Effect of cyclophosphamide on the rat thymus.

A single large dose (200 mg/kg) of cyclophosphamide (Cy) caused thymic involution. Thymic cortex was almost depleted of lymphocytes while medullary thymocytes were not affected. This resulted in thymic inversion on day 4. During the period of involution the cortex was invaded by positive acid phosphatase granules containing macrophages. Later the cytoplasms of these macrophages were loaded with lipid droplets and residual materials. Extensive vacuolization of the cytoplasms of epithelial and special cells were revealed. Regeneration began on day 4. Repopulating cells appeared primarily in subcapsular region. Regeneration was complete by day 11 as examined by morphological technics. While FCS dependent rosette formation capacity remained below the control level even on day 16.