= Morphogen
{wiki}
A <morphogen> is a signal whose spatial <concentration> helps specify different <biological cell> fates or behaviors during <biological development>. A <morphogen gradient> provides a positional cue: <biological cells> at different distances from a source experience different local <concentrations> and can follow different developmental pathways. A spatial <concentration> variation alone does not establish that a substance is a <morphogen>; its graded developmental effects matter. See https://www.ncbi.nlm.nih.gov/books/NBK26825/[Alberts et al., Molecular Biology of the Cell, 4th edition (2002): morphogens as graded developmental signals].
The beautiful idea is that one distributed signal can help organize several regions of a developing body. A continuous <morphogen gradient> can supply the input for distinct domains of <gene expression>, rather than requiring a separate signal for every position. A <morphogen response threshold> is a useful idealization of that interpretation: high, intermediate and low <concentrations> are assigned different outcomes. The <morphogen reaction-diffusion equation> describes one way to generate a <morphogen gradient>; the equation is a model of transport and reaction, while the <morphogen> is the signal being transported.
Detection and interpretation also depend on the receiving <biological cell>. For an extracellular <morphogen>, <receptors> and <signal transduction> connect the local <concentration> to a response; <receptor binding occupancy> is only one step in that connection. The same <morphogen gradient> can be interpreted differently in different tissues because their developmental context differs. Thus a <morphogen response threshold> summarizes a particular response in a particular context, rather than being a universal property of the signal. See https://www.ncbi.nlm.nih.gov/books/NBK9968/[Scott F. Gilbert, Developmental Biology, 6th edition (2000): positional information and context-dependent interpretation].
An important experimental example came in 1988. Wolfgang Driever and Christiane Nüsslein-Volhard used antibodies to examine the bicoid <protein> in early fruit-fly embryos. They reported a <concentration> profile approximately described by an <exponential function>, highest at the front, with the <protein> concentrated in <cell nuclei>. Their abstract proposed <diffusion> from localized <messenger RNA> and dispersed degradation as a likely origin of the <morphogen gradient>, rather than proving a unique transport mechanism. https://pubmed.ncbi.nlm.nih.gov/3383244/[Driever and Nüsslein-Volhard, A gradient of bicoid protein in Drosophila embryos, Cell 54 (1988), 83–93: primary abstract]{ref}
In a companion study, Driever and Nüsslein-Volhard genetically changed the amount and distribution of bicoid <messenger RNA>, measured the resulting <protein> <morphogen gradient>, and compared developmental maps. Increased local bicoid <protein> levels shifted prospective anterior structures toward the rear; decreased levels shifted them toward the front. This linked a measured <concentration> profile to positional effects in <biological development>. The toy calculations below illustrate that logic with arbitrary numbers, not measured bicoid parameters. https://pubmed.ncbi.nlm.nih.gov/3383245/[Driever and Nüsslein-Volhard, The bicoid protein determines position in the Drosophila embryo in a concentration-dependent manner, Cell 54 (1988), 95–104: primary abstract]{ref}
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