Cells form the building blocks of all animals and diversity in animal forms can be attributed to the way these building blocks arrange themselves while proliferating.These things depend on cross talk between cells and which is termed as Signalling.This language of cross talk involves a series of players,where one component relay the received signal to the one downstream to it ,forming a chain reaction or signalling pathway.

Signalling pathways transduce signal from external medium into the nucleus,which results in turning ON or OFF of concerned target genes of that pathway.Cells use different signalling pathways depending on the stage of development.All these pathways may differ in mechanistic rules but share some basic principles.Generally a pathway is turned on the moment ligand finds its target receptor and leading to the translocation of nuclear effector / effectors in Nucleus resulting in gene regulation.For simplicity we can take a example of Wnt/ wingless pathway in Drosophila where in binding of wingless ligand to Frizzled receptor turn on the pathway and there by leading to formation of non phosphorylated form of Beta catenin ,which can now form complex with TCF and enter nucleus to activate transcription of genes.

Cell-cell signalling pervades all aspects of development, not just in vertebrates, but in all animals (metazoa).Currently 17 signalling pathways are recognised and each has difeerent set of transducing components.

Early development and later
1. Wnt pathway
2. Receptor serine/threonine kinase (TGFb) pathway
3. Hedgehog pathway
4. Receptor tyrosine kinase (small G proteins) pathway
5. Notch/Delta pathway
Mid-development and later
6. Cytokine receptor (cytoplasmic tyrosine kinases)
pathway
7. IL1/Toll NFkB pathway
8. Nuclear hormone receptor pathway
9. Apoptosis pathway
10. Receptor phosphotyrosine phosphatase pathway
Larval/adult physiology
11. Receptor guanylate cyclase pathway
12. Nitric oxide receptor pathway
13. G-protein coupled receptor (large G proteins) pathway
14. Integrin pathway
15. Cadherin pathway
16. Gap junction pathway
17. Ligand-gated cation channel pathway

Reference:
JOHN GERHART
1998 Warkany Lecture: Signaling Pathways in Development
TERATOLOGY 60:226–239 (1999)

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