Four centrioles imaged in one electron micrograph of a thin section, cut from a Chinese hamster fibroblast grown in tissue culture. The two centrioles that appear circular are the 'mother' centrioles, in the sense that they are older, while the two in longitudinal section, which appear rectangular, are 'daughters.' They were initially formed only hours earlier as this cell began the S-phase of its cell cycle, and they are not yet fully mature. The mother centrioles bind 'pericentriolar material', which initiates the growth of microtubules. As the cell enters mitosis, the two pairs of centrioles will separate, so one mother and one daughter centriole will be situated at each pole of the growing mitotic spindle. Thus, each daughter cell will inherit two centrioles, each of which can help to organize the formation of a daughter centriole during the subsequent S-phase, so the cycle can continue. While the centriole cycle was initially described by classical light microscope cytology, this organelle is so small in most cells that details of its duplication were almost invisible. Electron microscopy provided the space resolution necessary to see what centrioles really looked like and to characterize the process of their duplication cycle. This micrograph captured in a single image all four of the centrioles in a cell that was about to enter mitosis and provided a clear demonstration of the spatial relationships among them. Other work at about the same time revealed aspects of the process by which multiple centrioles formed in cells that were differentiating to make the many basal bodies necessary to form the cilia the coat the free surface of some epithelial cells. Image originally appeared in: McGill M et al. J Ultrastruct Res. 1976;57:43-53.