Ectopic bone formation in the heart and brain of young and old male Fischer-344 rats may result from circulating ossified particles

BACKGROUND: We previously reported ossified particles (OSP, i.e., bone-like particles) in the circulation of rodents and humans. Given their diameters and other characteristics, OSP may pose as emboli and contribute to vascular dysfunction and calcification. Their presence in the circulation is an enigma and leads one to speculate as to their ultimate fate. Since valvular calcification of the heart is a known pathology, we theorize that OSP can gain access to tissues and organs and elicit ectopic bone formation. The objective of the study was to assess whether OSP were detectable in the two most vital organs, i.e., the heart and the brain. We hypothesized that the volume of bone in the myocardium would augment with advancing age; however, given the blood-brain barrier, we did not anticipate the presence of bone in this organ. METHODS: The heart and brain from young (6-mon; n=11) and old (24-mon; n=12) male Fischer-344 rats were collected following euthanasia, weighed to determine mass (g), fixed in 10% formalin for 3 days at 4ºC, and stored in 70% EtOH at -20ºC until analysis. Both tissues were subsequently scanned (55 kVp, and 145 μA) at 15μm via microCT (μCT 45; Scanco Medical, Inc. Switzerland). To determine bone volume, the entire heart and brain were analyzed. One-Way ANOVAs were used to determine significant differences in body mass (g), heart mass (g), brain mass (g), bone volume in the heart (mm 3 ), bone volume in the brain (mm 3 ), bone volume relative to heart mass (mm 3 /g), and bone volume relative to brain mass (mm 3 /g). A p-value of 0.05 was set a priori. Data are presented as Means ± Standard Deviation. RESULTS: Body mass (404±30g vs. 347±30g, respectively), heart mass (1.11±0.11g vs. 0.99±0.10g, respectively) and brain mass (2.05±0.15g vs. 1.91±0.10g, respectively) were higher ( p<0.05) in old vs. young rats. Bone volume in the heart (0.035±0.044mm 3 vs. 0.005±0.004mm 3 , respectively) and brain (1.19±1.16mm 3 vs. 0.38±0.31mm 3 , respectively) were higher ( p<0.05) in the old vs. young rats, and this increase remained when bone volume was normalized to heart mass (old, 0.030±0.035 mm 3 /g vs. young, 0.004±0.003 mm 3 /g) and brain mass (old, 0.577±0.567 mm 3 /g vs. young, 0.202±0.173 mm 3 /g). The microCT scans were limited to a 15μm resolution, eliminating detection of bone of lesser thickness. Thus, the bone volumes reported herein are presumably underestimated. CONCLUSION: Bone volume in the heart and brain increased as a function of advancing age in male rodents and may contribute to a variety of diseases (e.g., myocardial infarction, stroke, cognitive impairments, neurological disorders, etc.). We speculate that circulating OSP gain access to tissues and organs, leading to ectopic bone formation. The presence of bone in the brain was unexpected and suggest that the smallest OSP (≤ 5μm in diameter) can cross the blood-brain barrier. National Science Foundation: ECCS 1710948; American Heart Association: 16IRG27550003 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.