Microdissection and scanning electron microscopy of corrosion casts showed the structures of the vascular bed of the human pancreas to consist mainly of the capillary plexuses of the exocrine lobules, extralobular ducts and endocrine islets.
A considerable number of the exocrine lobules were found to contain one to four marked endocrine islets larger than 30μm in diameter. These intralobular islets received one or more arterioles (afferent vessels) and emitted conspicuous insulo-acinar portal vessels which continued into the lobular capillaries, suggesting insular control over the functions of the exocrine acini of the pancreas. Direct drainage of the intralobular islets into the veins was never reproduced. Not excluding the possibility that some lobules might contain smaller, unidentifiable islets, there nonetheless were many lobules which directly received arterioles. These lobules are free of control by an islet.
Rarely, an islet was located in the interlobular tissue space or along an extralobular duct. Such an extralobular islet issued no portal vessels, and drained into the interlobular or periductal veins. The surface of this type of islet comprised a thin network of fine capillaries. A possibility was suggested that this cortical network might be homologous with the lobular capillaries. No portal route was observed between the islets and extralobular ducts. Few connections were noted between the capillary plexuses of the lobules and ducts.