Figure 4. Mechanical stimulation is required for the maintenance of peri-arteriolar niches for lymphoid progenitors in the bone marrow.

The same differences that were evident among the frequencies of cell populations were also evident in absolute numbers. (a-c) The effect of voluntary running for 4 weeks on femoral cortical thickness (a), the percentage of LepR⁺ cells that were Oln-mTomato⁺ (b), and the frequencies of hematopoietic stem and progenitor cells in the bone marrow (c; 6–10 mice per treatment in 3–5 independent experiments). (d-g) The effect of hindlimb unloading for 2 weeks on femur cortical thickness (d), the percentage of LepR⁺ cells that were Oln-mTomato⁺ (e), the frequencies of stem and progenitors cells in the bone marrow (f), and the percentage of Osteolectin⁺ cells that incorporated a 48 hour pulse of BrdU (g; 5 mice per treatment in 3 independent experiments). (h) Single channel current recordings in Oln-mTomato⁺ cells at the indicated applied pressures. O and C denote the open and closed states of the channels. (i) Normalized open probability–pressure relationship from Oln-mTomato⁺ cells recorded with (blue) or without (white) 40 μM Yoda1 in the patch pipette (6 mice in 6 independent experiments). (j) Yoda1 mediated Ca⁺⁺ response in Oln-mTomato⁺ cells isolated from Oln^mT/iCreER; Piezo1^fl/fl or Oln^mT/+; Piezo1^fl/fl littermate control mice (5 mice per genotype in 3 independent experiments). (k) Western blot of Osteolectin⁺ cells from Oln^mT/iCreER; Piezo1^fl/fl or Oln^mT/+; Piezo1^fl/fl mice (representative of three independent experiments). (l-n) The effect of Piezo1 deletion in Osteolectin⁺ cells on cortical thickness (l), the percentage of LepR⁺ cells that were Oln-mTomato⁺ (m), and the frequencies of stem and progenitor cells in bone marrow (n; 6 mice per genotype in 3 independent experiments). All data represent mean ± SD. Statistical significance was assessed using paired t-tests (a, b, g, l, m) and matched samples two-way ANOVAs followed by Sidak’s (c-e, j, n) or Holm-Sidak’s (f) multiple comparisons adjustments.