Five male rats aged 0, 30, 60, 90, and 180 days were euthanized by cervical dislocation under anesthesia. The body and thymus weights were measured. The thymus was quickly removed and immediately immersed in 4% paraformaldehyde. Thymus samples were shaken overnight at 4°C and rinsed with phosphate-buffered saline overnight. Fixed tissues were dehydrated by immersion in a series of ethanol and xylene solutions and embedded in paraffin. Sections of 4 μm thickness were prepared. Anti-annexin A5 (MS-2, ×5,000) was used as the primary antibody (PRID: AB_2827744; https://antibodyregistry.org/AB_2827744). We obtained the antibody from rabbit serum and used it for immunohistochemistry and western blotting [15 (link), 19 (link)]. ImPRESS REAGENT (anti-rabbit IgG) (Vector Laboratories, Funakoshi, Tokyo, Japan) was used for signal detection. The diaminobenzidine substrate solution (Roche, Basel, Switzerland) was added dropwise to the tissue sections. The reaction was carried out at room temperature, and the degree of coloration was observed under a microscope. The color reaction was stopped with distilled water, followed by immersion in hematoxylin solution for approximately 1 min and rinsing with tap water. The specimens were dehydrated and mounted with a sealant (M-X) (Matsunami Glass, Osaka, Japan).