mCX45 with mRNAs for at least five other connexins (mCx26, mCx30.2, mCx32, mCx43, mCx47) detected but without immunocytochemical evidence for the corresponding protein within ultrastructurally-defined gap junctions. Those mRNAs appear to be down-regulated or destroyed by micro interfering RNAs (miRNAs) that are cell-type and cell-lineage specific. Retina Neurons within the retina show extensive coupling, both within populations of one cell type, and between different cell types. Discovery Naming Gap junctions were so named because of the "gap" shown to be present at these special junctions between two cells. With the increased resolution of the transmission electron microscope (TEM) gap junction structures were first able to be seen and described in around 1952. The term "gap junction" appeared to be coined about 16 years later circa 1969. A similar narrow regular gap was not demonstrated in other intercellular junctions photographed using the TEM at the time. Form an indicator of function Well before the demonstration of the "gap" in gap junctions they were seen at the junction of neighboring nerve cells. The close proximity of the neighboring cell membranes at the gap junction lead researchers to speculate that they had a role in intercellular communication, in particular the transmission of electrical signals. Gap junctions were also proven to be electrically rectifying and referred to as an electrical synapse. Later it was found that chemicals could also be transported between cells through gap junctions. Implicit or explicit in most of the early studies is that the area of the gap junction was different in structure to the surrounding membranes in a way that made it look different. The gap junction had been shown to create a micro-environment between the two cells in the extra-cellular space or "gap". This portion of extra-cellular space was somewhat isolated from the surrounding space and also bridged by what we now call connexon pairs which form even more tightly sealed bridges that cross the gap junction gap between two cells. When viewed in the plane of the membrane by freeze-fracture techniques, higher-resolution distribution of connexons within the gap junction plaque is possible. Connexin free islands are observed in some junctions. The observation was largely without explanation until vesicles were shown by Peracchia using TEM thin sections to be systematically associated with gap junction plaques. Peracchia's study was probably also the first study to describe paired connexon structures, which he called somewhat simply a "globule". Studies showing vesicles