Morphogen
[2] The concept of the morphogen has a long history in developmental biology, dating back to the work of the pioneering Drosophila (fruit fly) geneticist, Thomas Hunt Morgan, in the early 20th century.
[7] In developmental biology, 'morphogen' is rigorously used to mean a signalling molecule that acts directly on cells (not through serial induction) to produce specific cellular responses that depend on morphogen concentration.
This definition concerns the mechanism, not any specific chemical formula, so simple compounds such as retinoic acid (the active metabolite of retinol or vitamin A) may also act as morphogens.
As a result, in fly embryos transcription factors such as Bicoid or Hunchback can act as morphogens because they can freely diffuse between nuclei to produce smooth gradients of concentration without relying on specialized intercellular signalling mechanisms.
Although there is some evidence that homeobox transcription factors similar to these can pass directly through cell membranes,[13] this mechanism is not believed to contribute greatly to morphogenesis in cellularized[clarification needed] systems.
In most developmental systems, such as human embryos or later Drosophila development, syncytia occur only rarely (such as in skeletal muscle), and morphogens are generally secreted signalling proteins.
The nuclear targets of signal transduction pathways are usually transcription factors, whose activity is regulated in a manner that reflects the level of morphogen received at the cell surface.
'Low-threshold' target genes require only low levels of morphogen activity to be regulated and feature enhancers that contain many high-affinity binding sites for the transcription factor.
The general mechanism by which the morphogen model works, can explain the subdivision of tissues into patterns of distinct cell types, assuming it is possible to create and maintain a gradient.
The type species, Morphogenia struhli, was named in honour of Gary Struhl, the US developmental biologist who was instrumental in demonstrating that the decapentaplegic and wingless genes encode proteins that function as morphogens during Drosophila development.
