Lymphoreticular System

The rat thymus remains prominent into young adult­hood and involutes thereafter. Occasionally, the thymus has involuted to the point where it is no longer identifi­able by as early as 1 year of age, particularly in males. Hemosiderin progressively accumulates in splenic mac­rophages throughout life, particularly in breeding females. Splenic white pulp has a prominent marginal zone, compared to mice (Fig. 2.1). Rats lack tonsillar lymphoid tissue.