A morphogen is a signalling substance that influences the arrangement of tissue development in a process known as morphogenesis. Morphogens act directly on cells to bring about a characteristic shift in their development. However, they act at a distance from their source of production. The magnitude of their effect is directly proportional to the concentration. Several principles underpin the general role of morphogens. First, they should be capable of moving through the tissue. They should form a concentration gradient within the tissues and maintain it despite genetic and environmental changes.
The movement of morphogens and the development of a concentration gradient are so important that organisms have developed several mechanisms for these purposes. Cells have morphogen receptors on their surfaces as well as other cell surface molecules, including heparan sulphate proteoglycans. Conversely, the movement of morphogens is reported to be facilitated by cytonemes, transcytosis, and argosomes. Morphogen gradients are usually formed by restricted diffusion.
In embryonic development, morphogens work by forming the positions of different specialized cell types in a tissue. Expressing cells secrete a morphogen that establishes the positional worth of a cell by creating a concentration gradient that spans the area where the cell is located. The extent of the gradient at every point within the field is determined by the expanse between the morphogen secreting cells and the target cells. Mirror images can arise and duplicate by introducing an ectopic source of a morphogen.