The beta cells are the cells that synthesise and secrete insulin. They are found in the Islets of Langerhans, which (as any quizzer should know) are found in the pancreas. In fact they are also, and possibly more correctly, known (according to Wikipedia) as the pancreatic islets.
The beta cells should not be confused with the Betz cells, which are in the brain. Named after the Ukrainian scientist who described them in 1874, they're a type of neuron; they're also the largest cells in the central nervous system.
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