Genetically determined thymus enlargement in the early life in BUF/Mna rats

BUF/Mna (B) rat is a mutated strain, having much larger thymus than WKY/ NCrj (W), ACI/NMs (A), and F344 (F) rats throughout their life-span. Rats of the latter 3 strains have normal sized thymuses, being less than 5.3 in the thymus to body weight ratio (mg/g), when they were killed at 6 weeks of age. Genetic segregation of large thymus size in the B strain at 6 weeks of age was studied by crossing B rats with W, A or F rats. All of 3 types of the F1 hybrid rats between the B strain and the other strains showed intermediate thymus ratios between those of both parental strains. In F2 rats between the B and W strains, the distribution of thymus ratios showed about 5 different peaks. These findings might indicate that two polymeric autosomal loci, thymus enlargement-1 (Ten-1) and thymus enlargement-2 (Ten-2), can enlarge the thymus size in B rats. Histometrically, whole thymus and cortex areas of the B rats were 2-5 times larger than the W rats during 6-12 weeks of age, but medulla areas were slightly different between the strains, showing that larger thymuses in B rats were mainly due to the enlarged cortex areas.