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Why are hematopoietic structures "hiding" in the bones?
Last reviewed: 02.07.2025
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Throughout their evolution, blood stem cells have “found” a place for themselves that is inaccessible to ultraviolet radiation.
What do we know about the hematopoietic system? At school we learned that blood cells arise in the red bone marrow, which is present in the pelvic bones, ribs, sternum, cranial and long tubular bones, and inside the vertebrae. Bone marrow structures are represented by stem cells that produce other new cells – erythrocytes, as well as platelet 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, similar structures are located inside the kidneys.
Harvard scientists have suggested that the hematopoietic system of various representatives of the fauna was formed in the zone most protected from sunlight during their development. Dr. Friedrich G. Kapp and his colleagues noted that in fish, similar structures are covered by another cellular layer, represented by melanocytes. These cells secrete the pigment melanin, which is 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 there, we would not be able to protect our skin from the damaging effects of ultraviolet radiation. As a result, the DNA of the skin would be damaged, which would cause the development of malignant processes or the death of structures. Most likely, the layer of melanocytes in fish also serves as protection for the hematopoietic system.
An article published in Nature describes an interesting experiment. Fish that had been stripped of their pigment cells were exposed to ultraviolet radiation: the number of stem structures in them decreased, compared to fish that had a melanocyte layer. But normal fish could also be vulnerable if the ultraviolet light hit them from below, not from above: the lower part of the kidneys does not have melanocyte protection.
After a thorough analysis of fish evolution, scientists came to the conclusion that a layer of melanocytes is indeed necessary to protect the hematopoietic system. This protection is especially evident when observing the development of a frog. At the tadpole stage, the stem structures make the transition along the "kidneys - bone marrow" route: throughout all stages of development, the hematopoietic system continues to be protected from ultraviolet radiation.
Of course, the internal bone cavity is not the only secluded place where 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 vertebrates colonized land - the hematopoietic system "left" the kidneys and "settled" in the bone marrow, where it has been happily located ever since.
Information obtained from the website https://phys.org/news/2018-06-blood-cells-bones.html