Cell And Developmental Biology Lecture 12

Front 1

1) Morphogens? What are they? What do they allow?

2) The french flag model?

Back 1

1) •A substance that can elicitdifferent responses in cell depending on its concentration is calleda “MORPHOGEN”.

•Morphogengradients allow cells to acquire positional information.

2) A simple pattern of three different states.

Each cell has the potential to differentiate into either one of the three states, the position of each cell is defined by the morphogen concentration, positional value is interpreted by the cells (above or below certain morphogen concentration thresholds) which differentiate to form a pattern.

Signal source at one end of the array ofcells. Diffusion of signal establishes gradientof morphogen. Morphogen gradient allows cells toacquire positional information about where they are relative to source. Cells respond appropriately to serialthreshold of morphogen concentration.

Front 2

1) Stages of early development? (5)

Back 2

1) CLEAVAGE(no growth, rapid, reductional development- individual cells are getting smaller, overall embryo size stays the same)

BLASTULA(a fluid filled cavity forms- blasteole)

GASTRULA(cell movements, formation of 3 germ layers)

NEURULA(neural tissue forms)

TAILBUD(Phylotypic stage- the stage that is typical of our phylum (e.g-chordata forms the notochord which is a a cartilaginous skeletal rod).

Front 3

1) Early vertebrate embryos consist of three germ layers? What do they develop into? How are they arranged in the amphibian blastula stage embryo?

Back 3

1) 1.Theectoderm- Forms the epidermis and central nervous system

2.Themesoderm- Forms the notochord, dermis, skeleton, muscle, kidney,heart and blood

3.Theendoderm- Forms the gut, liver, pancreas and lungs

From top (at the animal pole) to the bottom (vegetable pole- have bigger cells, more yolk, unpimented) of the embryo:

Ectoderm --> Mesoderm --> Endoderm

Front 4

1) Studies to show that the formation of the mesoderm requires cell signalling?

Back 4

1) Marginal zone explants (theequatorial region) from early blastula embryos only form epidermis; however,marginal zone explants from later blastula embryos form mesoderm.

We can conclude that some eventduring blastula stages changes the fate of these marginal zone cells.

Signals from the vegetal pole(endoderm) induce mesoderm inanimal cap (ectoderm) explants. By labelling the animal cap cellswith a fluorescent dye, it was found that all the mesoderm is derived from the animal cap. No mesoderm arises from the vegetal cells. Wecan conclude that the vegetal cells produce a mesoderm inducing signal.

Front 5

1) Vegetal cells produce two discrete signals?

Back 5

1) Peter Nieuwkoopshowed that vegetal cells produce two distinct signals. One signal induced extreme ventralmesoderm (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 DVand VVcells as the source mesoderm inducing signals. However, they do not induce thefull range of mesodermaltissues e.gmuscle and kidney

Front 6

1) Cell lineage studies and VMZ and DMZ?

Back 6

1) Cell lineage studies show thatduring normal development most of the skeletal muscle comes from the VMZ (Ventral marginal zone) but inisolation it forms very little.

Combinations of late blastula /early gastrula VMZ and DMZ (dorsal marginal zone) will give rise to all mesodermaltissue types. Lineage studies show that in thesecombinations most of the muscle andkidney come from VMZ derived cells.

Front 7

1) "Three signal model"

Back 7

1) The “three-signal model”provided a framework to think about the signals important duringmesoderm induction and patterning.

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.

Front 8

1) TGFβ signalling and mesoderm formation?

Back 8

1) TGF βrelated molecules can act as morphogens

•All the cells of the animal hemispherecan respond to mesoderm inducing signals. This forms the basis of an assay for mesoderm inducing molecules.•TGFβfamily members activin and nodal are active in this assay.

A low concentration of activin or nodal induce ventral mesoderm.

A high concentration of activin or nodal induce dorsalmesoderm.

Nodal is expressed as a dorsal toventral gradient in the vegetal cells of blastula stagedembryo.

Front 9

1) Transforming growth factor-β signalling?

Back 9

1) •There are over 60 TGFβ-related ligands in the human genome.

•TGFβs aresecreted dimericproteins with many biological activities in the adult and developing embryo.

•TGFβssignal through cell surface serine/threonine kinase receptors.

•The intracellular effectors of the TGFβ signalling pathway are SMAD proteinswhich are activated by phosphorylation.

•On signalling SMAD proteins translocateto the nucleus where they work with other proteins to activate andrepress gene transcription.

**Good diagram on slide 18/21

Front 10

1) Evidence that Activin-like signals are required for mesoderm induction?

Back 10

1) A dominant negative form of the activinreceptor was made: this mutant receptor lacks theintracellular kinasedomain.

When expressed in cells the dominantnegative activin receptor dimerises with normal endogenous receptors, and no signal can be transmitted becauseno phosphorylationcan take place. All activin andnodal signalling is blocked.

When mRNA coding for the mutant receptoris injected into Xenopus embryos, all nodal signalling is blockedand the embryos that develop form nomesoderm.