Scientists have discovered that blood circulating around the body contains bone-like particles. They found the particles in blood samples from healthy humans and rats.
They suggest that the bone-like particles could be evidence of ossification in bone marrow vessels.
The team also proposes that the particles may contribute to conditions such as atherosclerosis, a process that causes plaques, or deposits of fats and other substances, to build up inside arteries.
A recent paper in the journal Microcirculation gives a detailed account of the study.
Although most of the bone-like particles were very small, a significant proportion were large enough "to block small blood vessels throughout the vascular tree," write the authors.
The study follows previous research that uncovered a process in bone marrow whereby blood vessels "progressively and theoretically convert into bone tissue with advancing age."
That finding was the work of Rhonda D. Prisby, Ph.D., an associate professor of kinesiology at the University of Texas at Arlington, who is also the lead and corresponding author of the new study.
Bone-like particles and vascular calcification
Prisby wondered if the processes of ossification in bone marrow blood vessels could give rise to bone-like particles that are capable of entering the bloodstream.
"By examining seemingly unrelated images and linking the details of them together," says Prisby, "I was able to posit the presence of bone-like particles in the blood."
Therefore, for the new investigation, she and colleagues used ultra-high microscopy to examine ossified bone marrow blood vessels in more detail.
They also "sought to confirm, characterize, and quantify the presence of [bone-like particles] in peripheral whole blood samples from humans and rats."
Cardiovascular diseases are the leading cause of death worldwide and were responsible for around 31% of all global deaths in 2016, according to the World Health Organization (WHO).
Vascular calcification is a complex and dynamic process — involving several mechanisms — that deposits calcium in the walls of blood vessels.
The process hardens the vessels, reduces their elasticity, and raises the risk of cardiovascular diseases and deaths from such diseases.
Probing the process of vascular calcification
In their study paper, Prisby and colleagues explain that the processes of vascular calcification are unclear. However, scientists suspect that the processes involve several types of cell in the vessel wall and the tissue that surrounds it.
The authors give an example involving smooth muscle cells. These cells in the vessel wall can transition from a contractile type to a bone-generating type and thence to a form that is "noncontractile, migratory, and proliferative."
This and other examples suggest that vascular calcification can start inside blood vessels.
Given that bone marrow vessels also undergo calcification, but most of the research literature on vascular calcification describes that which occurs outside the skeleton, the team decided to investigate further.
The purpose of the new study was to describe the features of bone marrow blood vessels that had become bone-like and to confirm what the team suspected — that the bloodstream contains bone-like particles, likely as a result.
The researchers used advanced microscopy techniques to examine human bone marrow blood vessels. These methods revealed two types of bone-like vessels: "transitioning" and "ossified."
They observed how transitioning vessels had bone-like particles stuck to their surfaces, meaning that, in theory, the particles could "gain access to and circulate within the blood."
'Sharp tips and edges'
As well as examining human bone marrow vessels, the researchers examined blood samples from younger and older healthy human volunteers and rats.
They found bone-like particles in samples from both species. In neither species did age appear to make a difference to the percentage of bone-like particles.
Although most of the bone-like particles had a diameter no bigger than 15 micrometers, some were bigger and "of sufficient size to serve as emboli."
Prisby suggests that scientists should consider the potential role of bone-like particles when studying the processes of vascular calcification, heart attack, and stroke.
"Some of the ossified particles have sharp tips and edges that could damage the lining of blood vessels. This damage could initiate events leading to atherosclerosis [...] which can restrict blood flow over time."
Rhonda D. Prisby, Ph.D.