Extended Data Figure 2. Relationship between type H capillaries and arterioles.

a, Tile scan (left panel) and different representations (merged channels) of image data in inset. Shown is a 4 week-old tibial metaphysis (mp) immunostained for CD31 (green), Emcn (red) and α-SMA (white). CD31^hi (green) Emcn- arteries with α-SMA+ coverage are directly connecting to type H ECs (arrowheads). Diaphysis (dp); endosteum (es); growth plate (gp).

b, Maximum intensity projections of 4 week-old Efnb2^GFP/+ (GFP, green) tibial metaphysis and diaphysis immunostained for CD31 (white) and Emcn (red). Panels in centre and on the right different representations (merged channels) of image data. Only Emcn- arterial ECs (arrowheads) and Emcn^hi type H ECs in metaphysis and endosteum (arrows) are positive for GFP, while diaphyseal type L ECs lack Efnb2^GFP expression.

c, Confocal images of Efnb2^GFP metaphysis showing GFP+ (green) Emcn^hi (red) type H ECs (arrows) connecting to α-SMA+ (white) cell-covered, Emcn- artery (arrowheads).

Dashed lines mark growth plate (gp) or compact bone next to endosteum (es). mp, metaphysis; dp, diaphysis. Nuclei, DAPI (blue).

d, RT-qPCR analysis of Efnb2 and Sox17 expression (normalised to Actb) in sorted tibial CD31^hi Emcn^hi relative to CD31^lo Emcn^lo ECs. Data represent mean±s.d (n=6; three independent experiments). P values, two-tailed unpaired t-test. UD; undetectable.