Direct cell-to-cell signaling **involves the direct physical contact between cells during communication. Gap junctions in animals and plasmodesmata in plants are tiny channels that directly connect to neighboring cells, which allow the cells to transfer signaling molecules that transmit their current state of homeostasis with one another.
Paracrine signaling **involves cells close to one another but not in direct contact. A cell releases a short-lived signal into a localized area that will induce changes in a nearby cell. Paracrine signals can diffuse only over relatively short distances.
Endocrine signaling **involves cells far apart in which a longer-lasting signal, called a hormone, is released into the extracellular fluid and travels widely throughout the organisms to target cells. Protein hormones are large molecules that must bind to receptors on the cell membrane. Steroid hormones are lipid-soluble molecules that are able to pass through the cell membrane and attach to an intracellular receptor. One example of endocrine signaling involves the release of human growth hormone (HGH) from the pituitary gland into the bloodstream, which targets bone and muscle cells to trigger growth.
Synaptic signaling **involves a specialized nerve cell, a neuron, and its target cell. This association is called a chemical synapse and involves the release of neurotransmitters from the neuron into the synaptic gap to target the target cell.
Autocrine signaling **occurs when a cell sends a signal to itself by secreting something that in turn binds to specific receptors on its own membrane. This plays an important role in cell development and the immune system.