Ultrastructural Changes of the Thymic Cortex of the Mouse on Age

Park, Kyung Ho; Park, Sam Yeul; Ahn, E Tay; Ko, Jeong Sik; Kim, Jin Kook

Abstract

In this study, the structural components of thymic cortex were compared in aging process. Thymic tissues of 1 week, 8 weeks, 6 months, 12 months, 18 months and 24 months-old ICR mice were fixed in the 2.5% glutaraldehyde- 1% paraformaldehyde solution, and refixed in the 1% osmium tetroxide solution. Dehydrated tissue blocks were embedded in the araldite mixture. Ultrathin sections were stained with uranyl acetate and lead citrate solution. Results from the electron microscopic observations are as follow; 1. In the thymic cortex of 1 week old mice, epithelial reticular cells incompletely surrounded thymoblasts and thymocytes, resulting variable spaces between thymocyte and cytoplasm of epithelial reticular cells. 2. Microenvironments of the thymus of 8 weeks to 6 months old mice showed typical thymic structure, and thymocytes and thymoblasts were completely surrounded with epithelial reticular cells. 3. In the thymic cortex of a 12 months old mouse, many epithelial reticular cells contains vacuolated cytoplasm and apoptotic thymocytes. 4. Epithelial reticular cells of 18 months old mouse were shrunken, and intercellular spaces between them and thymocytes and/or thymoblasts are increased. Deep cortex shows activated macrophage and mature plasma cells with a well developed rough endoplasmic reticulum. 5. Epithelial reticular cells of a 24 months old mouse were shrunken, since decreased cytoplasmic volume and increased lipidic globules. Deep cortex showed many activated macrophages and mature plasma cells. Epithelial reticular cells showed increasing electron density in their cytoplasmic plates and lipomatous atropy on aging. In conclusion, on aging, the components of microenvironment of thymus are more or less involute for preventing immunological actions in the thymic parenchyme. The digestion of cellular material results in a accumulation of lipid within the cells. Morphological change of epithelial reticular cell were characterized by the prominent cytoplasmic vacuolization. The cortical and subcapsular epithelial cells exhibited a loss of the cytoplasmic processes in all animals except in the 6 months old animals. At the same time, the thinning of thymic cortex, alterations of epithelial reticular cells, more macrophages, and other cell elements were pronounced, which may well be correlated with the age involution of the thymus.