Why do human embryos develop a fully functioning, beating heart, so early in development? Why is it that the human embryological heart starts beating long before the circulatory system and the bodily tissues that will be served by circulating blood have developed?
Embryologists have often pondered such questions. Now, two independent groups of researchers in Boston may have discovered the answers.
Scientists at Children’s Hospital, Brigham and Women’s Hospital, and the Harvard Stem Cell Institute have found that a beating heart is necessary for the production of blood stem cells. More specifically, the biomechanical forces produced by the early embryological heart trigger the production of chemicals which in turn trigger the cellular formation of hematopoietic cells, which are the stem cells that differentiate into blood.
In other words, mechanical stress triggers the release of chemicals which stimulate cellular development through signaling pathways. The scientists found that one of the most important of these chemicals is nitric oxide, which is produced in the body by mechanical stress and which is one of the key biochemical regulators of a number of physiological processes, not the least of which is the regulation of blood vessel elasticity and growth. Nitric oxide is naturally produced by the body throughout life, and this latest discovery that it plays a key role in increasing stem cell production could have implications for people with immune disorders, blood cancers and other diseases that require bone marrow transplantation. Currently, matching donors are available for only approximately a third of all the patients who require bone marrow transplantation.
According to Dr. Leonard Zon of the Division of Hematology/Oncology at Children’s Hospital in Boston and director of their stem cell research program, "Basically we cannot offer optimal therapy to two-thirds of patients." Using zebrafish embryos, Dr. Zon and his colleagues created a mutant strain of embryos in which a heartbeat and circulation were absent, and which were also found to be deficient in hematopoietic stem cells. The scientists then discovered that by increasing nitric oxide in the mutant fish embryos, they were able to restore blood stem cell production, and conversely, by inhibiting nitric oxide they were once again able to demonstrate a reduction in the number of blood stem cells. The researchers then conducted the same experiments in mouse embryos and concluded that these phenomena are common across vetebrate species.
As Dr. Zon explains, "Nitric oxide appears to be a critical signal to start the process of blood stem cell production. This finding connects the change in blood flow with the production of new blood cells."
As all embryologists know, the embryonic human heart begins to beat in a regular rhythmic pattern by the 6th week of embryonic development, at which time the septum primum begins to appear, which will later subdivide into the left and right chambers of the heart. It was never fully understood, however, why this early cardiac development precedes development of the full circulatory system and of the tissue throughout the body which the circulating blood will feed. Now it seems as though this advanced cardiac specialization so early in embryogenesis is necessary for the formation of the blood stem cells which will later produce the various lineages of blood cells throughout the body.
In early mammalian embryos, the blood progenitor cells first develop within the walls of the aorta but later migrate into the bone marrow. In this latest study, when the scientists used a drug to block nitric oxide in pregnant mice, the developing embryos were not easily able to form hematopoietic stem cells. The scientists then discovered that an increase in blood flow not only yields an increase in nitric oxide production, but also an increase in activity of the eukaryotic gene RUNX1, which is a "master regulator" of blood stem cells.
A second, independent team of researchers made corroborating discoveries. According to George Q. Daley, M.D., Ph.D., director of the Pediatric Stem Cell Transplantation Program at Children’s Hospital in Boston, and director of the Laboratory for Systems Biology of the Center for Excellence in Vascular Biology at Brigham and Women’s Hospital, "In learning how the heartbeat stimulates blood formation in embryos, we’ve taken a leap forward in understanding how to direct blood formation from embryonic stem cells in the petri dish." According to Dr. Guillermo Garcia-Cardena, director of the Laboratory for Systems Biology of the Center for Excellence in Vascular Biology at Brigham and Women’s Hospital, who also participated in the study, "These observations reveal an unexpected role for biomechanical forces in embryonic development. Our work highlights a critical link between the formation of the cardiovascular and hematopoietic systems." Also collaborating on the study with Dr. Daley’s group were researchers at the Indiana University School of Medicine.
These findings have applications not only in prenatal development and embryogenesis but also in the maintenance of health and the treatment of disease in mature adults. Such a discovery – that the chemical stimulus from nitric oxide produced by the mechanical stress of blood flow is what triggers hematopoietic stem cell production – would also have implications for athletes, as well as for the benefits that moderate physical exercise can impart to anyone. Shear mechanical stress is now seen to hold a new medical importance, since it is the friction created by fluid flowing through the circulatory system which exerts physical pressure on the surface of the cells lining the vessels, which in turn stimulates the expression of chemical regulators of blood formation, which in turn triggers the production of the hematopoietic stem cells.
Biomechanical forces represent the convergence of physics and biology, and although such forces are not usually studied by physicans, they have often been a topic of interest among physicists and mathematicians, as every calculus student will at some point encounter the 18th century mathematician Daniel Bernoulli who is remembered today for his mathematical modeling of fluid dynamics, and Bernoulli’s equation is often applied to the flow of blood through arteries and veins. Of perhaps greater interest to physicans today than the precise physics and mathematics underlying such principles, however, are the prospects that new "drugs" could be engineered which could either mimic the action of blood flow on precursor cells, or stimulate the nitric oxide signaling pathways for therapeutic benefits in patients with blood and other diseases that might otherwise require transplantation.
Perhaps the natural benefits of physical exercise could also be employed toward such an end, conscientiously and therapeutically, with a greater respect for the complex molecular mechanisms that are responsible for the cardiovascular benefits of exercise. Among other researchers, Dr. Douglas Seals of the University of Colorado at Boulder has already been publishing extensive studies for years on the role of nitric oxide in physical exercise, on which he has repeatedly reported that the increased blood flow which results from physical exercise is what increases shear stress on the surface of the endothelium, which in turn triggers adaptive responses in gene expression and in the phosphorylation of nitric oxide synthase, which is the enzyme responsible for nitric oxide production. Endothelium-derived nitric oxide has thus already been well understood to play a number of important anti-atherosclerotic roles, not the least of which include anti-inflammatory and anti-thrombotic effects as well as vasodilation. It is nitric oxide, or the absence thereof, which is primarily responsible for determining vascular tone. As Dr. Seals was quoted as saying in 2008, "There are multiple lines of evidence that regular aerobic exercise improves the function and health of arteries largely by improving the bioavailability of nitric oxide." Indeed, vasoconstrictor and vasodilator proteins in the vascular endothelium are quantitatively measurable, and for years Dr. Seals has been publishing studies on the correlation of vascular endothelial dysfunction to aging, as nitric oxide and nitric oxide synthase progressively diminish in the absence, over years and decades, of aerobic activity. Now, the missing link has been found, making the connection between nitric oxide and stem cell stimulation.
Exercise and physical fitness have long been recognized as important factors both in the prevention and in the treatment of cardiovascular disease, and now the complex role of stem cells in such phenomena is gradually being understood in more and more detail. A mechanically stimulated chemical phenomenon which regulates the earliest developmental stages of life may now also be harnessed and utilized for the maintenance and restoration of health at all stages throughout the entire human life span.
The results of these studies appeared today in both the journals Cell and Nature.