Why are the hematopoietic structures "hiding" in the bones?

Stem blood cells throughout their evolution "looked" for themselves a place inaccessible to the penetration of ultraviolet radiation.

What do we know about the hematopoietic system? At school, we learned that blood cells originate in the red bone marrow, which is present in the pelvic bones, in the ribs, sternum, skull and long tubular bones, inside the vertebrae. Bone marrow structures are represented by stem cells that produce other new cells - erythrocytes, as well as thrombocyte precursors (megakaryocytes) and immunocytes. But what do we know about the mechanism of hematopoiesis, and why does it occur inside the bone? After all, in fish, for example, such structures are located inside the kidneys.

Harvard scientists suggested that in various species of fauna, as they developed, the hematopoietic system was formed in the zone most protected from sunlight. Dr. Friedrich G. Kapp and his colleagues noted that in fish such structures are covered by another cellular layer represented by melanocytes. These cells secrete a pigmentary substance melanin, capable of neutralizing ultraviolet radiation. Melanocytes can be present almost everywhere, despite the fact that we only know about their presence in the skin. Indeed, if these cells were not, we could not protect our skin from the damaging effects of ultraviolet radiation. As a result, the DNA of the skin would be damaged, which would lead to the development of malignant processes, or the destruction of structures. Most likely, the layer of melanocytes in fish also serves as a protection for the hematopoiesis system.

An article published in Nature publishes an interesting experiment. Pisces, free from pigment cells, subjected to ultraviolet radiation: the number of stem structures in them decreased, compared with fish in which the melanocytic layer was present. But even normal fish could be vulnerable if the ultraviolet light was on them from below, and not from above: the lower part of the kidneys does not have melanocytic protection.

After a careful analysis of the evolution of fish, scientists came to the conclusion: a layer of melanocytes is really necessary to protect the hematopoietic system. This protection is especially evident when observing the development of a frog. At the stage of the tadpole, the stem structures make a transition along the "kidney-bone marrow" route: during all stages of development, the hematopoiesis system continues to be protected from ultraviolet radiation.

Of course, the internal bone cavity is not the only secluded place in which cells can hide from the sun. But we must not forget about other conditions necessary for normal hematopoiesis. Most likely, at some stage of evolution - for example, when the vertebrate animals mastered sushi, - the hematopoietic system "left" the kidneys and "settled" in the bone marrow, which is still safe.

Information is available at https://phys.org/news/2018-06-blood-cells-bones.html