ER morphological analysis associated with interstitial cells of Cajal and smooth muscle cells in the murine stomach

In the gastrointestinal tract, several modulators are involved in generating complex motility patterns, including smooth muscle cells (SMCs), interstitial cells of Cajal (ICCs), and PDGFRα+ cells. Notably, ICCs are excitable cells that generate characteristic oscillations in pacemaker activity mediated by the cytoplasmic Ca²⁺ concentration. Since the primary source of cytoplasmic Ca²⁺ is the endoplasmic reticulum (ER) or sarcoplasmic reticulum (SR), its distribution and localisation in the cytoplasm may indicate characteristic aspects that help in understanding its physiological functions. Furthermore, caveolae, which are invaginations of the plasma membrane (PM) observed in both ICCs and SMCs, are key sites for Ca²⁺ sparks, and their functions have been well studied in muscle cells. However, the mechanism by which caveolae and ER interact to regulate Ca²⁺ in ICCs remains unclear compared with that in SMCs. In this study, microdomains comprising the PM, caveolae, and ER/SR were analysed using focused ion beam/scanning electron microscopy (FIB/SEM) to provide new insights into ICC functional analysis. Additionally, as mitochondria could regulate local Ca²⁺ concentration, mitochondria-associated membrane (MAM) contacts with the ER were also analysed in ICCs. Novel ER ultrastructures with distinct characteristics and distributions were identified in each cell type by FIB/SEM. Furthermore, reconstructed three-dimensional (3D) images enabled measurement of the distance between the PM and ER, as well as the MAM areas, thus contributing to a better understanding of ICC physiological features. These new morphological insights may help resolve controversial interpretations in physiological and pharmacological studies of ICCs.