In the complex landscape of human biology, the sense of smell often occupies a paradoxical position. While frequently overlooked in favor of sight or hearing, its sudden absence—whether through viral infection, environmental trauma, or the steady march of time—can profoundly diminish a person’s quality of life. Unlike the delicate and often irreplaceable neurons of the central nervous system, however, the sensory neurons housed within the nasal cavity possess an extraordinary, near-miraculous capacity for self-renewal. These cells are the frontline soldiers of the olfactory system, constantly exposed to the harsh realities of the external environment, from airborne toxins to infectious pathogens.
Now, a pioneering study from the Tufts University School of Medicine and the Graduate School of Biomedical Sciences (GSBS) has shed new light on how this regenerative engine operates. By developing a sophisticated, three-dimensional model of olfactory tissue, researchers have overturned long-held assumptions about the stem cells responsible for maintaining our sense of smell. The study, published recently in the journal Cell Reports Methods, suggests that a specific type of stem cell previously dismissed as largely dormant may, in fact, be a central protagonist in the story of nasal tissue repair.
The Frontline of Sensory Regeneration
The nasal cavity is a unique biological theater. Unlike the brain, which is shielded by the skull and the blood-brain barrier, the olfactory epithelium is in perpetual contact with the outside world. This exposure makes the sensory neurons responsible for smell highly susceptible to damage. In a healthy individual, these neurons are replaced regularly throughout their lifespan, a process of neurogenesis that remains one of the most robust examples of regeneration in the adult body.
However, this regenerative capability is not invincible. Viral infections—most notably COVID-19—as well as exposure to industrial toxins, cigarette smoke, and the natural physiological changes associated with aging can severely hamper the ability of these cells to replicate. When the birth of new neurons fails to keep pace with the death of old ones, the result is anosmia (the total loss of smell) or hyposmia (a partial loss).
To investigate why this process fails and how it might be restored, the Tufts research team, led by senior author Brian Lin, a research assistant professor in the Department of Developmental, Molecular and Chemical Biology, sought to create a more accurate representation of the nasal environment. Their solution was the development of a three-dimensional olfactory organoid—a "mini-organ" grown in a lab that mimics the structure and function of actual mouse tissue.
Reevaluating the Role of "Dormant" Stem Cells
Central to the study’s findings is the interaction between two primary types of stem cells found in the nose: horizontal basal cells (HBCs) and globose basal cells (GBCs). For years, the prevailing scientific consensus suggested a clear division of labor. GBCs were seen as the active "workhorses," constantly dividing to produce new sensory neurons under normal conditions. In contrast, HBCs were viewed as a reserve force—dormant cells that only sprang into action following a catastrophic injury to the tissue.
The Tufts study challenges this binary view. Using their new 3D model, the researchers discovered a much more nuanced and interdependent relationship between the two cell types.
"Our research suggests that these two stem cells may be interdependent," explains Dr. Lin. "One type that we thought was largely dormant—HBCs—may actually play a crucial role in supporting the production of new neurons and the repair of damaged tissue."
Through meticulous observation of the organoid cultures, the team identified a specific subpopulation of HBCs characterized by the production of a protein known as KRT5. Rather than sitting idly by, these KRT5-positive HBCs appeared to actively facilitate the generation of new olfactory neurons. The evidence became clear when the researchers experimented with the composition of the organoids. When these specific HBCs were selectively removed from the cultures, the production of new neurons was significantly impaired.
This discovery shifts the understanding of the olfactory system’s architecture. It suggests that HBCs are not merely a "break glass in case of emergency" backup system, but are essential players that provide the necessary environmental cues and support for GBCs to function correctly. This interdependence means that if HBCs are compromised, the entire regenerative cycle of the nose could grind to a halt.
The Impact of Aging on Sensory Health
The implications of this research extend beyond general biology into the realm of geriatric medicine. It is a well-documented phenomenon that the sense of smell often declines as people age, a condition that can lead to poor nutrition, depression, and an inability to detect environmental hazards like gas leaks or spoiled food.
To explore this, the Tufts team utilized their 3D model to compare cells harvested from young mice against those from older specimens. The results provided a stark visual of the aging process at a cellular level. They observed a marked decline in the ability of cells from older mice to generate new neurons within the organoid framework.
"We found a decline in the ability of the older mice cells to generate new neurons," says Lin. "We think this is due to a decrease in the GBC population as we age, but we need to do more work to test this hypothesis."
The ultimate goal of this line of inquiry is "rejuvenation." If the researchers can confirm that a dwindling population of GBCs—or a failure in the supportive role of HBCs—is the primary cause of age-related smell loss, they can begin to develop therapies aimed at revitalizing these stem cell niches.
An Easy-To-Use Model
One of the most significant achievements of the study is not just the biological discovery itself, but the tool used to reach it. The lead author of the study, Juliana Gutschow Gameiro, joined the Tufts team as a visiting Ph.D. student from the State University of Londrina in Brazil. Her mission was to develop a model that was not only effective but also accessible.
In the world of high-level biomedical research, many experimental models require prohibitively expensive equipment and highly specialized facilities. Gameiro and Lin were determined to create a 3D olfactory organoid that could be replicated in labs with more modest resources. This "democratization" of the research tool is particularly timely.
"Because loss of smell is associated with COVID-19, as well as with Parkinson’s disease and other conditions, a much larger number of researchers from a variety of different fields have begun researching olfactory epithelial cells in the last few years," says Lin.
The pandemic acted as a catalyst for olfactory research, as millions of people worldwide experienced sudden sensory loss. Furthermore, the link between a declining sense of smell and early-stage neurodegenerative diseases like Parkinson’s and Alzheimer’s has made the study of the nasal epithelium a priority for neurologists. By providing an easy-to-create model, the Tufts team is empowering a broader spectrum of the scientific community—including those who may not be stem cell specialists—to investigate how these neurons regenerate and why that process eventually fails.
Next Step: A Human Organoid
While the mouse-tissue model has provided invaluable insights, the researchers are already looking toward the next frontier: the human nose. The transition from animal models to human tissue is a critical step in translating laboratory findings into clinical treatments.
The ultimate vision is to develop a human olfactory organoid that can serve as a platform for drug screening. Such a model would allow pharmaceutical researchers to test potential treatments for anosmia or age-related sensory decline in a controlled, lab-grown environment that accurately reflects human biology.
Organoids offer several advantages over traditional research methods. They are faster and less expensive than whole-animal studies and provide a more complex, realistic environment than simple 2D cell cultures. While organoids have already been successfully developed for other organs, such as the lungs and kidneys, the human olfactory system has proven particularly elusive.
The primary hurdle is the difficulty of obtaining pure olfactory tissue from human subjects. "It’s challenging to get pure olfactory tissue from humans," Lin notes. Currently, the process involves anesthetizing a patient and using a tool similar to a COVID-19 testing swab, which must be pushed deep into the nasal cavity to reach the olfactory epithelium.
However, even when tissue is successfully collected, it is often a mixture of different cell types. Unlike the relatively "clean" samples used in their mouse model, human samples are frequently contaminated with respiratory stem cells, which are structurally similar but functionally distinct from olfactory stem cells. Separating these two populations is a complex task that currently limits the scalability of human olfactory organoid production.
The research team’s next challenge is to refine a simple and cost-effective technique for isolating these human olfactory stem cells. Once they can reliably separate these cells and "coax" them into growing in the laboratory, the path will be clear for a new generation of treatments.
As the scientific community continues to grapple with the long-term effects of the pandemic and the challenges of an aging global population, the work being done at Tufts offers a glimmer of hope. By uncovering the hidden partnership between "dormant" and active stem cells, these researchers are not just mapping the biology of the nose—they are laying the groundwork for a future where the loss of smell is no longer an inevitable part of the human experience.
