Photomed Laser Surg. 2016 Nov;34(11):550-555.

Photobiomodulation Therapy Promotes Expansion of Epithelial Colony Forming Units.

PubMed link



Objective: This preliminary study examined effects of low-dose light/laser therapy, now called Photobiomodulation (PBM) therapy, on epithelial colony forming units (eCFUs) in keratinocytes from skin and mucosa. Increased eCFUs would provide a larger pool of cells with improved potential for tissue repair and regeneration. This study also examined basic PBM parameters such as wavelengths, light sources, and dose in promoting eCFUs.

Background Data: Regenerative medicine is at the brink of exploiting the tremendous potential offered by advances in stem cell biology. The two distinct aspects for utilizing stem cells in these applications, either resident (endogenous) or transplanted (exogenous), rely on cells being amenable to expansion and being directed toward mature, functional tissues. Despite significant progress in the fundamental understanding of stem cell pluripotency, there remain fundamental challenges in applying these basic insights into clinical practice.

Methods: Two epithelial keratinocytes cell lines from skin and mucosa were subjected to PBM treatments with various devices (LED and Laser), wavelengths, and doses that were assessed for expansion potential with the colony forming unit assay.

Results: This study noted a dose-dependent effect of 810 nm laser on increasing eCFUs, both in terms of size and number of colonies. Comparisons of different wavelengths and light sources noted better efficacy of collimated and coherent lasers compared to LEDs and broad-band light.

Conclusions: PBM therapy promotes expansion of eCFUs representing increased progenitors and stem cell populations capable of contributing to tissue repair and regeneration. Further exploration of the precise mechanisms would enable optimization of PBM clinical protocols to harness the regenerative potential of stem cells for wound healing and tissue regeneration.

Key Words: Photobiomodulation (PBM) therapy; Colony forming units (CFU); Wound healing; Stem cells and Laser.

PMID: 27841965; DOI: 10.1089/pho.2015.4054



The ultimate goal of regenerating and reorganization of wounded or diseases tissues is based on utilizing pools of cell populations capable of into physiologically functional units. The inherent ability of stem cells to self-renew and differentiate, the use of stem cells for regenerative medicine has profound implications (1). These approaches are predicated on the presence of an amenable pool of cells and the field has actively explored both exogenously transplanted cells as well as naturally presented, endogenous stem cells. There are several ongoing advances in enabling utilization of stem cells in regenerative medicine broadly divided into biological (eg; pharmacological approaches, regulatory nucleic acids, peptides and growth factors) and biophysical (biophotonics, ultrasounds, radiofrequency) approaches (2). Among these, a recent emerging technique to harness stem cells for tissue regeneration is termed Photobiomodulation (PBM) therapy utilizes low-dose light illuminations to stimulate wound healing and tissue regeneration (3). Two distinct PBM mechanisms have been elucidated to date, firstly, an intracellular pathway involving cytochrome C oxidase in the mitochondria that results in increased adenosine triphosphate (ATP) and reactive oxygen species (ROS) that results in activation of diverse signaling pathways (4-7). The second extracellular pathway has been identified that involves activation of ubiquitous, multifaceted latent growth factor complex TGF-β1 by a redox-mediated mechanism following low dose laser treatments (8). This latter pathway was effectively utilized to drive differentiation of dental and bone marrow derived mesenchymal stem cells along a lineage-restricted fate (8, 9).

Our recent study extended these observations to examining epithelial regeneration in the skin and mucosa. Utilizing two keratinocyte cell lines from the skin and mucosa, we examined three critical parameters for development of a robust PBM clinical treatment namely, the light source, wavelength and dose. An epithelial colony forming unit (eCFU) assay was used to examine the ability of PBM treatment in expanding the pool of amenable stem cells that could promote skin regeneration involving surface epithelium and specialized appendages including hair, sebaceous and sweat glands (Figure 1). The eCFUs represent the number of stem cells in a given population that are capable of forming individual colonies by self-renewal and differentiation (10). We observed that increase in colony formation was evident when low doses of light (1 J/cm2) treatments were applied that was noted at higher energy (3 J/cm2). These observations highlight the importance of the hormetic, biphasic, Arndt-Shultz dose curve evident with PBM therapy (11). We next examined a few popular PBM wavelengths and noted that both 660 and 810 nm were effective at promoting eCFUs. Finally, we examined the effect of various light sources on epithelial colony formation. It was noted that treatments with laser were slightly better that PBM treatments with LED and broad-band light sources. This suggests that coherent sources are likely more efficient than non-coherent light sources in evoking certain biological responses. Nonetheless, it should be emphasized that all three light sources were noted to be capable of promoting eCFUs and may offer other distinct advantages for clinical translation such as better safety profile with LEDs and broad-band light compared to lasers

In summary, this study highlighted the ability of PBM therapy to promote expansion of epithelial stem cells from skin and mucosa that would be beneficial for wound healing, skin and mucosal rejuvenation and hair growth among other applications. Further mechanistic insights into directed differentiation of these expanded cells could enable innovative, new therapies harnessing the regenerative potential of resident, endogenous stem cells.




Figure 1: Outline of eCFU expansion by PBM treatments that could aid in regeneration of skin and mucosa including lining epithelium, sebaceous or sweat glands and hair follicles. The image (left panel) shows a population of epithelial keratinocytes (orange) that include few stem cells (blue). These stem cells can be expanded by PBM treatment to form increased eCFUs (middle panel) representing a larger pool of amenable stem cells capable of multiple, functionally specialized cell fates (right panel) for skin and mucosal regeneration.




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