a, Intra-operative site from a fibula-harvesting procedure in a 9-year-old male patient. The periosteum was split and detached from the cortical bone. Typical spotting haemorrhages along the cortical shaft appeared immediately after removal of a compress (filled arrowheads). b, Intra-operative site in a 17-year-old male patient following femoral fracture and malunion before axis correction, with spotting trans-cortical haemorrhagess (filled arrowheads). c, Magnification of the white boxin b emphasizing localized haemorrhagess on the bone surface (filled arrowheads). d, 3D reconstruction of 7 T TOF MR angiography images from the right shank of a healthy 47-year-old male. Tibia and fibula (grey) surrounded by muscle tissue (flesh-coloured) and two vessel types (red and blue) running in parallel are visible. Scale bar, 50 mm. e, Higher magnification of the tibia (white box in d) showing pores in the CB (filled arrowheads) and two distinct vessel types in the BM (open arrowhead, blue; filled arrow, red). f, Longitudinal optical section through the tibia emphasizes the intracortical blood supply. The NA (filled arrow) traverses the bone shaft with the central sinus (CS) in close proximity (open arrowhead). g, Higher magnification of the white boxed area in f shows a canal in the CB (filled arrowhead) forming an ICL. h, Optical cross-section of the tibia illustrating close proximity of the NA (filled arrow) and the CS (open arrowhead). Canals in the CB are running mainly parallel to the bone shaft, and occasionally connect to the medullary cavity and bone surface (filled arrowheads). Scale bars, 20 mm (e–h). i, LSFM of a human femoral neck cross-section shows a large artery (CD31+/Sca-1+, filled arrow) entering the CB (white dotted line, autofluorescence, grey) from the periosteum (P), and an artery (filled arrowhead) running through trabeculae in the BM. Scale bar, 500 µm. j,k, Human femoral neck (autofluorescence, grey) containing direct trans-cortical vessels (dTCVs, CD31, turquoise; α-SMA, red, filled arrowheads) with an average diameter of 52.9 ± 9.6 µm (data are mean ± s.e.m. of 41 vessels). Scale bars, 100 µm.