There are about 3 million islets distributed in the form of density routes throughout the pancreas of a healthy adult human in diameter. Each is separated from the surrounding pancreatic tissue by a thin fibrous connective tissue capsule which is continuous with the fibrous connective tissue that is interwoven throughout the rest of the pancreas. Islets of Langerhans can also form superstructures called islet clusters surrounding large blood vessels. The roundness of islets along the pancreas has also been quantified as an index of sphericity. Islets closest to the spherical form are mainly found in the tail of the pancreas, whereas the least-spherical islets are found in the neck of the pancreas.