Fig 1-17a. Mitogenic pathways in the thyroid. Data from the thyroid cell systems are integrated into the present general scheme of cell proliferation cascades. In the first line, known activators of various cascades in dog and human thyroid cells are shown. Various levels indicate a time sequence and postulated causal relationships from initial interaction of extracellular signal with its receptor to endpoints: proliferation and differentiation expression. In dog but not in human thyroid cells, acetylcholine through muscarinic receptors activates the phospholipase C cascade. PCNA, proliferating cell nuclear antigen; cAPK, cyclic adenosine monophosphate-dependent kinase; CDK, cyclin-dependent kinase; DAG, diacylglycerol, + , stimulation; + , inhibition; ; ♦ + , induction; GFR, growth factor receptor; ODC, ornithine decarboxylase; PI3K, phosphatidylinositol 3-kinase; PKB, protein kinase B; PLC, phospholipase C; RSK, ribosomal S6 kinase.