Assessments were made of the thymus and spleen weights and the total nucleotide, nucleic acid, and protein content as well as the incorporation of [14C]leucine into protein and of [3H]orotate into RNA, in the thymus, spleen, liver, brain, kidney, lungs, heart, pancreas, and skeletal muscle of normal (+/+) and dystrophic (dy/dy) 129 ReJ mice aged 40, 60, or 90 days. The weights of the thymus and spleen were lower at all stages of dystrophy. Total nucleotide and RNA levels per thymus were reduced at 90 days, while total DNA content was decreased at 60 and 90 days. Protein concentrations per thymus were diminished at each stage of the disease. The specific activity of the free amino acid pool and total free nucleotide pool did not show any significant variations in the thymus at any phase of dystrophy. Incorporation of [14C]leucine into protein and of [3H]orotate into RNA was considerably lower in the thymus at each stage of the disease. Total nucleotide content per spleen was decreased at 40 days, with no change at 60 days and followed by an increase at 90 days in the dystrophic mice. DNA, RNA, and protein levels were all reduced in the spleen at each stage of the disease. The specific activity of the free amino acid pool and total free nucleotide pool, as well as the incorporation of [14C]leucine into protein and of [3H]orotate into RNA, showed similar changes in the spleen as noted in the thymus at each phase of dystrophy. These observations indicate that significant alterations in cellular growth occur not only in skeletal muscle and other nonlymphoid organs, but also in the lymphoid organs of dystrophic mice. Such changes in the cellular growth of lymphoid organs could be responsible for an impairment of immunologic responses reflecting thymic atrophy in murine muscular dystrophy.
