A gradual decrease in nucleolar size with the maturation of columnar epithelial cells in the adult rat intestine under normal and various experimental conditions.

The columnar cells, which form over 90% of the epithelium in the small intestine, undergo rapid and continuous renewal and maturation. Samples from duodenum, jejunum, upper, mid- and terminal ileum of young male rats were processed for histology. The average maximal nucleolar area was determined in 10-cell-wide bands of the basal, mid and upper levels of crypts and villi, respectively, by image analysis; in the duodenum, it was 2.8, 2.1, 1.7, 1.5, 1.3 and 0.8 (in micron2) in the respective epithelial levels from crypt base to villus top. Although villus size decreased by 68% from duodenum to terminal ileum, nucleolar area was similar at each respective epithelial level along the intestine. This indicated that nucleolar size was related to cell maturity, rather than to the size of epithelium. In other groups of rats, the duodenum was examined after administering specific inhibitors. Methotrexate (within a day) and cycloheximide (within 3 h) did not significantly affect nucleolar size, indicating that the decrease in size was not under the influence of immediate synthesis of nucleic acid or protein. On the other hand, tunicamycin (within a day) delayed the decrease and actinomycin D (within 3 h) caused a maximal decrease in all nucleoli. This implied that a glycoprotein factor and some changes in DNA were involved in the decrease in nucleolar size. The rate of protein synthesis in duodenum was then measured by grain count per cell area in autoradiographs made after 1 h of injection of [3H]leucine. From crypt base to villus base, the grain count doubled while the nucleoli decreased to nearly half of their size in the crypt base. When actinomycin D injection preceded the [3H]leucine administration, all nucleoli decreased markedly and the grain counts increased by about 30% in all epithelial levels. It thus appears that the decrease in nucleolar size stimulates protein synthesis, possibly by the release of ribosomal material or some other factor. Protein synthesis in turn has been shown to be related to cell maturation. It is concluded that the nucleolus is involved in some manner in the regulation of the maturation and renewal of the epithelial cells.