Bacteria are known to use quorum sensing as a mechanism coordinate a number of behaviors including population density control, biofilm formation, virulence and antibiotic resistance. Quorum sensing occurs when bacteria secrete signaling molecules that bind to receptors that then activate the transcription of genes, which have a biological effect on the bacteria as an entire unit.
Chen and his colleagues initially hypothesized that a similar mechanism might be used by hair cells after the research team plucked 200 hairs from mice in a specific pattern within a confined area, but more than 1,000 hairs grew back, including hairs beyond the specific region. As Julia van Kessel, an assistant research scientist at Indiana University Bloomington told Scientific American, “It’s a pretty new field, but if cells can signal to bacteria in the gut, as research has shown, you would think they could signal amongst themselves as well.”
In the paper, published in the prestigious journal Cell, the authors developed a model for this quorum sensing in mammalian cells. According to their research findings, it seems that plucking hair cells induces apoptosis of hair keratinocytes, the predominant cell type in the epidermis. After this “micro-injury stage,” the plucked or now damaged follicles release CCL2, a type of chemical chemokine that recruits a type of white blood cell, called macrophages, to the entire region.
During the quorum sensing stage, macrophages then secrete TNF-alpha, which is the molecule that “activates the regeneration of all hair follicles in the region, plucked or not.” Finally, in the regeneration stage, other secondary hair-growth propagation signals “induces more hair regeneration” beyond the region that was plucked including stem cell activation. Thus, the regeneration of hair follicles after injury requires the immune system to “repair” the damaged follicles and activate stem cells to result in new hair.
Interestingly, however, the research team determined that the range for the quorum signal to be detected (and thus the regeneration process described to be activated) was on the order of 1 mm. Thus, if the plucking pattern was greater than 6 mm in diameter, “hairs did not regenerate at all.” Indeed, diameters between 3 mm and 5 mm induced the regeneration of between 450 and 1,300 hairs.
In the future, the research team hopes to determine if quorum sensing occurs in other parts of the body or in other species. As van Kessel again said of the work and its greater significance, “I don’t think quorum sensing is limited to bacteria. It’s just that we’ve always studied them and they are easy to manipulate…This [paper] is definitely a new way of thinking about things.”
(Photo courtesy of Craig Sunter)