The first Islets of Langerhans are formed between E16-E18.5 of mouse development. The islets include five different endocrine cell types: alpha, beta, delta, pancreatic polypeptide-producing cells and epsilon cells. Interestingly, in response to a glucose challenge, islets of ventral pancreatic bud origin were found to release less insulin than islets of dorsal pancreatic bud origin. This implies that aside from different developmental pathways, there are also functional differences between the cells from the two pancreatic buds. In human Islets of Langerhans the different endocrine cell types are dispersed throughout the islet, unlike the mouse islet architecture, in which the beta cells are generally centrally located and surrounded by the other endocrine cell types. In murine islets, beta cells comprise 77% of the cells. In human islets, beta cells comprise 55% of the cells in the islet. An intricate network of blood vessels infiltrates the Islet of Langerhans and forms venules, arterioles and capillaries which enable direct release of hormones into the blood stream.