and Mesoderm Endoderm Formation (Induction)
Mesoderm and Endoderm Induction. Definitions and properties Nieuwkoop's Studies Using Nieuwkoop's Studies to Indetify Candidate Molecules and Molecular Pathways of Mesoderm Formation. TGF-s FGFs Present State of Affairs.
Nieuwkoop's Classic I nduction Experiments
Conclusions from Nieuwkoop's Experiments
1. M esoderm is induced when ectoderm and endoderm are juxtaposed. 2. Signals arise from endoderm and received by ectoderm. 3. The signal from the endoderm is secreted. 4. M esoderm induction is independent of protein synthesis. 5. The signal can only act over a short distance (4-5 cell diameter).
Methods to Identify the Endogenous Mesoderm Inducing Signal
1. Cut and Dump: Place animal cap tissue in the presence of potential mesodermal inducer-score for expression of mesodermal markers and cellular movements. 2. Functional Screen: Inject embryos with potential gene or cDNA library, cut animal caps score as above. 3. Expression Screen: a. Differential screen between ectoderm and endoderm. Alternatively b. Pick cDNA from tissue specific library and perform in situs.
Every molecule ever identified in 30+ years of screening that fits this criteria is either: 1. A member of the TGF-s superfamily 2. A member the of FGF superfamily
FGF Signaling
Genetics of Mesoderm Formation
Nodal
Mice homozygous for mutations of Nodal leads to death at gastrulation with lack or severe reduction of all mesodermal derivatives. Two forms of Nodal in zebrafish, Squint and Cyclops. Double homozygous mutants lack all endoderm and anterior and trunk mesoderm (have posterior somites? FGF signaling?).
Schematic outline of the Nodal signaling pathway
Shen, M. M. Development 2007;134:1023-1034
How far can Nodal Signal?
Biochemical evidence consistent with Cyclops acting over short range (1-2 cells) and Squint acting long range (6-10 cells).
1.Clonal analysis shows Nodal pathway activated in cell autonomous fashion. 2.Squint can be made in non-responding cells and act over no...