Regeneration of Tooth Roots With Borrowed Stem Cells in Pigs

Because a recent post about tooth-making stem cells in alligators generated so much interest, I found another recent paper that reports the regeneration of the tooth root structure in pigs. This is a proof-of-concept paper that demonstrated the feasibility of such a procedure.

The journal is Stem Cells and Development and the research team is from the Beijing Key Laboratory of Tooth Regeneration and Function Reconstruction in Beijing, China. The corresponding author is Songlin Wang from the Molecular Laboratory for Gene Therapy and Tooth Regeneration.

Tooth loss represents a growing problem in an aging population. Dental implants provide one solution, but without a good jaw bone into which these implants can be attached, implants have little chance of staying put. Regenerating a tooth root that can support a natural or artificial crown is the most important part of the tooth in maintaining tooth function.

In previous work, Wang and his collaborator Songtao Shi from UCLA have shown that stem cells from root apical papilla and periodontal ligament stem cells from exfoliated teeth can coat bioengineered surfaces and form tooth structures that can support artificial crowns in miniature pigs (see Sonoyama et al., PLoS One 1:e79-e92). However, aged patients sometimes have bone marrow stem cells that do not grow well in culture and respond poorly to bioengineering protocols. Therefore, Wang and his crew sought to demonstrate that mesenchymal stem cells from donor animals (allogeneic stem cells) could provide the same kind of benefit.

The two stem cell populations used in this paper was dental pulp stem cells (DPSCs) and periodontal ligament stem cells (PDLSCs). The DPSCs were cultured from exfoliated minipig teeth and grown in culture for two or three passages. The culture medium used, as far as I can tell, was the same one used the Gronthos in his PNAS paper that reported the isolation and characterization of DPSCs. That medium was a modified Eagle’s medium supplemented with 20% Fetal Calf Serum and 100 μM L-ascorbic acid 2-phosphate, 2 mM L-glutamine, 100 units/ml penicillin, and 100 μg/ml streptomycin. Gronthos then grew his cells at 37°C in 5% CO2 (see S. Gronthos, et al PNAS 97(25): 13625–13630). After 2-3 passages, the DPSCs were seeded on a hydroxyapatite tricalcium phosphate scaffold and grown in a bioreactor for 5-7 days

PDLSCs were grown in culture with approximately the same cocktail as the DPSCs and then plated on 60 mm dishes with vinylene carbonate (Vc). Vc induces the PDLSCs to grow s sheets that could be used to wrap the hydroxyapatite tricalcium phosphate structures that had been seeded with DPSCs.

These wrapped structures were implanted into the gums of minipigs and then used to secure dental implants.

Tooth Root Regeneration

After 6 months, the implants were assessed as was the integrity and strength of the surrounding tissue.

Gross, radiographic, and histological analysis of the bio-root 6 months after transplantation. (A, C) Gross view of the general shape of HA/TCP and the bio-root 6 months after transplantation (ellipse). (B, D) X-rays revealed that HA/TCP formed tissues without an obvious dental structure (ellipses), but the HA/TCP/DPSC/PDLSC sheet implant formed a hard root structure (ellipses). (E, F) No obvious boundary was observed between newly regenerated tissue and bone in the microcomputed tomography scan image of the HA/TCP group. (G, H) A hard root structure (arrows) was present and a clear PDL space found between the implant and surrounding bony tissue (triangle arrows). (I–K) HE staining showed some bone formation and HA/TCP remaining in the HA/TCP group (I), and PDL-like tissues were generated parallel to the dentin-like matrix structure in the autologous group (J) and allogeneic group (K). (L) Semiquantitative analysis showed that mineralized tissue regeneration capacity of autologous or allogeneic groups was significantly higher compared with the HA/TCP group. Percentage of mineralized tissues at 6 months after crown restoration was significantly higher than that before crown restoration in both autologous and allogeneic groups. No significant difference of regenerated mineralized tissue percentages was found between autologous and allogeneic groups. Scale bar: (I–K) 200 μm. B, bone; HA/TCP, hydroxyapatite/tricalcium phosphate; PDL, periodontal ligament; MT, mineralized tissue. *P<0.01 compared with autologous or allogeneic groups; #P<0.01 compared with autologous or allogeneic groups after crown restoration.
Gross, radiographic, and histological analysis of the bio-root 6 months after transplantation. (A, C) Gross view of the general shape of HA/TCP and the bio-root 6 months after transplantation (ellipse). (B, D) X-rays revealed that HA/TCP formed tissues without an obvious dental structure (ellipses), but the HA/TCP/DPSC/PDLSC sheet implant formed a hard root structure (ellipses). (E, F) No obvious boundary was observed between newly regenerated tissue and bone in the microcomputed tomography scan image of the HA/TCP group. (G, H) A hard root structure (arrows) was present and a clear PDL space found between the implant and surrounding bony tissue (triangle arrows). (I–K) HE staining showed some bone formation and HA/TCP remaining in the HA/TCP group (I), and PDL-like tissues were generated parallel to the dentin-like matrix structure in the autologous group (J) and allogeneic group (K). (L) Semiquantitative analysis showed that mineralized tissue regeneration capacity of autologous or allogeneic groups was significantly higher compared with the HA/TCP group. Percentage of mineralized tissues at 6 months after crown restoration was significantly higher than that before crown restoration in both autologous and allogeneic groups. No significant difference of regenerated mineralized tissue percentages was found between autologous and allogeneic groups. Scale bar: (I–K) 200 μm. B, bone; HA/TCP, hydroxyapatite/tricalcium phosphate; PDL, periodontal ligament; MT, mineralized tissue. *P

As you can see in panel E and F, control implants that had no cells and only hydroxyapatite calcium triphosphate showed no tooth-like structures, but in G and F, the structures with cells showed a consistent tooth structure with a periodontal ligament (see broad arrow).  In panels J and K, there is obvious bone formation with periodontal ligament in the autologous and allogeneic stem cell transplants.

Cross sections of the implants also showed that not only did these structures look like real tooth root structures, but they contained structures proteins indicative of real tooth root structures.  Dentin sialophosphoprotein (mercifully abbreviated to DSPP) is present in the cell-seeded implants, but in on the hydroxyapatite calcium triphosphate-only implants.

Clinical assessment of implants failed to detect any gingivitis or periodontal disease associated with the implants.

This experiment shows that stem cell-seeded scaffolds can regenerate tooth root structures.  Since this worked in minipigs and not simply rodents, these results strongly suggest that such a strategy could work in humans.  Clinical trials anyone?

Human Neural Stem Cell Line Heals Spinal Cord-Injured Rats

Spinal cord injuries represent one of the most intractable problems for regenerative medicine. When the spinal cord is injured, a tissue that is normally isolated from the bloodstream, now comes into contact with a variety of inflammatory factors and cells that increase the destruction of the original lesion. The spinal responds with a glial scar that plugs the lesion and prevents further exposure of the spinal cord to damaging inflammation, but the scar is also filled with molecules that repel neuronal axon growth cones. This spells curtains for neuronal regeneration, and finding a cell type that can negotiate around the glial scar and find the original muscle is a genuine tour de force.

Given this to be the case, there have been many experiments in rodents to examine the efficacy of various stem cell populations to as treatments for spinal cord injuries. A recent paper in Stem Cell Research and Therapy (van Gorp et al., 2013, 4:57) has examined human fetal spinal cord-derived neural stem cells (HSSCs) and their ability to restore motor function in rats with spinal cord injuries to the lower back. Because this group examined movement and spinal cord tissue samples, this paper contributes something significant to our knowledge of HSSC-mediate healing of spinal cord injuries.

The HSSC line used in this paper is neural stem cell line NSI566RSC, which was extracted from the spinal cord of an 8-week old “fetus.” I have placed fetus in quotes because at eight weeks, the fetus is actually a very old embryo, since the end of the eighth week is end of embryonic development. I realize that these types of age calculations have room for error, and therefore, the baby might very well have been at the early fetal stage. However, the baby’s mother terminated her pregnancy (yes it was an abortion and no I am not cool with that) and donated the dead baby’s tissue to UC San Diego for research purposes.

Sprague-Daley rats were subjected to spinal cord injuries at the level of the third lumbar vertebra. Three days later, half of the rats were given saline injections into their spinal cord and the other half were given HSSC injections into their spinal cords. The animals were evaluated for two months after the treatments on a daily basis. After two months, the rats were sacrificed (put down) and the spinal cord tissue was extensively analyzed.

Of the 35 animals employed in this study, 3 were excluded because of paw injuries or drug toxicity. Eight weeks after the cells were implanted, the rats were tested with a CatWalk apparatus to determine their gait. The rats injected with HSSCs showed a much more normal gait than those injected with saline. To give you some idea of the improvement, the rats that were not injured had a RCHPP or rostro-caudal hind paw positioning score of 0+/- 1.7mm, and the saline injected animals had an average RCHPP of -18 +/- 3.1 mm, and those injected with HSSCs had an RCHPP of -9.0 +/- 1.9 mm.

Despite these improvements, there were no significant differences in ladder climbing, stride length, overall coordination, or single-frame motion.

Next, Marsala and colleagues showed that the muscle spasms associated with spinal cord injury were slightly decreased by the implantation of HSSCs and not by injection of saline. To measure spasticity, the ankle or front paw is rotated and the electromyograph of the muscle is measured. The electromyograph or EMG measures the electrical activity of the muscle showed modest improvements in the HSSC-injected animals

Sensory sensitivity was improved in the HSSC-injected animals, and this improvement was progressive. When the rats were prodded below the level of the injury, where they should have no feeling, the HSSC-injected rats showed better response to the stimulation. This was the case with mechanical stimulation and thermal stimulation.

Post-mortem analysis also showed something interesting. When the fluid-filled cavity of the damaged spine was examined, the HSSC-injected animals had a significantly small cavity. Because the injected cells had been labeled with green fluorescent protein, they glowed under UV light and any neuronal cells derived from the injected HSSCs glowed green too. The lesioned areas in the HSSC-injected mice were repopulated with cells. Motorneurons, interneurons and glial cells were detected.

What to make of this study? The repopulation of the spinal cord and the growth of spinal nerve elements within the fluid-filled cavity is remarkable, but the lack of better motor function is disappointing. The recovery of sensory ability is significant, especially, since it is pretty clearly not due to spinal hypersensitivity.

There are two possibilities for the low motor recovery. First, there is a possibility that the these experiments were not conducted for as long a time period as they needed to be. Since the sensory ability improvement was progressive, maybe the motor recovery was too, perhaps? Secondly, maybe the grow and connection of motor neurons had trouble with the glial scar. Why the sensory nerves did not have such a problem and the motor neurons would is inexplicable at this time. However, another possibility is that the muscular targets of motor neurons are not as obvious in adult animals as they are in a developing animal. Finding ways to “paint” the muscles might be a way to increase motor neuron innervation in the future.

Thus, this cell line, NSI-566 RSC is certainly a potential treatment for spinal cord patients. A phase I trial is in the works.

Supreme Court Strikes Down Patenting of Human Genes

Can we patent human genes? After years of debate, the Supreme Court of the United States (SCOTUS) has ruled unanimously that the answer is “No.” The majority of Americans are simply unaware that approximately 25% of their genes have been patented by companies and research institutions over the last few decades by the U.S. Patent and Trademark Office. However, the SCOTUS’ decision has determined that your genes are not patentable.

There is a fine article about this in the June 13th edition of the Wall Street Journal.

The SCOTUS decision is a victory for scientists, physicians and patients who argued that such patents interfere with the practice of medicine, patient care and scientific research. In handing down its decision, SCOTUS has made one of the most significant rulings in this age of molecular medicine, since it ultimately decides who may own the fundamental building blocks of life.

In writing for the court, Justice Clarence Thomas, said the genes Myriad Genetics, Inc isolated are products of nature, which aren’t eligible for patents. Thomas penned, “Myriad did not create anything,” Justice Thomas wrote in an 18-page opinion. “To be sure, it found an important and useful gene, but separating that gene from its surrounding genetic material is not an act of invention.”

Justice Thomas and the court essentially followed the legal framework established by Solicitor General Donald Verrilli when he rejected the views of both the U.S. Patent and Trademark Office and the specialized tribunal that hears patent appeals, the U.S. Court of Appeals for the Federal Circuit.

Justice Thomas credited Myriad for a “medical breakthrough,” since they had identified, isolated, and characterized the BRCA1 and BRCA2 genes. These genes can predict if a woman has a 50% – 80% likelihood of breast cancer, in comparison to the average American woman’s 12% to 13% risk. However, Thomas opined that “”groundbreaking, innovative, or even brilliant discovery does not by itself satisfy” federal law’s requirements for a patent. To demonstrate his reasoning, Thomas made reference to a 1948 case, in which the court decided that a product that combined several different species of bacteria that were useful for improving soil-nitrogen levels was unpatentable because the bacteria themselves were naturally occurring.

Justice Thomas also wrote that “separating (a) gene from its surrounding genetic material is not an act of invention.” Thomas also rejected an argument put forward by the company seeking the patent, Myriad, that patenting BRCA1 and BRCA2 would promote innovation. Instead, Thomas and his colleagues thought that patenting these genes and other would stifle innovation and frustrate progress.

In a nutshell: even if a discovery is brilliant or groundbreaking, that doesn’t necessarily mean it’s patentable, according to SCOTUS.

Now what do I think? I think that this is probably good news for patients. Even though you are not aware of it, gene patenting has affected you. Once a company legally “owns” a human gene, they control who can conduct research on that gene and who can run tests on that gene. If you have a genetic disease and you need a genetic test to confirm that you have it, medical labs are limited on what genes they can offer tests for because of gene patents. This limits the range of services medical labs can offer to patients. Medical laboratories that offer particular genetic tests are only allowed to do so because they pay royalties to the companies that own the genes, and this jacks up the cost of those tests. Consequently, many labs do not offer genetic tests in order to spare themselves the cost, time, paperwork, lawyers’ fees, and hassles.

Gene patents also stifle research. You see once a company owns a patent on a gene, they sit on the patient and do not conduct any research on those genes. Gene patents also prevent other scientists from researching the gene as well. This ties the hands of medical geneticists who want to define the exact mechanisms by which particular mutations cause or contribute to specific genetic diseases. Since many diseases have a genetic component, gene patents get in the way of further research. Dr. Iris Schrijver, president of the Association for Molecular Pathology, which opposes gene patents, made this observation:

Because variation in gene sequences plays an important role in the development and progression of many diseases, through gene patents, patent holders can essentially gain ownership of the understanding of some diseases and of certain areas of patient care itself.

Fortunately, SCOTUS has put the kibosh on such occurrences. Now, we hope that there will be a new era of genetic research where our genes are not claimed by one company or another, and researchers are free to work on whatever gene they choose.

As a postscript, Justice Thomas did leave the door open for companies to patent synthetically made versions of genes.  This would allow companies the ability to patent creations of their own for further use and research and development.  As noted in the Wall Street Journal piece:

Still, a footnote gave Myriad little reason to cheer. Justice Thomas added that the court took no position on whether cDNA met the other requirements for a patent, such as being “nonobvious.” He referred to a brief filed by the Obama administration, which observed that “given the prevailing level of knowledge in biotechnological fields, future patent applications directed to cDNAs and other synthesized DNA molecules may be rejected as obvious.”

Alligator Stem Cells and Tooth Replacement

Mammals usually have one set of baby teeth (also known as milk teeth) and after those are lost, we have one set of adult teeth and these are not replaced if they are lost. This condition is called “monophyodont.” Reptiles and sharks, however constantly replace their teeth. This condition is called “polyphyodont.” Alligators and crocodiles are among one group of reptiles that replace their teeth throughout their lives, and because the development of these creatures has been studied to some extent, it is known that the ability of these creatures to replace their teeth on a regular basis results from a resident stem cell population. Studying that stem cell population more closely might provide clues for tooth replacement in humans.

American Alligator
American Alligator

A research team led by scientists at the Keck School of Medicine professor of pathology Cheng-Ming Chuong at the University of Southern California. Dr. Chuong and his collaborators from around the world have identified unique cellular and molecular mechanisms behind tooth renewals in American alligators.

Chuong explained, “Humans naturally have only two sets of teeth – baby teeth and adult teeth. Ultimately, we want to identify stem cells that can be used as a resource to stimulate tooth renewal in adult humans who have lost teeth. But, to do that, we must first understand how they renew in other animals and why they stop in people.”

Even though humans cannot replace their adult teeth, a tissue called the dental lamina remains, which is known to be crucial for tooth development.

Why are alligators potentially a good model system for tooth replacement in mammals? First author of this study, Ping Wu, explained it this way, “Alligator teeth are implanted in sockets of the dental bone, like human teeth. They have 80 teeth, each of which can be replaced up to 50 times over their lifetime, making them the ideal model for comparison to human teeth.”

Through the use of microscopic imaging techniques, Chuong and others found that each alligator tooth is a complex unit of three components: a functional tooth, a replacement tooth, and the dental lamina, all other which are at different developmental stages.

The tooth units are built to enable a smooth transition from dislodgement of the functional, mature tooth to replacement with a new tooth. Further imaging studies strongly suggested that the dental lamina contains a stem cell population from which new replacement teeth develop.

“Stem cells divide more slowly than other cells, said co-author Randall B. Widelitz, who serves as an associate professor of pathology at USC. Widelitz continued, “The cells in the alligator’s dental lamina behaved like we would expect stem cells to behave. In the future, we hope to isolate those cells from the dental lamina to see whether we can use them to regenerate teeth in the lab.”

The researchers also intend to learn what molecular networks are involved in repetitive renewal and hope to apply the principles to regenerative medicine in the future.

The authors also noted that novel cellular mechanisms are used during the development of the tooth unit. Also, unique molecular signaling speeds growth of replacement teeth when functional teeth are lost.

See P. Wu PNAS 2013; DOI: 10.1073/pnas.12132110.

RNA Molecule Protects Stem Cells During Inflammation

During inflammation and infection, bone marrow stem cells that make blood cells (so-called hematopoietic stem cells or HSCs) and progenitor cells are stimulated to proliferate and differentiate into mature immune cells. This especially the case for cells of the so-called “myeloid lineage.

Hematopoietic Stem Cells (HSCs) are able to differentiate into cells of two primary lineages, lymphoid and myeloid. Cells of the myeloid lineage develop during the process of myelopoiesis and include Granulocytes, Monocytes, Megakaryocytes, and Dendritic Cells. Circulating Erythrocytes and Platelets also develop from myeloid progenitor cells.

Hematopoiesis from Multipotent Stem Cell

Repeated infections and inflammation can deplete these cell populations, which leads to serious blood conditions and increased incidence of cancer.

A research team from the California Institute of Technology, led by Nobel Prize winner, David Baltimore, has discovered a small RNA molecule called microRNA-146a (miR-146a) that acts as a safety valve to protect HSCs during chronic inflammation. These findings also suggest that deficiencies for miR-146a might contribute to blood cancers and bone marrow failure.

Baltimore and his colleagues bred mice that lacked miR146a. MicroRNAs are very short RNA molecules (around 22 base pairs long) that regulate the activities of other genes. They control the expression of genes at the transcriptional and post-transcriptional level. In the case of miR146a(-) mice, whenever these mice were subjected to chronic inflammation, the total number and quality of their HSCs declined steadily. In contrast, miR-146a(+) mice were better able to maintain their levels of HSCs despite long-term inflammation.

The lead author of this work, Jimmy Zhao, said, “This mouse with genetic deletion of miR146a is a wonderful model with which to understand chronic inflammation-driven tumor formation and hematopoietic stem cell biology during chronic inflammation.”

Zhao also noted the surprising result that the deletion of one microRNA could cause such a profound and dramatic pathology. This underscores the critical and indispensable function of miR-146a in protecting the quality and longevity of HSCs. This work also establishes the connection between chronic inflammation and bone marrow failure and diseases of the blood.

Even more exciting is the prospect of synthesizing anti-inflammatory drugs that could treat blood disorders. In fact, it is possible that artificially synthesized miR146a might be an effective treatment if small RNAs can be effectively delivered to specific cells.

Zhao also noted the close resemblance that this mouse model has to the blood disorder human myelodysplastic syndrome or MDS. MDS is a form of pre-leukemia that causes severe anemia and a dependence on blood transfusions. MDS usually leads to acute myeloid leukemia. Further study of Zhao and Baltimore’s miR146a(-) mouse might lead to a better understanding of MDS and potential new treatments for MDS.

David Baltimore, senior author of this paper, said, “This study speaks of the importance of keeping chronic inflammation in check and provides a good rationale for broad use of safer and more effective anti-inflammatory molecules. If we can understand what cell types and proteins are critically important in chronic-inflammation-driven tumor formation and stem cell exhaustion, we can potentially design better and safer drugs to intervene.”

See Jimmy L Zhao, Dinesh S Rao, Ryan M O’Connell, Yvette Garcia-Flores, David Baltimore. MicroRNA-146a acts as a guardian of the quality and longevity of hematopoietic stem cells in mice.  DOI: May 21, 2013.  Cite as eLife 2013;2:e00537.

Postscript: This paper is especially meaningful to me because my mother died of MDS. The fact that a better model system for MDS has been established is an essential first step in finding a treatment for this killer disease.

Using Over-The-Counter Abortion Pills to Trick Your Pregnant Girlfriend

Another WordPress blogger who runs a site entitled “saynsumthn” has provided the following harrowing story:

Another recent example which received national attention was the arrest and indictment of a Florida man who tricked his pregnant girlfriend into taking abortion pills to abort her pregnancy. Authorities say that 28-year-old John Andrew Welden did not want to be a father, so when his girlfriend, Remee Jo Lee, got pregnant, Welden faked a prescription for an abortion pill, switched a label so the medication appeared to be a common antibiotic, and gave her the drug. The drug did its job. The unborn baby died. Now Welden, is facing the possibility of life behind bars without parole, charged with murder under a rarely used federal statute known as the “Protection of Unborn Children Act.”

Now folks, even though Plan B does not seem to have a lot of side effects, its over-the-counter status means that it is as easy to get as chewing gum. Look for this sort of thing to happen a lot more often with the legalization and over-the-counter status of Plan B. Rapists, child molesters, incestuous family members can simply slip their victims Plan B to cover up their crime.

Giving Plan B over-the-counter status is a bad idea and this demonstrates it. Welden should have been charged with drug tampering and administering medicine without a license, but since Plan B is over the counter, he cannot be charged with such a crime even though he ought to be. What happens when these monsters start giving Plan B to pregnant women after the time of its efficacy? What birth defects await us?

Oh wait, plan B is not supposed to cause an increase in birth defects because it’s the same hormones that are in many daily birth control pills. But what about the dosage? The Princeton University Student Health Center’s own fact sheet states:

When dedicated ECPs are not available, certain ordinary birth control pills can be used in specified combinations as emergency contraception. In either case, the regimen is one dose followed by a second dose 12 hours later, where each dose consists of 1, 2, 4, 5, or 6 pills, depending on brand. Currently, 19 brands of combined oral contraceptives are approved in the United States for use as emergency contraception.

Now wait a minute people – emergency contraceptive pills or ECPs can be replaced by 1-6 regular birth control pills taken at 12 hour intervals. But these do not cause birth defects? Has this been studied? Not really.

What about Ella or ulipristal acetate, which is an antiprogestin that antagonizes the activity of progesterone on the uterus? The relative of this drug is mifepristone, which is a component of the “French abortion concoction” RU-486. This will cause birth defects if it fails to elicit abortion. How is it that Ulipristal doesn’t?

Additionally, why is it that Plan B is classified as a pregnancy Category X drug, meaning that it could lead to serious problems when taken during pregnancy, such as miscarriages or birth defects?

I do not believe the talking points concerning Plan B. Making it over the counter was a political decision pure and simple.

Discarded White Blood Cells Induce Relocation of Blood Stem Cells

Researchers at the Fundación Centro Nacional de Investigaciones Cardiovasculares or CNIC in Madrid, Spain have discovered that the clearance of the white blood cells called neutrophils induces the release of blood cell making stem cells into the bloodstream.

Our blood consists of a liquid component known as plasma and cells collectively known as “formed elements.” Formed elements include red blood cells and a whole encore of white blood cells. Red blood cells contain hemoglobin that ferry oxygen from the lungs to the tissues. White blood cells come in two flavors: granulocytes, which contain granules, and agranulocytes, which are devoid of granules.

Granulocytes are a subgroup of white blood cells characterized by the presence of cytoplasmic granules. Granulocytes are formed in the bone marrow and can be classified as basophils, eosinophils, or neutrophils. These particular cell types are named according to their distinct staining characteristics using hematoxylin and eosin (H&E) histological preparations. Granules in basophils stain dark blue, eosinophilic components stain bright red, and neutrophilic components stain a neutral pink.


The most abundant white blood cells is known as a neutrophil. Neutrophils comprise 50-70% of all white blood cells and are a critical component of the immune system. When immature, neutrophils have a distinct band-shaped nucleus that changes into a segmented nucleus following maturation. Neutrophils are normally in circulating blood, but they migrate to sites of infection via chemotaxis under the direction of molecules such as Leukotriene B4. The main function of neutrophils is to destroy microorganisms and foreign particles by phagocytosis.

Granulocytes-blood smear

Because neutrophils are packed with granules that are toxic to microorganisms and our own cells, damaged neutrophils can spill a plethora of pernicious chemicals into our bodies. To prevent neutrophils from aging and becoming a problem, they live hard and die young. in the vicinity of 1011 neutrophils are eliminated every day. They are rapidly replaced, however, and the means of replacement includes stem cell mobilization from the bone marrow to the bloodstream.

Workers in the laboratory of Andrés Hidalgo have discovered what happens to the discarded neutrophils. Earlier work in mice showed that injections of dead or dying neutrophils increase the number of circulating blood cell-making stem cells. Therefore, something about dead neutrophils causes the hematopoietic stem cells to move from the bone marrow to the bloodstream. By following marked, dying neutrophils, Hidalgo and his coworkers showed that the neutrophils went to the bone marrow to die. While in the bone marrow, the dying neutrophils were phagocytosed (gobbled up) by special cells called macrophages.

Once these bone marrow-located macrophages phagocytose aged neutrophils, they begin to signal to hematopoietic stem cells in the bone marrow, and these signals drive them to move from the bone marrow to the bloodstream to replenish the neutrophil population.

Hidalgo admits that even though his research has produced some unique answers to age-old questions, it also poses almost as many questions as it answers. For example, Hidalgo and his colleagues showed that neutrophils follow a circadian or day/night rhythm and this has implications for diseases. For instance, the vast majority of heart attacks are in the morning. Does this have something to do with neutrophil aging cycles?

“Our study shows that stem cells are affected by day/night cycles thanks to this cell recycling . It is possible that the malign stem cells that cause cancer use this mechanism to relocate, for example, during metastasis,” said Hidalgo.

Daily changes in neutrophil function could be part of the reason that acute cardiovascular and inflammatory events such as heart attack, sepsis or stroke tend to occur during particular times of the day.

“Given that this new discovery describes fundamental processes in the body that were unknown before, it will now be possible to interpret the alterations to certain physiological patterns that occur in many diseases,” Hidalgo said.

See Cell 2013; 153(5): 1025 DOI:10.1016/j.cell.2013.04.040.