Diabetes mellitus is caused by inadequate synthesis or secretion of insulin.
The endocrine pancreas may not produce and secrete sufficient insulin.

Amyloidosis
of islets of Langerhans in a cat, 25x
(Dept. of Pathology, Faculty of Veterinary Medicine, University
of Utrecht.)

Amyloidosis
of islets of Langerhans in a cat, congo red staining (right) and
in polarized light (left), 25x
(Dept. of Pathology, Faculty of Veterinary Medicine, University
of Utrecht.)
Severe inflammation (e.g. pancreatitis) or neoplasia of the exocrine pancreatic tissue can also lead to loss of islet function. In these cases diabetes is usually complicated by exocrine pancreatic insufficiency.