1. Hypertrophy of the islands of Langerhans occurs in two forms: (1) the simple type which is nothing more than an increase in size of preexisting islands; and (2) the columnar type, in which the islands are composed of anastomosing columns of cylindrical cells.
2. Regeneration of the islands of Langerhans takes place by means of a budding off of columnar cells from the ducts. The process is analogous to the development of the islands in the fetal pancreas.
3. Newly formed islands of Langerhans are composed of cylindrical cells similar in all respects to those constituting hypertrophied islands of the columnar type. The two structures are identical, the latter being a later phase of the former.
4. Newly formed islands of Langerhans are capable of a greater hypertrophy than preëxisting islands. Both types of insular hypertrophy are usually of a compensatory character, the columnar, or regenerative type, being the more important.
5. Hypertrophy and regeneration of the islands of Langerhans occur most frequently in diabetes mellitus (34 per cent. of 100 cases studied). These changes are usually associated with sclerosis or hyaline degeneration of other islands.
6. Hypertrophy and regeneration of the islands of Langerhans are occasionally observed in cirrhosis and carcinoma of the pancreas. In some of the most advanced cases, however, the islands have been spared and neither hypertrophy nor regeneration are present.
7. Regeneration of the islands of Langerhans has been noted in five cases in which there was no evidence of diabetes and where the pancreas was for the most part normal. In four of these cases, columnar hypertrophy of the islands was also observed. All five cases were associated with abnormalities of the larger ducts—obstruction, chronic inflammation, and adenomatous proliferation of the lining epithelium.
8. The occurrence of hypertrophy and regeneration of the islands of Langerhans affords considerable evidence in favor of their anatomic and functional independence.