Identification of interstitial cells of Cajal. Significance for studies of human small intestine and colon.

BACKGROUND

Interstitial cells of Cajal (ICC) were described a century ago by Ramón y Cajal a.o. as primitive neurons in the intestines. In the period 1900-1960 a large number of light microscopical studies of ICC were published, in which ICC were identified by heir characteristic morphology. After 1960 electron microscopical studies emphasized similarities between ICC and fibroblasts. In our early studies of ICC in the external musculature of mouse small intestine, we identified ICC by their characteristic morphology and topography, and we analyzed the relation between ICC, autonomic nerves and smooth muscle. These studies strongly suggested that ICC were fundamental regulators of external muscle function. These hypotheses have since been supported by independent morphological and electrophysiological evidence, strongly suggesting a pacemaker role of some ICC populations as well as other regulatory functions (mechanoreceptive, mediating inhibitory nervous input). In spite of this possible fundamental importance for G-I motility, ICC have not been adequately described or even identified in human intestine, and hence, never included in ultrastructural studies of G-I neuropathology. This survey presents the concepts of ultrastructure necessary for identification and morphological studies of ICC in human intestine.

METHODOLOGY

Several light microscopical methods have been claimed selective for intestinal ICC (and nerves), including vital methylene blue, Golgi methods, silver impregnations and ZIO-methods. Unfortunately, even the most reliable of them (ZIO, vital methylene blue) do not work at all in some animal species and in some, regions of the intestinal wall. In our hands, the best results have been obtained in mouse small intestine with vital methylene blue staining (AP only) and with ZIO methods (all levels of the musculature). None of them have yet proved their value in human intestine, although we have occasionally stained ICC-SMP in human colon with ZIO methods. Optimal ultrastructural preservation of ICC for TEM studies in laboratory animals, has been achieved by fixation by vascular perfusion. In humans, the ultrastructural preservation was satisfactory, but not ideal, after optimizing the immersion fixatives (Karnovsky-types, addition of picrate).

RESULTS AND DISCUSSION

General architecture of musculature externa. ICC differ markedly in their organization and ultrastructure from one part of the gut and from one region to another. For understanding the extent and distribution of ICC plexuses it is therefore critical to preserve a stringency and uniformity with respect to terminologies and descriptions of topography. Auerbach's plexus (AP) designates the ganglionated plexus between the main muscle layers in small and large intestine, whereas we prefer to use the term myenteric plexus for all nerves in the external muscle. The deep muscular plexus (DMP) designates the two-dimensional, non-ganglionated nerve plexus between the thick outer and thin inner subdivision of the circular muscle layer in the small intestine. In our material a similar plexus was not present in colon. The submuscular plexus (SMP) indicates the non-ganglionated, two-dimensional nerve plexus present at the submucosal surface of the colonic circular muscle. The circular muscle in small- and large intestine of large animals (dogs, humans) is organized into lamellae, separated by main septa piercing the whole layer. Both in the small intestine and colon the innermost circular muscle cells, are smaller, and, particularly in colon, more irregular than the bulk of the circular muscle. Identification of ICC in laboratory animals. By TEM ICC are distinguishable as a separate cell type in the small- and large intestine of all species investigated.(ABSTRACT TRUNCATED)