The scientific community has long sought to uncover the identity of a true dermal stem cell. While many unique cell populations have been described in the literature, some of which are predictive of a dermal stem cell, the exact location and behavior of this cell has largely eluded us. In a landmark paper published in December 2014, Dr. Jeff Biernaskie and his colleagues at the University of Calgary used a number of eloquent in vivo lineage tracing experiments to locate and track this cell over the course of single hair cycles and multiple consecutive hair cycles.
Dermal papilla (DP) are unique cells of mesenchymal origin located at the base of the hair follicle that initiate entry of the follicle into the first growth phase, known as anagen. To achieve this, the DP send signals to epithelial precursor cells found within the “bulge,” a specialized region adjacent to the middle of the follicle. This signal instructs bulge epithelial precursor cells to proliferate. The result is the gradual formation of a new hair; it elongates and thickens, rising up from the follicle root and eventually protruding through the surface of the skin.
Over the course of a human’s life, the DP become fewer in number and undergo a form of cellular atrophy, both of which contribute to arresting hair follicle growth, hair thinning and, in the case of males, pattern balding. Scientists have studied them intently. Despite the fact that the DP are responsible for stimulating hair growth, they are only weakly proliferative; this has raised the question of how they are maintained over the life of a human and whether a more primitive progenitor exists that divides to replenish the DP compartment. It has been hypothesized that such a progenitor could be found in the dermal sheath (DS), a blanket of cells that wraps the follicle, as it has a much higher proliferative capacity.
In 2001, a researcher at the Hospital for SickKids in Toronto, Dr. Freda Miller, established a primitive population of cells in vitro from the dermis (termed skin-derived precursors or SKPs). SKPs were originally shown to have the potential to differentiate into cells of multiple lineages including neurons, glia, smooth muscle cells, and fat cells. Biernaskie joined Miller’s lab as a post-doctoral fellow shortly after this discovery and continued experimenting with SKPs. He showed that SKPs originate from a population of Sox2+ cells and are able to differentiate into various dermal cell types. They are also able to self-renew, maintain multipotency, and induce hair morphogenesis (as measured through ex vivo hair follicle reconstitution assays).
The behaviour of SKPs suggests a dermal stem cell exists; however, it does not provide insight into where it may reside in the hair follicle niche. Biernaskie left Miller’s lab in 2009 to form his own lab in Calgary to continue his work in the area of tissue morphogenesis and regeneration. Sticking to the theory that such a stem cell would be found in the dermal sheath, Biernaskie’s first step towards locating it was to purify DS cells from rodent skin using fluorescent activated cell sorting (FACS). He did this using smooth muscle actin (SMA), which is strongly expressed in the dermal sheath but largely downregulated in the DP. Using a mouse model that expresses a red marker in DS cells, Biernaskie and his team isolated DS cells in anagen phase from the backs of mice. By excluding CD34 and alpha-8 integrin, which are expressed by the vasculature and arrector pili, respectively, it was possible to purify SMA-positive cells specific to the dermal sheath.
Once placed in culture, these cells formed SKP colonies and did so at a frequency 4-fold greater than the SMA-negative fraction. Like SKPs, they had the ability to self-renew over multiple passages: a hallmark of stem cells. Importantly, they gave rise to Sox2+ positive cells indicative of mature DP cells, suggesting that the SMA-positive fraction was capable of functionally reconstituting both cells of the DP and dermal sheath. In order to substantiate these results, in vivo lineage tracing experiments were performed by labeling hair follicle DS cells with yellow fluorescent protein (YFP). In doing so, researchers were able to observe the fate of DS cells over the course of a full hair cycle.
This is where things get interesting. Biernaskie’s group discovered that a small subset of YFP+ cells was retained over consecutive cycles. Furthermore, they found that these cells actively escaped cell death by migrating away from the transient portion of the hair follicle that naturally collapses each time a new hair is grown. These cells then homed to the periphery of the DP at the end of the hair cycle and reintegrated into the dermal cup (the portion of the dermal sheath that surrounds the follicle root). Upon closer inspection, these cells were observed to form an abundant number of slender processes that extended outwards and wrapped around the dermal papilla. Biernaskie termed these cells hair follicle dermal stem cells (hfDSCs).
In total, the group analyzed more than 120 follicles. On average, there were ~3 YFP+ cells retained per cycle (range of one to six). Based on this, the group concluded that the hfDSC “pool” in each follicle is approximately three to six cells at any given time. To confirm that these hfDSCs exhibit bipotency within the hair follicle, and determine the function of their progeny, Biernaskie and colleagues examined the fates of single cell clones generated within individual hair follicles. Of the 240 hair follicles analyzed, 79% contained hfDSCs that were undergoing cell division. These stem cells generated progeny that entered the DP, DS, and dermal cup. Interestingly, a subset of hfDSCs remained mitotically inactive (21%), which could be a dormant population that acts as a reserve.
The work of Biernaskie and his team provide direct evidence of a dermal stem cell population that functions to maintain the DS and DP compartments. This population appears to be rare and long-lived and takes residence in the dermal cup of the hair follicle. In vivo tracing experiments illustrate a hair follicle that is remarkably dynamic, and unveil new modes of communication between cells in the hair follicle niche. The processes radiating out from hfDSCs into the DP compartment provide evidence of a physical communication process that, until now, was unknown.
Both the Centre for Commercialization of Regenerative Medicine and one of its industry consortium members, Replicel, have been exploring therapeutic paradigms for the treatment of hair loss. Pattern balding (androgenetic alopecia) is a market of particular interest, given that there are currently no less than 400 million men on the planet who are afflicted with it. Males in Asia seem to have a keen interest in hair loss products, so this will be a key market to go after commercially given its size. Therapy for hair loss could be drug- or biologic-based, where small molecules or growth factors are formulated into creams and lotions, or cell-based, as is being pursued by Replicel, where skin progenitors are transplanted directly into the scalp.
There are a small number of cell lines around the world that can be used to screen for agents that promote hair growth. The ability of these lines to identify positive hits depends on the degree to which they recapitulate the in vivo hair follicle niche. It will be interesting to see whether Biernaskie and colleagues can isolate and characterize the elusive hfDSC. If a cell line can be created in vitro, it would be an invaluable tool for screening and the identification of therapeutics for treating hair loss in the future.