French flag model

During early development, morphogen gradients generate different cell types in distinct spatial order.

The French flag model was championed by the leading Drosophila biologist, Peter Lawrence.

Christiane Nüsslein-Volhard identified the first morphogen, Bicoid, one of the transcription factors present in a gradient in the Drosophila syncytial embryo.

Some of the earliest and best-studied morphogens are transcription factors that diffuse within early Drosophila melanogaster (fruit fly) embryos.

The difficulties with all gradient based models of morphogenesis were extensively reviewed by Natalie and Richard Gordon and include seven[5] specific points:

Model of concentration gradient building up; fine yellow-orange outlines are cell boundaries. [ 1 ]
The diffusion of a morphogen in an elongated compartment. The three colors blue, white and red represent three concentration ranges of the initial morphogen, which can then activate three genes in a differentiated manner, and thus be at the origin of three different parts of the organism.