Abstract: Objective To observe the microstructure characteristics of spleen at different ages of SD rats. Methods Seven rats were randomly sampled from a same cohort of normal male SD rats at different growing stages (1 day and 1, 2, 3, 6,12, 24 and 36 months of age). The tissue blocks from whole spleens were taken to make the methyl acrylic resin embedded sections. After the sections were stained with periodic acid-Schiff's reagent and hematoxylin, the microstructure characteristics were observed. Result There was no obvious structure at 1 day, the spleen was composed of dense cells, including the splenic cord and the periarteriolar lymphoid sheath. At 1 month of age, the splenic sinus was formed generally and the boundary was not obvious with splenic cord, and the marginal zone was clearly and megakaryocytes were plents. For the 2 months, the splenic follicle was formed, all key structures had been formed and the capsule was thickening. For the rats of 3 to 12 months, the percentages of the white pulp and the red pulp in the spleen became stable. For the rats of 24 to 36 months, all structure's boundary were blurred. Conclusion The development of structure in rat spleens is characterized of a continuous, establishing, improving and regressing process. The rat spleen grows fully before second month.

Key words: Rat, Spleen, Histology, Microscopic observation