INS and type 2 diabetes mellitus: In line with an earlier report that STZ-induced type 2 diabetic rats exhibited hyperglycemia, weight loss, elevated serum insulin, increased insulin resistance, and abnormal nerve conduction (Zhang et al. 2021), as shown in Fig. 1A, the axons of SC in our STZ diabetic rat model were characterized by morphological deformation, irregular arrangement, and pits and vacuoles under transmission electron microscopy.