Figure 9. Schematic representation of the regulation of parathyroid cell proliferation by extracellular Ca2+ concentration, involving an inhibitory pathway via the calcium-sensing receptor, and a stimulatory pathway via an unknown transmembrane transduction mechanism. Normally, pathway 1 predominates over pathway 2 in parathyroid tissue. In presence of parathyroid hyperplasia with calcium-sensing receptor down-regulation pathway 2 could become dominant and favor parathyroid cell proliferation over suppression.