The thymus has an important role in the immune reaction. The lymphocytes localized both in thymic cortex and in medulla, known as T cells, proliferate in an environment of the epithelial reticular cells. T cells educated to react against non-self materials enter the circulation and function in peripheral immunity. Thymus of the mouse has almost developed and grown to its natal size by the 18 days gestation. After the birth thymus grows at a somewhat slower rate until puberty. It subsequently shows a decrease both in size and in weight. That is due to the decrease in the lymphocyte-accumulated region and to the partial replacement by fat tissue. The process is so-called age"involution". Fat cells are classified as connective-tissue cells. They must develop and proliferate in the capsule or in the trabecula of the thymus after birth, because no typical fat cells containing lipid droplets are observed at birth.We examined when fat cells appeared in the thymus after the birth. Fat cells are known to express leptin, so that the appearance of leptin mRNA was detected by the PCR method. The expression of leptin was detected as early as in the thymus of the 3 week-old mouse and increased with the age. It is sugested that fat tissue development begins immediately after the birth and precedes to the age involution in the thymus, although that the cortex and medulla develops until puberty
