Biomedical Engineering Reference
In-Depth Information
Fig. 3.1
Schematic representation of morphogen-based signalling in developing organisms. An
organiser generates a morphogen gradient that reaches neighbour cells. The neighbour cells differ-
ently interpret the signal depending on their position in the gradient. Intermediate cells (
lower
pathway
) and macromolecules of the extracellular matrix (
upper pathway
) contribute to the main-
taining and control of the gradient
that, during the development of an embryo, signals could diffuse from cell to cell to
organise tissues.
According to the current opinion, for a molecular signal to be recognised as a
morphogen two main criteria need to be fulfilled [
1
] :
•
It should be able to act directly at a distance.
•
It has to induce distinct biological/morphological responses at different concentrations.
Most studies on morphogens have therefore been performed on developing
organsims and, in particular, on invertebrates (e.g. the fruit fly
Drosophila melano-
gaster
) [
3
]. Here, morphogens that determine the tissue patterning (e.g. the anteri-
or-posterior axis of the body) have been the first to be identified and include
transcriptional factors able to activate specific genes in the cells such as the Hedgehog
(Hh) and the decapentaplegic (dpp) factors.
Studies have shown the ability of Hh to act both directly at a short distance or to
activate dpp that is a long-distance factor of tissue patterning [
1
] .
However, it is also important to highlight that the ability of the morphogens to
activate specific genes in the target cells also depends on the expression of specific
proteins in the latter [
1,
4
] .
3.2
Morphogens in Vertebrates
The proof of the existence of morphogens in vertebrates has not been as straightfor-
ward as in invertebrates [
5
]. While studying the development of limbs, a morphogen
gradient able to control their position in relation to a group of mesenchymal cells
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