Question

Adults with severe anemia will sometimes have yellow marrow convert back to red marrow. What kind of feedback is this and how do you know?

Answer 1

Red bone marrow is involved in production of red blood cells as it contains hematopoietic mesenchymal stem cells. In case of severe anemia, the red bone marrow will increase production of red blood cells. Yellow bone marrow contains non-hematopoietic mesenchymal stem cells that can accumulate lipids. Thus, yellow bone marrow is the storage site of lipids. In infants and children, the bones have red bone marrow. More yellow bone marrow is formed when the person ages. However, when there is an increased demand for red blood cells in the body, the yellow bone marrow is converted to red bone marrow.

When infants grow into adults, the bones stop growing robustly. Hence, at this stage the red bone marrow will decrease its activity. At this stage, fats cells will enter the bone marrow and the yellow bone marrow is formed. However, when there is severe anemia, the requirement for red blood cells increases. This is because there is oxygen depletion in the body due to low red blood cells. The oxygen deficiency or hypoxia will stimulate erythropoietin (EPO) production in kidneys. This EPO will stimulate the inactive hematopoietic stem cells in yellow bone marrow to divide. Stem cells are capable of self-renewal and thus, number of hematopoietic stem cells increase in the yellow bone marrow, leading to red bone marrow formation. The production of RBC is also stimulated to replenish the loss due to anemia. This will increase the oxygenation in the body. Thus, yellow bone marrow is converted to red bone marrow.

This type of feedback mechanism is a positive feedback regulation. Feedback loops are present in the body to maintain homeostasis. They are of two types-positive feedback and negative feedback. In positive feedback, the product of a reaction causes an increase in the reaction while in negative feedback the product of reaction decreases the reaction. In this case of severe anemia, the anemia has reduced the number of red blood cells (product). This decreases levels of RBC are infact stimulating the formation of red blood cells by converting the yellow inactive bone marrow to active red bone marrow. This, the formation of red bone marrow reaction is a positive feedback, that results in increased production of RBC. On the other hand, if there was decreased production of RBC, then it would be a negative feedback reaction. The formation of red bone marrow will reinstate the RBC level in the body, thereby maintaining homeostasis.