMP, EGF, FGF, Hedgehog, IGF, Nodal, PDGF, TGFß + Wnt
What did Pieter Nieuwkoop (1917-1996) observe when he isolated the animal and vegetal fragments from axolotl blastulae ?
Which fragment makes the futur mesoderm ?
What did he conclude from this ?
- animal and vegetal fragments –> differentiate into epidermis + endoderm respectively
- mesoderm –> also formed when animal and vegetal fragments were combined
- mesoderm –> only formed by the animal fragment
Concl: mesoderm formation –> induced by the vegetal fragment
How did Nieuwkoop suggest that mesoderm induction occured ?
- mesoderm –> induced in the equatorial region of amphibian blastulae by a signal from the vegetal hemisphere
- this signal –> induces expression of mesoderm specific genes such as brachyury
How can the Xenopus blastula stage animal caps be used as an assay to identify mesoderm inducing factors (MIFs) ?
What results will this yield ?
Why do caps w/ MIFs elongate ?
By incubating them in media containing candidate molecules.
Animal caps –> only form epidermis when incubated alone (specification) Vs form mesodermal tissues (e.g. notochord and muscle) when a MIF is added
Caps w/ MIFs elongate because the induced notochord undergoes convergent-extension.
What are the 2 main families of MIFs ?
Which proteins belong to these different families ?
Are any of these proteins limited to the vegetal hemishphere of Xenopus blastulae ?
- Transforming Growth Factor ß (TGFß) family: Activin, BMP 2/4, Growth & Differentiation Factor 3, Nodal, Vg1 (GDF1)
- Fibroblast Growth Factor (FGF) family: FGF1/2/4/8
- Only GDF3, Nodal, and Vg1 are localised to the vegetal hemisphere of Xenopus blastulae
Which is the key signalling molecule for mesoderm induction ?
Inhibition studies in zebrafish, frogs + mice suggest that Nodal is the key signalling molecule.
How does mesoderm induction occur ?
- Nodal = primary mesoderm inducing signal released by the vegetal hemisphere –> induces brachyury (bra) expression in the mesoderm
- brachyury –> encodes a T-box transcription factor that regulates expression of fgf4
- FGF signalling (ERK2*) –> maintains expression of brachyury
- +ve feedback circuits like this –> commonly used by embryonic genetic networks
What are morphogens ?
How are they synthesized/destroyed ?
How do cells use them for communication/induction ?
- molecules that induce multiple cell fates in a concentration dependent manner
- synthesised by source cells, morphogens diffuse across a layer of unspecified cells to form a gradient (w/ highest concentration closest to the source)
- a “sink”may destroy the morphogen
- cells determine their fate by reading the concentration of the morphogen
What model did Lewis Wolpert propose to explain how morphogens worked in 1969 ?
The “french flag” model (cells above a threshold concentration T1 become blues cells, those between T1 and T2 white cells, and those below T2 red cells).
How is the spinal chord organized embryologically ?
- distinctive dorsal-ventral pattern –> can be revealed by the distribution of distinct transcription factors
- ventral midline lines just above notochord + contains a specialised structure = floor plate
- above floor plate, in distinctive order –> a number of different ventral interneurons (PV0-PV3) + motoneurons
Which structure regulates spinal chord patterning ?
The notochord.
How do we know that cell fate within the ventral half of the neural tube is specified by the notochord ?
Because the notochord induces ectopic floor plate + motor neurons when grafted to the side of the neural tube.
Motor neurons can also be induced when a floor plate is grafted into the side of the neural tube.
What does Shh from the notochord induce ?
The floor plate (+Shh expression in the floor plate).
What does Shh from the floor plate generate ?
How do neural tube cells respond to different [Shh]s ?
- A gradient across the ventral half of the neural tube
- concentration = high ventrally + low dorsally
- neural tube cells –> respond to different concentrations of Shh by activating expression of different genetic programs, specifying at least five different cell-types.
