Islets cells are the major part of pancreas. The beta cells constitute the predominant type of cells in the islets. The beta cells are particularly important because they make insulin. Degeneration of the beta cells is the main cause of type I (insulin-dependent) diabetes mellitus. Islets cells are the major part of pancreas. The beta cells constitute the predominant type of cell in the islets. The beta cells are particularly important because they make insulin. Degeneration of the beta cells is the main cause of type I (insulin-dependent) diabetes mellitus.