lecture 12 - mesoderm induction and patterning

Question 1

What are signals as morphogens?

Answer 1

How do cells know where they are in the developing embryo and what they should develop into?
Diffusion from a source can establish a concentration gradient of a secreted signal .
Sensing the concentration of the signal in the immediate environment tells a cell where it is relative to the signalling source.
Cells are able to sense the local concentration of the signal and respond in different ways depending on concentration.
A substance that can elicit different responses in a cell depending on its concentration is called a “MORPHOGEN”.
Morphogen gradients allow cells to acquire
positional information

Question 2

Describe Morphogens: The French flag model

Answer 2

simple pattern of 3 different states (colours) Each cell has the potential to develop as blue, white or red
Signal source at one end of the array of cells.
Diffusion of signal establishes gradient of morphogen.

Position of each cell is determined by concentration of morphogen
Morphogen gradient allows cells to acquire positional information about where they are relative to source.

Positional value is interpreted by cells which differentiate to form a pattern
Cells respond appropriately to serial threshold of morphogen concentration

The concentration gradient can be interpreted in many different ways.
Could generate other patterns with different colours or more threshold responses

Question 3

What are the stages of early development?

Answer 3

CLEAVAGE (no growth, rapid)
BLASTULA (a fluid filled cavity forms)
GASTRULA (cell movements, formation of 3 germ layers)
NEURULA (neural tissue forms)
TAILBUD (Phylotypic stage)

Question 4

Describe the structure of early vertebrate embryos

Answer 4

Early vertebrate embryos consist of three germ layers.
The ectoderm-Forms the epidermis and central nervous system
The mesoderm-Forms the notochord, dermis, skeleton, muscle, kidney, heart and blood
The endoderm-Forms the gut, liver, pancreas and lungs

Question 5

How is the mesoderm formed?

Answer 5

Formation of the mesoderm requires cell signalling.
Marginal zone explants (the equatorial region) from early blastula embryos only form epidermis; however, marginal zone explants from later blastula embryos form mesoderm.
We can conclude that some event during blastula stages changes the fate of these marginal zone cells.
Signals from the vegetal pole (endoderm) induce
mesoderm in animal cap (ectoderm) explants
By labelling the animal cap cells with a fluorescent dye, it was found that all the mesoderm is derived from the animal cap. No mesoderm arises from the vegetal cells. We can conclude that the vegetal cells produce a mesoderm inducing signal.

Question 6

How are the ventral mesoderm and dorsal mesoderm induced?

Answer 6

Peter Nieuwkoop showed that vegetal cells produce two distinct signals. One signal induced
extreme ventral mesoderm (mainly blood and smooth muscle.) One signal induce extreme
dorsal mesoderm (mainly notochord). Small dorsovegetal (DV) region induces extreme dorsal
mesoderm.Large ventrovegetal (VV) region induces extreme ventral mesoderm.
These experiments identify DV and VV cells as the source mesoderm inducing signals.
However, they do not induce the full range of mesodermal tissues e.g muscle and kidney
Ventral mesoderm:
Blood
Smooth muscle
Dorsal mesoderm:
Notochord
+ a little skeletal muscle

Question 7

Describe the role of the VMZ and DMZ

Answer 7

Cell lineage studies show that during normal development most of the skeletal muscle comes from the VMZ but in isolation it forms very little.
VMZ - mostly blood and smooth muscle
DMZ - mostly notochord
Combinations of late blastula / early gastrula VMZ and DMZ will give rise to
all mesodermal tissue types. Lineage studies show that in these combinations most of the muscle and kidney come from VMZ derived cells.

Question 8

What is the 3 signal model?

Answer 8

Blastula stage
1) DV signal induces DMZ
2) VV signal induces VMZ
Late blastula / early gastrula
3) DMZ source of signal that 
     patterns the VMZ
Results in formation of 
full range of dorsal and 
ventral mesoderm

Question 9

Describe how TGF β related molecules can act as morphogens

Answer 9

All the cells of the animal hemisphere can respond to mesoderm inducing signals. This forms the basis of an assay for mesoderm inducing molecules.
TGFβ family members activin and nodal are active in this assay.
A high concentration of activin or nodal induce dorsal
mesoderm
A low concentration of activin or nodal induce ventral
mesoderm

Question 10

Describe growth factor-β signalling

Answer 10

There are over 60 TGFβ-related ligands in the human genome.
TGFβs are secreted dimeric proteins with many biological activities in the adult and developing embryo
TGFβs signal through cell surface serine/threonine kinase receptors.
The intracellular effectors of the TGFβ signalling pathway are SMAD proteins which are activated by phosphorylation.
On signalling SMAD proteins translocate to the nucleus where they work with other proteins to activate and repress gene transcription.

Question 11

Describe Evidence that Activin-like signals are

required for mesoderm induction

Answer 11

A dominant negative form of the activin receptor was made: this mutant receptor lacks the intracellular kinase domain.
When expressed in cells the dominant negative activin
receptor dimerises with normal endogenous receptors,
and no signal can be transmitted because no phosphorylation
can take place. All activin and nodal signalling is blocked.
When mRNA coding for the mutant receptor is injected
into Xenopus embryos, all nodal signalling is blocked
and the embryos that develop form no mesoderm.