In the first mechanism, a diffusible signal is sent through the extracellular space, and is received by a cell-surface receptor, which further transmits the signals by way of second messengers. In the second mechanism, cells directly contact each other through transmembrane proteins located on their surfaces. In the third mechanism, the cytoplasm of two cells is connected through gap junctions ,allowing the the signal to pass directly from one cell to another cell. In plants, direct connections between cells are called plasmodesmata . Although one of the classic models for signaling involves diffusion, there is new evidence that inductive signals may in fact be actively transported within and between cells, and that cellular projections may be involved in long distance communication between cells.
Pattern formation concerns the processes by which cells acquire positional information.
The morphogen gradient model involves the production and release of a diffusible chemical signal called a morphogen . Morphogen release creates a concentration gradient, with high concentrations of morphogen close to the source, and low concentrations farther away from the source. Exposure to different threshold levels of morphogen leads to different cell fates. In this way, different amounts of one chemical signal can create a complex pattern. The very first step in patterning the embryo of the fruit fly, Drosophila melanogaster , is a good example of pattern formation by a gradient.