<P> Cell signaling has been most extensively studied in the context of human diseases and signaling between cells of a single organism . However, cell signaling may also occur between the cells of two different organisms . In many mammals, early embryo cells exchange signals with cells of the uterus . In the human gastrointestinal tract, bacteria exchange signals with each other and with human epithelial and immune system cells . For the yeast Saccharomyces cerevisiae during mating, some cells send a peptide signal (mating factor pheromones) into their environment . The mating factor peptide may bind to a cell surface receptor on other yeast cells and induce them to prepare for mating . </P> <P> Cell signaling can be classified to be mechanical and biochemical based on the type of the signal . Mechanical signals are the forces exerted on the cell and the forces produced by the cell . These forces can both be sensed and responded by the cells . Biochemical signals are the biochemical molecules such as proteins, lipids, ions and gases . These signals can be categorized based on the distance between signaling and responder cells . Signaling within, between, and amongst cells is subdivided into the following classifications: </P> <Ul> <Li> Intracrine signals are produced by the target cell that stay within the target cell . </Li> <Li> Autocrine signals are produced by the target cell, are secreted, and affect the target cell itself via receptors . Sometimes autocrine cells can target cells close by if they are the same type of cell as the emitting cell . An example of this are immune cells . </Li> <Li> Juxtacrine signals target adjacent (touching) cells . These signals are transmitted along cell membranes via protein or lipid components integral to the membrane and are capable of affecting either the emitting cell or cells immediately adjacent . </Li> <Li> Paracrine signals target cells in the vicinity of the emitting cell . Neurotransmitters represent an example . </Li> <Li> Endocrine signals target distant cells . Endocrine cells produce hormones that travel through the blood to reach all parts of the body . </Li> </Ul> <Li> Intracrine signals are produced by the target cell that stay within the target cell . </Li>

Chemical signals that act from cell to cell are called