Exogenous IGF-1 enhances thymic function, thymocyte proliferation, and thymopoietic throughput. Recombinant human IGF-1 was administered by continuous infusion using subcutaneously placed osmotic pumps to deliver a dose of 100 μg/day for a total of 2 weeks in 8- to 12-week-old thymus-intact female C57BL/6 mice. Mice were killed at the end of the 2-week infusion for analysis, or had pumps replaced to deliver another 2-week course of continuous IGF-1 at 100 μg/day for a total course of 4 weeks. Thymus data (A-D) were obtained after 2 weeks of IGF-1 treatment: (A) Normalized thymus weight to body weight ratios; (B) major thymocyte subsets defined by CD4⁺ and CD8⁺ staining patterns, from least mature to most mature: DN (CD4⁻ CD8⁻), DP (CD4⁺ CD8⁺), and CD4SP (CD4⁺ CD8⁻) and CD8SP (CD4⁻ CD8⁺); (C) early thymocyte subpopulations defined by lineage⁻ and CD44 and CD25 costaining patterns, with maturation progression from DN1 through DN4; and (D) total thymus TRECs after 2 weeks of IGF-1 treatment. (E) Effect of IGF-1 on thymocyte proliferation. At the specified time points during IGF-1 administration, BrdU was administered intraperitoneally and BrdU uptake in the specified thymocyte subpopulations was assessed as a measure of cell-cycle entry. Shown are representative data of 2 independent experiments with 4 mice per group per time point. *P < .05 between IGF-1– and diluent-treated mice. (F) Effect of IGF-1 on thymopoietic throughput. IGF-1 was administered together with BrdU (0.8 mg/mL) spiked into the drinking water for continuous BrdU administration. At the specified days of IGF-1/BrdU coadministration, thymocyte populations were analyzed for BrdU uptake. Shown are representative data of 2 independent experiments with 4 mice per group per time point. P < .05 between IGF-1– and diluent-treated mice for all subsets at all time points. Error bars represent SEM.