The number and proliferation of ETPs and DN thymocytes in the involuted thymus is increased after Fgf7 treatment. (A) Mean ± SD number of cells in the thymi of 15- to 18-month-old mice 15 days after 1 round (Fgf7 × 1, n = 8 mice) or 3 rounds (Fgf7 × 3, n = 6 mice) of Fgf7 or PBS treatment. The average number of total cells in the thymi of young (4-6 weeks) mice is shown for comparison. (B) Average ± SD number of ETPs in the thymi of the mice shown in panel A. (C) Frequency of Ki-67–expressing ETPs in the thymi of 15- to 18-month-old Fgf7-treated mice described in panel A. (D) Mean ± SD number of DN2, DN3, and DN4 cells in the thymi of the mice described in panel A. (E) Frequency of Ki-67–expressing DN2, DN3, and DN4 cells in the thymi of mice described in panel A. Mean ± SD values presented above are based on data pooled from 4 independent experiments. The P values are based on comparison between cells isolated from Fgf7- and PBS-treated old mice.