Transplantation of young gut microbiota restored lymphoid/myeloid cell ratio and increased the proportion and absolute number of HSCs in aged mice. (A) A flowchart of animal treatments. (B) WBC concentration in PB, as determined by an automated hematologic cell counter, in mice in the antibiotics group, aged group, FMT-AA group, FMT-YA group, and the young control group. (C) Hematopoietic lineage differentiation in PB. (D-H) Proportions and concentrations of B cells, CD4⁺ T cells, CD8⁺ T cells, myeloid cells, and granulocytes in PB. (I) Count of WBCs in BM. (J-L) Proportions and absolute numbers of HSPCs, LT-HSCs, and common lymphoid progenitors (CLPs) in BM. (M-N) 5-bromo-2′-deoxyuridine (BrdU) incorporation was detected by anti-BrdU antibody in HSPCs and LT-HSCs. (O-P) Apoptosis was measured by annexin V staining in HSPCs and LT-HSCs. Each point represents data from a single mouse (n = 6 per group). Graphs show mean ± standard error of the mean (SEM), with statistical significance determined by Student t test. ∗P < .05, ∗∗P < .01, ∗∗∗P < .001. n.s., nonsignificant; TNC, total nucleated cell.