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Discovery of a bone-like blood particle in the peripheral circulation of humans and rodents. OBJECTIVE: To characterize ossified bone marrow blood vessels and confirm the presence of ossified particles (OSP) in humans and rodents. METHODS: Human bone marrow blood vessels were processed for scanning and transmission electron microscopy. Whole blood samples were collected from younger (26-39 years; n = 6) and older (55-63 years; n = 6) volunteers and male Fischer-344 rats (1 month, n = 7; 6 months, n = 7; 12 months, n = 7; 18-months, n = 6; 24 months, n = 8). OSP in the whole blood samples were sorted and imaged with microscopy to determine diameter, circularity, and solidity. Additionally, the chemical composition of OSP was determined via elemental analysis. RESULTS: SEM revealed two types of ossified bone marrow blood vessels: that is, "transitioning" and "ossified." OSP were adhered to the surface of transitioning vessels and theoretically gain access to and circulate within the blood. The majority of OSP were ≤15 μm in diameter, but many were of sufficient size to serve as emboli (ie, >15 μm).OSP were predominately oblong in shape and several had jagged tips and edges. CONCLUSIONS: We introduce a novel, bone-like blood particle that may be diagnostic of bone marrow blood vessel ossification. Further, OSP may associate with several disease states (eg, atherosclerosis).