Lecture 20 Morphogens and Patterning Flashcards
Morphogenesis is
One of three fundamental aspects of developmental biology
others are control of cell growth and differentiation
Morphogens are
Signalling molecules
Shape the form of a tissue
Morphogenesis is the
Patterning of multicellular organisms during development
Establishing of a tissue pattern requires
- Local production of a morphogen
- Degradation of the morphogen
- Diffusion of the morphogen
Variation of production, degradation and diffusion of a single morphogen can lead to
The establishment of a variety of patterns
Two or more morphogens and their interactions can lead to the formation of
Very complex patterns
Wolpert defines cell fate as a function of the
Morphogen concentration (French flag model)
The French flag model
- Source secretes a morphogen
- Morphogen forms a concentration gradient in neighbouring tissue
- Cell fate is determined by the different concentrations
Morphogens guide pattern formation:
Level of morphogen concentration determines whether gene A B or C is activated
This leads to the generation of morphologically different cell types
They are arranged in a way that depends on the localisation of the source
Butterflies as an example of Turing’s mathematical model of morphogenesis
Number of activated Wnt receptors determine the genes transcriptionally activated
High threshold: white
Moderate threshold: black
Low threshold: yellow
Shh in the spinal cord primordium
Shh is a morphogen pattern in the spinal cord
Shh is released from the floorplate and notochord (FP, NC)
Shh binds to Ptch
Shh-Ptch allows Smo signalling to activate Gli
Different interneuronal and motoneuronal population are induced depending on the Shh activation threshold
The number of activated Shh receptors determine the
Transcription factors, which lead to the induction of 5 different neuronal populations
In addition to Shh what is also secreted from the spinal roof plate?
BMPs/Wnt
BMPs/Wnt countergradients induce
Different neuronal identities in the spinal cord
Interneurons V0-V3 and D1-D6
Motorneurons
Critical evaluation of the diffusion based model
French flag and turing models based on assumptions:
- Production and degradation of a morphogen must be linked
- Transport allows patterning of entire tissue
- Tissue integrity and size are maintained
The inhibitor model (gierer-meinhardt, 1972)
- Morphogenetic fields control development of tissues
- Production of morphogen coupled to production of inhibitor
- Morphogenesis dependent upon speed
Gierer-meinhardt suggested the development of tissues is dependent upon
Diffusion speed
Inducer/activator = slow diffusion, short range
Inhibitor = fast diffusion, long range
Hydra is studied for
Regeneration
Polar and continuously renews itself (stem cells)
Mechanical dissociation leads to regeneration of entire new body column
Induction and maintenance of a morphogenetic field translates into
Tissue polarity and positional information
Hydra supports the
French flag model
Morphogen/inhibitor ratio at time of cutting determines whether a head or foot will grow
A graft of head tissue onto a hydra foot
Induces a second head
But only induces a head in areas of hydra where inhibitor is low (so can’t induce a second head close to the first)
HydraWnt3 induces
The head
Homolog of vertebrate Wnt3a gene
Expressed at the mouth
Paracrine signalling molecule
HydraDkk is the
Inhibitor of HyWnt3
Represses head formation
Long range inhibitor
Morphogens determine
The pattern in developing tissue
Morphogens are substances which are
Locally produced and secreted to influence the development of neighbouring tissue (Alan Turing)
Morphogens form a
Gradient
Cells determine their fate depending on
Morphogen concentration (Wolpert)
A stable gradient is achieved through
Activator - inhibitor interaction (Gierer and Meinhardt)
Interplay of activator and inhibitor allows
Pattern formation of tissue development
Morphogenetic gradients are required in
Development and regeneration (hydra)
