**Abstract**

Insulin resistance in key target organs and beta cell dysfunction due to gluco- and lipotoxicity, are the two main factors driving type 2 diabetes mellitus pathogenesis. Recently, it has been suggested that ectopic fat deposition in the pancreas, named non-alcoholic fatty pancreas disease, occurs in metabolic syndrome, and may play an etiological role in islet dysfunction and damage the exocrine pancreas, increasing its susceptibility to pancreatitis and pancreatic cancer. In this chapter, we present transmission electron microscopy (TEM) as a valuable method to detect early changes in the ultrastructure of pancreatic cells during the development of the metabolic syndrome in mice fed with a western diet (WD). Mice fed with a WD develop pathological ultrastructural alterations in the exocrine and endocrine cells. We demonstrate how to use image segmentation methods and ultrastructural morphometry to analyze and quantify structural changes in cellular organelles and evaluate the presence of lipid droplets, autophagic structures, and vacuolization. Since ultrastructural lesions can be detected early during the progression of the metabolic syndrome, are in many aspects subtle, and by far precede cell apoptosis, necrosis, fat infiltration, and overt functional changes, TEM is not only a suitable but probably the crucial method for detecting early pancreas dysfunction.

**Keywords:** pancreas physiology, exocrine cells, endocrine cells, ultrastructure, metabolic syndrome, type 2 diabetes mellitus, western diet
