Key-Notes: basic concepts in Developmental Biology
Key-Notes
7. Pattern formation
Pattern formation
All embryos of a given species have a similar structure or body plan.
How does it occur?
During development each cell must differentiate according to its position in the embryo, so that the "correct" cell types arise in the correct place. In other words, cells must know where they are in relation to other cells in the embryo. This is achieved by giving each cells a positional value in relation to the principle embryonic axes.
Regional specification describes any mechanism that tells a cell where it is in relation to other cells in the embryo, so that it can behave in a manner appropriate for its position. Regional specification is essential for pattern formation.
Keys question in developmental biology are:
How cells become aware of their position? What is the nature of the positional information they receive?
Several model systems indicate that cells may acquire positional values on the basis of their distance from a source of a morphogen.
Morphogens
A Morphogen is a secreted substance that can influence cell fate (.i.e. specify multiple cell fates) having different effects at different concentrations. In its simplest model, the positional information along an axis can be generated by the synthesis of a morphogen at a source at one end of the axis, and diffusion away from the source would set up a morphogen gradient.
Cells at different position along the axis would receive different concentrations of the morphogen and this would induce different patterns of gene expression at different concentration thresholds. Such concentration-dependent patterns of gene expression would represent the "address" or positional identity of the cell.
Morphogen gradients have been shown to incorporate a range of mechanisms including short-range signal activation, transcriptional/translational feedback, and temporal windows of target gene induction.
Among the main morphogens that play a critical role in cell–cell signals in both development and disease, we found members of the Wnt family, fibroblast growth factor (Fgf), hedgehog (Hh), transforming growth factor beta (TGFb), and retinoic acid (RA).
Do not confuse a morphogen with morphogenesis. Morphogenesis is the process by which structures form during development, reflecting different types of cell behaviour.
Read the book chapter "The developmental mechanics of cell specification" for a deeper understanding of this topic.