Fig. 2: VCAM1⁺ sinusoids are a “colonization niche” in the diaphysis.

a the graphs show the number of hematopoietic cells detected in Fig. 1g (left) and percentage of perfused vessels (right) defined by the presence of Ter119⁺ red blood cells in the vessel lumen of the E18.5 bone marrow. Each dot corresponds to one femur section from one fetus. b Experimental scheme for fate mapping yolk-sac-derived cells using Csf1r-MeriCreMer and Ai14tdTomato^Tom/Tom mice. The right panel shows the percentage of the indicated cells that are yolk-sac-derived (TdTomato⁺) by flow cytometry. c Representative images and quantifications of TdTomato⁺ yolk-sac-derived osteoclasts (CD61⁺). Each dot corresponds to one fetus in one experiment. d The left panels show whole-mount images and representative maps from 5 µm optical sections of E16.5, E17.5 and E18.5 femurs showing progressive colonization by the indicated myeloid cells. The cell diameter and cell type are represented by the diameter and color of the spot. The right panels are representative images showing progressive upregulation of VCAM1 in sinusoidal and perisinusoidal cells in the central diaphysis as the bone marrow matures. e number of the indicated cells in the E16.5, E17.5 and E18.5 femur sections shown in d. Each dot corresponds to the average of 1 femur from 3 fetuses. Data are presented as mean values +/- SD. f Quantifications of the VCAM1⁺ area (black line) and VCAM1 intensity (red line) in the samples shown in c. n = 3 femur sections from 3 fetuses per time point. Data are presented as mean values +/- SD. g Representative zoomed-in optical section and map of an E18.5 femur in d showing that myeloid cells selectively map to VCAM1⁺ sinusoids in the central diaphysis and not to VCAM1^- sinusoids. h Graph showing the distance of a respective myeloid cell population (one dot = one cell) in 3 E16.5 sections from each fetus to the closest VCAM1⁺ sinusoid. Statistical differences were calculated using the Kruskal-Wallis multiple comparisons test. i microscopy images showing expression of VCAM1 in CD31+ vascular endothelial and PDGFRβ stromal cells. j experimental scheme for conditional Vcam1 deletion in endothelial cells using Cdh5-Cre^ERT:Vcam1^flox/flox mice pulsed with tamoxifen at E15.5 (prior to colonization of definitive hematopoiesis). k, l quantifications (k) and raw images (l) showing colonization by Ly6C + CD11b+ cells (monocytes and neutrophils) and VCAM1 expression in CD31+ cells in femur sections from Cdh5-Cre^ERT:Vcam1^flox/flox mice (KO) or littermate controls (LC). n = 5 for both conditions. Each dot corresponds to one section from one fetus. For all panels, P values were calculated using two-tailed Mann-Whittney analyses. Source data are provided as a Source Data file.