This chapter contains introductory remark under the headings of, What ICC is, Structure of GI tract, Nomenclature of ICC, and Shape and size of ICC.
Santiago Ramon y Cajal, who contributed to the establishment of the neuron theory and was awarded the Nobel Prize for Medicine and Physiology in 1906, regarded the demonstration of the histological basis of autonomic innervation and how nerves transmit signals to effector tissues as a major challenge. He described a fine cellular network that he designated as “cellules interstitiellesor neurones sympathiques interstitiels” in association with the terminal arborization of the autonomic nerves of intestines, glands, and blood vessels stained with methylene blue or the Golgi method. Cajal considered these cells as primitive nerve cells that mediate nerve impulses from the terminal portions of the sympathetic nerves to the smooth muscle cells. Since then, interstitial cells of Cajal (ICC), as referred to by later microscopists, have been a subject of a historical debate in respect of their cytological nature.
A breakthrough in ICC research was triggered by the novel hypothesis proposed by Thuneberg 1982, which suggested that ICC act as pacemaker cells and that they conduct impulses in the gut musculature in an analogous fashion to that in the heart. This hypothesis greatly stimulated both morphological and physiological studies of ICC.
c-Kit-immunostaining proved to depict exactly the same feature of the cells as demonstrated by methylene-blue or Golgi methods originally used for the detection of ICC. Therefore, ICC are defined here as c-Kit immunoreactive cells showing bipolar or multipolar shape within the gastrointestinal tract.
The presence of ICC has been reported in a wide variety of species, including the frog, lizard, turkey, opossum, bat, rabbit, hedgehog, pig, horse, and conventional experimental animals such as the mouse, rat, guinea-pig, and in the monkey and human. In the human, ICC have been found throughout the digestive tract from the esophagus to the inner sphincter region of the anus.
The cell shape of ICC appears to be determined by several factors, including the presence or absence of a nerve plexus, their relationships to those plexuses and the frequency of connections between ICC themselves.
The terminology adopted in this Atlas is based on the Thuneberg’s invention of classifying ICC depending on the tissue layer with which they are associated, and follow the modern practice of avoiding attribution of the individual name of discoverer of cells and tissues, and the addition of new subtypes.
The myenteric plexus is distributed throughout GI tract and has been known to plays a central role in regulating the motor activity of the GI tract. Accumulation of evidence shows that ICC associated with the myenteric plexus (ICC-MP) act as the primary pacemaker cells both in the stomach and small intestine and as secondary pacemaker in the colon. Thus, the specific features of the myenteric plexus in each organ are key in the movement of the external muscle layer.