Electron Microscopic Studies on the Innervation of the Smooth Muscle and the Interstitial Cell of Cajal in the Small Intestine of the Mouse and Bat

Abstract

Innervation of smooth muscle was EM studied in the intestinal muscle coat of the mouse and bat [Miniopterus schreibersi]. Interstitial cells of Cajal were found in intercellular connective tissue especially in the circular muscle layer of the mouse. Many nexuses were demonstrated, especially in the same layer between the smooth muscle cells themselves and between the interstitial cells and smooth muscle. Intercellular connective tissue of the muscle coat contained considerable numbers of small axon bundles which were provided with vesiculated varicosities or terminals on their surface. Vesiculated varicosities bore a naked surface directed towards adjacent smooth muscle cells, suggesting the probable release of transmitter substances acting upon the latter over a wide gap of hundreds to thousands .ANG. (synapse per distance). A subsynaptic cistern was often found beneath the sarcolemma of smooth muscle cells. In mouse intestinal muscle, small axon bundles and synapses per distance were prominent, whereas single axons and close contact synapses were seldom seen. Vesiculated varicosities on the border of the neuropil of the myenteric ganglion suggested the synapse per distance. Interstitial cells of Cajal were characterized by slender cytoplasmic processes, ample free ribosomes, numerous pinocytotic vesicles and a basement membrane which incompletely or almost completely covered them. They formed a close contact synapse (200 .ANG. wide gap) with the vesiculated varicosity of the axon on 1 hand and nexuses with adjacent smooth muscle cells on the other hand, thus forming a terminal link between the autonomic nerve and smooth muscle. Ultrastructural properties of interstitial cells suggest that they might represent a particular type of undifferentiated or immature smooth muscle cells. Smooth muscle cells abutting on the myenteric plexus often protruded a tongue-shaped process towards the ganglion or the axon bundle connecting the ganglia: the process either adhered to the Schwann sheath or came in direct contact with naked axons, with a narrow gap intervening.