Historical and Topological Perspective of Connexins
Preceding to the discovery of gap junction, it was observed that there might exist a pathway for direct cell–cell communication. Based on the work of Weidmann, it was observed that the space constant for the spread of current extends beyond the expected value for a single Purkinje fibre. Accordingly, it was suggested that this phenomenon might be due to the existence of a low-resistance intercellular channels. Further evidence supporting the existence of such intercellular channels was provided by the discovery of electric transmission at the giant crayfish motor synapses. These and other observations established that the cells of most invertebrate and vertebrate tissues are directly linked together by communicating channels mediated by low-resistance intercellular channels. In vertebrates, most of the cells form gap junctions except red blood cells, spermatozoa, and skeletal muscle. However, the progenitors of these cells are known to form gap junctions. Direct evidence for the existence of intercellular communicating channels was provided by the electron microscopic studies. It was observed that in excitable tissues, a current transfer between the adjacent cells only occurs when the plasma membrane of these cells was in close proximity and no such electric transmission was detected when the cells were not in close proximity with each other. Further studies confirmed the existence of such intercellular channels, and these were assigned several names, like nexus, macula communicans, and finally gap junction.