**1. Introduction**

### **1.1. Beta cells (β-cells)**

Beta cells are a type of cell in the pancreas located in the so-called islets of Langerhans. They make up 65-80% of the cells in the islets.

The Islets diameter is about 50 to 300 micrometers. They are composed of several types of cells. At least 70 percent are beta cells, which are localized in the core of the islet. These cells are sur‐ rounded by alpha cells that secrete glucagon, smaller numbers of delta cells that secrete soma‐ tostatin, and PP cells or F cells that secrete pancreatic polypeptide. All of the cells communicate with each other through extracellular spaces and through gap junctions. This arrangement al‐ lows cellular products secreted from one cell type to influence the function of downstream cells. As an example, insulin secreted from beta cells can suppress glucagon secretion.

A neurovascular bundle containing arterioles and sympathetic and parasympathetic nerves enters each islet through the central core of beta cells. The arterioles branch to form capilla‐ ries that pass between the cells to the periphery of the islet and then enter the portal venous circulation.
