Islets of Langerhans

The left picture: simple diagram of an Islet of Langerhans. 

The right picture: photo of an Islet of Langerhans under a microscope.

 

The Islets are comprised of 2 types of cells, alpha cells and beta cells.  The alpha cells produce glucagon and the beta cells produce insulin, and both of these hormones are secreted directly into the small blood vessels passing through the pancreas.