Fig. (14) Electron microscopic radioautogram of the adrenal medullary tissues of a senescent adult mouse at postnatal month 24 (year 2), labeled with 3H-thymidine, showing DNA synthesis (a few silver grains) in the 2 nuclei at lower left as well as in a few mitochondria of the noradrenalin cell at center and 2 other several adenalin cells at left and lower left and right. x 6,000.