Exogenous IGF-1 expands peripheral LSK and increases cell cycle entry of bone marrow and peripheral LSK before numeric increases in thymocyte populations. (A) IGF-1 was administered by continuous infusion for 2 weeks into 8- to 10-week-old thymus-intact C57BL/6 mice. At days 4, 7, 10, and 14 of IGF-1 infusion, cohorts of mice were killed and analyzed for thymocyte and peripheral (combined spleen and lymph node) LSK enumeration. Shown are the quantitative fold differences for each cell population between IGF-1–treated and diluent-treated control mice at each time point during IGF-1 administration. Data points are based on the combined data of 2 independent experiments, with 6 mice per group per time point. Quantitative differences between IGF-1 and control mice were statistically significant (P < .05) for all populations except for bone marrow LSK at all time points and thymocyte subpopulations at day 4 only. Dotted lines indicate 1:1 ratio in cell numbers between IGF-1–treated and diluent-treated control mice. ETP indicates early thymic progenitor (lineage⁻, CD44^hi, CD25⁻, c-kit⁺); LSK, lineage⁻, Sca-1⁺, c-kit⁺ progenitor cells. (B) BrdU uptake after intraperitoneal BrdU administration was measured in bone marrow and peripheral LSK at day 4 of IGF-1 administration in C57BL/6 mice. Shown are representative data from 3 independent experiments, with 4 mice per group. Error bars represent SEM. *P < .05 between IGF-1– and diluent-treated mice.