The Islets of Langerhans
The pancreas is a solid gland that is approximately 10 inches long, and located behind the stomach close to the duodenum [6]. The human pancreas is essentially composed of two kinds of secretory cells that are both involved in nutrient handling. 98% of the cells are of the exocrine type which secrete a food-processing enzyme mixture into the duodenum. The remaining 2% are of the endocrine type which serve a metabolic function by secreting a mix of nutrient-generated hormones into the portal vein [2]. The small endocrine portion is of vital importance in maintaining glucose homeostasis, mainly through the action of the 51-amino acid peptide insulin [2]. In the human, the endocrine part consists of 105 to 106 cellular aggregates scattered throughout the exocrine parenchyma, where each aggregate contains about a thousand endocrine cells. These structures were first described by Paul Langerhans (1849-1888) in his doctoral thesis in 1869 [2]. He was unable to determine the function of these cells. Finally, in 1889 the endocrine function was ascribed to the pancreas, when Minkowski and von Mering performed their classic pancreatectomy experiments and established the relation of the gland to carbohydrate metabolism and liver [2].
Source: http://www.digestivedisorders.org.uk/leaflets/pancreax.html
Characteristics of the Islet of Langerhans
The adult human islet is characterized by an average diameter of 140 um. It consists of a compact mass of epithelial cells pervaded by a dense network of anastomosing capillaries [2]. Four endocrine cell types can be distinguished: alpha cells, beta cells, delta cells, and pancreatic polypeptide (PP) cells. The alpha and beta cells were both first described by Lane in 1907 [2] on the basis of their different histochemical staining characteristics.