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RNA synthesis was examined in the epithelial cells of the mouse pyloric antrum using radioautography 20 min after injection of either 3H-uridine or 3H-orotic acid. The epithelium of the mouse antrum was known to invaginate into blind tubular units composed of mucous cells arranged from base to top into a gland, an isthmus, and a pit. These were subdivided into segments and, after radioautography, silver grains were counted over cell nuclei in each segment. Following 3H-uridine injection, silver grains were present over all nuclei but were more abundant over those of the isthmus than of the gland or the pit. When nuclei were examined in the electron microscope, nucleoplasmic as well as nucleolar silver grains were more numerous in the isthmus than in the pit or gland. Following 3H-orotic acid injection, silver grains were again present over all nuclei; but maximal incorporation appeared to be in pit cell nuclei where, by electron microscopy, it was mainly assigned to the nucleoplasm. When the incorporation was calculated per whole nucleus, however, it was less in pit cell than in isthmal cell nuclei. Even so, the proportion of label in pit cell nuclei was much greater than after 3H-uridine injection. The interpretation of these findings is based on the fact that isthmal cells are immature, whereas cells migrating from the isthmus to become gland or pit cells show increasing differentiation. The immature cells of the isthmus incorporate both uridine and orotic acid more effectively than do the differentiated cells of pit and gland. Since silver grain counts over nuclei provide an index of the rate of RNA synthesis, this synthesis proceeds more actively in the isthmus than in the pit or gland. This is true of ribosomal RNA synthesis, as shown by nucleolar grain counts, and of other RNA's synthesis, as shown by nucleoplasmic grain counts. It seems, however, that while uridine is involved in the synthesis of all types of RNA, orotic acid is mainly implicated in the synthesis of the heterogeneous RNA from which the messenger RNA arises.
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