Although some studies have been conducted over the past few decades, the existence of mitochondria-rich cells (MRCs) in reptiles is still obscure. This is the first study to uncover the presence of MRCs in the small intestine of Chinese soft-shelled turtles. In this study, we investigated the ultrastructural characteristics of MRCs and the secretion of different ion transport proteins in the small intestine of Pelodiscus sinensis. Transmission electron microscopy revealed that the ultrastructural features of MRCs are clearly different from those of other cells. The cytoplasmic density of MRCs was higher than absorptive epithelial cells (AECs) and goblet cells (GCs). MRCs possessed abundant heterogeneous mitochondria and an extensive tubular system in the cytoplasm, however, the AECs and GCs completely lacked a tubular system. Statistical analysis showed that the diameter and quantification of mitochondria were highly significant in MRCs. Mitochondrial vacuolization and despoiled mitochondria were closely associated with autophagosomes in MRCs. The multivesicular bodies (MVBs) and the exosome secretion pathway were observed in MRCs. Immunohistochemical staining of ion transport proteins indicated positive immunoreactivity of Na+/K+_ATPase (NKA) and Na+/K+/2Cl- cotransporter (NKCC) at the basal region of the mucosal surface. Likewise, the immunofluorescence staining results showed a strong positive localization of NKA, NKCC, and carbonic anhydrase (CA) at the basal and apical region of the mucosal surface of small intestine. Our findings suggest that MRCs provide support and regulate cellular ions for intestinal homeostasis and provide energy for cellular quality control in intestine.
Keywords: Chinese soft-shelled turtle; MRC; Na+/K+/2Cl− cotransporter; Na+/K+_ATPase; carbonic anhydrase; small intestine; ultrastructure.