The specific activities of tyrosine hydroxylase and choline acetyltransferase are more than 10-fold higher in PC12 pheochromocytoma cells at high cell density (8 × 105cells/cm2) as compared to those at low density (104cells/cm2). Co-cultivation of cells at high and low densities or cultivation of cells at the same density but either singly or in large clumps of 50-200 cells indicates that it is a specific intercellular contact rather than release into the culture medium of a conditioning factor which is responsible for the increased activities observed. © 1979.
Lucas, C. A., Edgar, D., & Thoenen, H. (1979). Regulation of tyrosine hydroxylase and choline acetyltransferase activities by cell density in the PC12 rat pheochromocytoma clonal cell line. Experimental Cell Research, 121(1), 79–86. https://doi.org/10.1016/0014-4827(79)90446-4