The de novo and salvage pathways for the synthesis of pyrimidine residues of RNA predominate in different locations within the mouse duodenal epithelium.

RNA synthesis was examined by radioautography in mouse duodenal epithelium using 3H-uridine as a tracer of the salvage pathway and 3H-orotic acid as a tracer of the de novo pathway. The incorporation of the two precursors was estimated by counting silver grains in light-microscopic and electron-microscopic radioautographs at successive levels of crypt and villus. With both precursors, silver grains were found over all epithelial nuclei, but in numbers varying by location. Thus, after 3H-uridine injection, the number of grains was high over nucleolus and nucleoplasm in the base of the crypt, declined gradually in the middle and top of the crypt, and was low along the villus. After 3H-orotic acid, the number of grains was fairly low throughout, but peaked over the nucleoplasm in lower villus cells. The 3H-uridine reaction over nucleolus and nucleoplasm in crypt cells was interpreted as synthesis by the salvage pathway of ribosomal RNA and heterogeneous RNA, respectively, whereas the 3H-orotic acid reaction over the nucleoplasm of some villus cells indicated that these cells synthesized heterogeneous RNA by the de novo pathway.