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Lymphoproliferative Cycles in the Thymus Cortex

Department of Medical Microbiology, School of Medicine, University of California, Davis, California 95616

Abstract

The belief held for many years that small lymphocytes are terminal cells of lymphocytopoietic differentiation has recently been challenged by observations that antigens and nonspecific lymphomitogens can stimulate peripheral small lymphocytes to enlarge and proliferate. The resulting blastoid cells may form descendant clones of antibody-forming cells or small lymphocytes (1). These complex, perhaps cyclical, differentiative actions of small lymphocytes outside the thymus prompted efforts to reexamine lymphocyte differentiation within the thymus.

Animals used in this study comprised 6-week-old male mice of Swiss-Webster (SW) and C3H strains and 1-month-old guinea pigs of the Hartley strain. Animals of adolescent age were used because a steadier state of thymocytokinetics obtains during this period of interphase between organ growth and involution than at other ages (2).

Counts of lymphocyte mitoses in thymus sections of SW mice killed at 2-hr intervals during a 24-hr period were used to estimate mean mitotic indices of 1.88 ± 0.30% in cortex and 0.38 ± 0.08% in medulla.