The endocrine portion of the pancreas takes the form of many small clusters of cells called islets of Langerhans.
In histological sections, islets are seen as relatively pale-staining groups of cells embedded in a sea of darker-staining exocrine tissue.
Islets are richly vascularized, allowing their secreted hormones ready access to the circulation
Additionally, they are innervated by parasympathetic and sympathetic neurons, and nervous signals clearly modulate secretion of insulin and glucagon.
Four cell types can be identified: