Age-dependent changes in the cellularity and ultrastructure of the spleen of Fischer F344 rats.

The age-related changes in the cellularity (cells/gram of tissue) of the spleens and thymuses of Fischer F344 male rats were determined. A decline in the weight of the thymus with age was observed as previously reported by others. The decline was most drastic between 4 and 20 months of age. The spleen, however, increased in weight with age. The increase was almost linear between 4 and 30 months of age. Yet when the number of cells recovered from each organ as a function of age was determined, a decrease for both the thymus and the spleen was observed with increasing age. It was surprising to find that fewer cells were recovered from the spleens of old animals even though the weight of the spleen of the old animals was greater than the spleens from the younger animals. The ultrastructure of the splenic white pulp of rats ranging from 4 to 30 months of age was studied to determine the possible cause for the age-related decrease in cellularity of the spleen. The white pulp of the 4-month-old rats contained a large number of small lymphocytes, and the number of cells was found to decrease with increasing age. The 30-month-old animals had less than 20% the number of lymphocytes in the white pulp as the 4-month-old animals, and the white pulp exhibited an increased number of reticular cells and macrophages with enlarged cytoplasm. The decreased cellularity and increased structural disturbance might be significant in the age-related decline of spleen lymphocyte functions.