The RCJ arises as the consequence of a high rate of cell division in the cap initial cells (resulting in a columellar structure within the cap) and a low rate in the adjacent tier of cells in the QC. The QC, first discovered by Clowes (1956) with use of radiolabelled DNA autoradiography, is a distinct central region of the RAM, which consists of slowly dividing cells (Jiang and Feldman, 2005).