Gap junctions are
cellular channels which go through the
cell membrane and connect with the channels of another cell. They are different from normal cellular channels in that they aren't specific to dumping or taking up any specific kind of
molecule. Rather, they allow two adjacent cells to share any chemical between them, providing intimate
intercellular communication and an easy way to maintain
homeostasis. Since regular intercellular communication via
hormones,
metabolites, and so forth is often slow, gap junctions provide instantaneous communication that is invaluable to the overall organism's survival. Indeed, most every cell in a
multicellular organism has gap junctions with its adjacents, the few exceptions are
skeletal muscle and free standing cells like
red blood cells.
The makeup of gap junctions is different from that of regular channels, too. Each component passageway, called a connexon, is made up of six transmembrane proteins, called connexins, arranged in a hexagon. Each connexon lines up with a connexon on the adjacent cell, and the hole between the six connexins allows intermingling of cytoplasm. A single connexon would allow relatively little interchange, so real gap junctions are made up of clusters of aligned connexons. Since the connexons are little hexagons, the gap junction itself looks something like a honeycomb in cross section.
Homeostasis is seen best in organs, which need to deal with widely varying conditions as quickly and uniformly as possible. It's gap junctions that allow organs to function as a unit at all, whether by diffusing oxygen through heart tissue that isn't served directly by a capillary or by spreading a toxin across enough liver cells that none of the cells are killed. Broken connexin genes cause terrible systemic diseases; visceroarterial heterotaxia, for instance, comes from a single mutant heart connexin.
Since gap junctions are responsible for cells communicating with their neighbors, so when an individual cell needs to die for the good of the collective, that need is communicated through the gap junction. Hormones hit too many cells at once to communicate this need effectively, and we know that there aren't any free standing "killer cells" to do the job. Cancer occurs when cells refuse to die or differentiate, and instead multiply out of the body's control. W. Loewenstein and Y. Kanno have proposed that the reason the cells do not differentiate or stop growing is that their gap junction communication has been turned off. Essentially, the unreachable cancer cells behave as though they were unicellular organisms in an environment perfect for their unbounded proliferation.
Specifically, it's proposed that temporary gap junction inhibition is responsible for tumor formation, and stable down regulation is responsible for the metastatic state. In other words, temporary inhibition can cause benign tumors, but permanent loss of the junctions will result in cancer. A few labs have results that add weight to this hypothesis, including a species of mouse with a connexin gene knocked out which displays a much higher rate of liver cancer than control mice.
Importantly, gap junctions are only found between cells in a multicellular organism, and would be useless or even dangerous for a unicellular one. Sexual reproduction between unicellular organisms is done by a special protein (often coded for by a plasmid) that doesn't allow anything to pass that isn't needed for reproduction. In this way both cells are protected from toxins or excess metabolites that the other might be carrying. If both cells set up connexon clusters for reproduction, those things could move through it, possibly damaging one or both cells.
One last cool way that nature has used the gap junction is in some neural tissue, specifically neurons that are tuned for speed rather than learning. Neurotransmission is ordinarily done via chemical means, neurotransmitters bursting from axon to dendrite accross a synapse. As it turns out, neurons can synapse through a gap junction instead, which is known as an electrical synapse. Since the ion flow can just go through the junction, the two connected neurons fire much more quickly than if chemical neurotransmission occurred -- it's as though the two neurons fire as one. Electrical synapses are found in quantity on the retina, and more sparsely throughout the rest of the brain.
You may wonder why the whole brain isn't made out of electrical synapses rather than chemical ones, as speed can be a huge evolutionary advantage. I discussed this with a neuroscience professor I once had, and she mentioned that selectivity doesn't work with electrical synapses. That is, an electrical synapse must fire when any adjacent neuron fires, while chemical synapses will only fire when a certain threshold of other neurons firing is reached. I also figured that since gap junctions allowed flow of most any ion, the learning exhibited by NMDA and other kinds of long term potentiation wouldn't work at all, making learning much more difficult.
There are creatures that solely use electrical synapses; many insects do, and some fish. Houseflies have simple little gap junction brains with no learning capability, but the speed of their neurotransmission is what allows them to so quickly dodge your hand when you try to smash them.