Our blood may come from a previously unknown source, mouse study suggests

(ORDO NEWS) — Our body is so complex that even the most vital and well-studied systems still surprise us. 

For example, blood can have not one, but two types of cell origin in mammals, according to a study done on mice.

“Historically, people thought that most of our blood comes from a very small number of cells that eventually become blood stem cells, also known as hematopoietic stem cells,” explains Harvard University cell biologist Fernando Camargo, one of researchers who conducted research on mice.

“We were surprised to find another group of progenitor cells that are not derived from stem cells. They produce most of the blood from fetal life to young adulthood and then gradually begin to decline.” These cells are known as embryonic multipotent progenitors (eMPPs).

Hematopoietic stem cells are formed early in development from cells that line the arteries. Previously, it was believed that EMPs separated from hematopoietic stem cells at some early stage of their development.

Using a newly developed genetic barcoding strategy, Harvard University biomedical scientist Sachin Patel and colleagues were able to follow the dividing cells and see that hematopoietic stem cells and EMPs originated from the same tissue. 

To do this, the researchers inserted pieces of easily detectable DNA sequences into a place in the genome of a mouse cell that was passed on to all their cellular descendants.

This allowed them to trace the lineage of all target cells, finding that eMPPs divide into the cells responsible for the majority of lymphoid cells (a certain type of white cell) in developing mice. These eMPP cells appear to be the mothers of many immune blood cells, including white blood cells (B and T cells).

Although hematopoietic stem cells can also produce these immune cells (as shown in the model below), they do so on a much more limited scale. They tend to produce more cells, which lead to the formation of megakaryocytic parts of the blood – cells that produce the components necessary for blood clotting.

“Next, we are trying to understand the consequences of mutations that lead to leukemia by studying their effects in both blood stem cells and EMP in mice,” says Camargo. “We want to see if the leukemias that arise from these different origins are different – lymphoid or myeloid type.”

What’s more, it appears that the contribution of eMPPs to blood supply weakens over time, which may explain the longstanding mystery of why our immune systems weaken with age.

Patel and his team also tested how this new knowledge could improve bone marrow transplantation, finding that EMT transplants did not take well in mice.

“If we could add a few genes to make EMPs stick around for a long time, they could potentially be a better source for bone marrow transplants,” explains Camargo.

“They are more common in young bone marrow donors than blood stem cells, and they are ready to produce lymphoid cells, which could lead to better immune system recovery and fewer infectious complications after transplantation.”

Of course, all of this will only apply if the results are the same in humans. The paths of development do not always coincide in different species of mammals.

Now a team of researchers is studying these maternal blood cells in humans and hope their findings will lead to new treatments to boost the aging immune system.

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