Novel volume-electron microscopic ultrastructural analysis of gastrointestinal excitability associated with...

Gastrointestinal smooth muscle cell excitability is regulated by a syncytium of smooth muscle cells, interstitial cells of Cajal (ICCs) and platelet-derived growth factor receptor α+ cells. The mechanism of Ca²⁺ upregulation, such as Ca²⁺-activated Cl^- channel (CaCC) for characteristic pacemaker activity of ICCs, is supported by the interaction between the endoplasmic reticulum as the Ca²⁺ source, the plasma membrane providing the membrane activity and mitochondria as a buffer. A concept of 'microdomains' consisting of these endoplasmic reticulum, plasma membrane and mitochondrial components has been suggested based on electrophysiological studies, laser microscopy and transmission electron microscopy. However, their entire structure and function cannot be understood without detailed 3-D information. In this article, 3-D analysis of the microdomain on CaCCs is reported to present a new interpretation of characteristic excitable Ca²⁺-dependent mechanisms. Novel volume-electron microscopy (EM) techniques, such as focused ion beam/scanning electron microscopy (FIB/SEM), are powerful tools to understand these mechanisms. These techniques can show cellular membrane contacts in sheet structures, and can calculate geometrical features, such as distance, surface area and volume. Geometric analysis of organelles and membranes with FIB/SEM represents a novel study of the gastrointestinal tract. Conventional transmission EM images and immunohistochemistry of new ICC subtypes are reviewed with respect to the relationship between anatomical and physiological functions. 3-D analysis of endoplasmic reticulum in motor neurons is also summarized as an example of other excitable cell types analysed by FIB/SEM.