Fig. (13) Electron microscopic radioautogram of the adrenal medullary tissues of a senescent adult mouse at postnatal month 12 (year 1), labeled with 3H-thymidine, showing DNA synthesis (several silver grains) in the nuclei as well as in a few mitochondria of a noradrenalin cell at center right and several adenalin cells at left and lower left and right. x 6,000.