Data availability The data that support the plots within this paper and other findings of this study are available from the corresponding author upon request. The bulk and single-cell RNA-seq data are available as part of the Gene Expression Omnibus (GEO) SuperSeries GSE134691. Author contributions S.C. and A.G. designed the study, performed experiments, interpreted results and wrote the manuscript. J.Á.N.-Á. designed the study and experiments and interpreted data.

Closed circulatory systems (CCS) underlie the function of vertebrate organs, but in long bones their structure is unclear, although they constitute the exit route for bone marrow (BM) leukocytes. To understand neutrophil emigration from BM, we studied the vascular system of murine long bones. Here we show that hundreds of capillaries originate in BM, cross murine cortical bone perpendicularly along the shaft and connect to the periosteal circulation. Structures similar to these trans-cortical-vessels (TCVs) also exist in human limb bones. TCVs express arterial or venous markers and transport neutrophils. Furthermore, over 80% arterial and 59% venous blood passes through TCVs. Genetic and drug-mediated modulation of osteoclast count and activity leads to substantial changes in TCV numbers. In a murine model of chronic arthritic bone inflammation, new TCVs develop within weeks. Our data indicate that TCVs are a central component of the CCS in long bones and may represent an important route for immune cell export from the BM.

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