Introduction

Skin cells

Skin is the body's outer layer which also include nails, hair and the glands and nerves on your skin. The whole layer acts as a physical barrier — protecting the body from bacteria, infection, injury and sunlight.

Skin contains 20 cell types that contribute to skin function and stratification. The epidermis is the outermost layer of the skin and consists of 5 sublayers: stratum basale, spinosum, granulosum, lucidum and corneum. Keratinocyte stem cells in the basal layer undergo self-renewal and differentiate upwards to form the upper the stratum corneum layer, where dead cells are constantly being sloughed off.

The basement membrane separates the epidermis from its underlying dermis. The dermis is supplied by a network of blood vessels, nerve endings and is home to appendages such as sebaceous glands, sweat glands, sensory nerves and hair follicles. Fibroblasts are the main cell type within the dermis and are responsible for the production of extracellular matrix (ECM) components that provide structural integrity, flexibility and elasticity to the skin.

Dermal aging

Aging of the skin is characterized by slower regeneration and eventual loss of skin structure and functionality. Skin aging is associated with a compromised protective role, specifically impaired wound healing and barrier function.

An increased inflammation, impaired water and thermal homeostasis leads to diminished elasticity, hydration, structural changes, and susceptibility to various dermal disorders.

Skin aging can be attributed to age-related intrinsic factors (chronoaging) and extrinsic causes. Intrinsic aging presented with reduced proliferative capacity of epidermal stem cells (keratinocytes) and manifested as fine lines, dry skin, altered pigmentation and loss of elasticity. Extrinsic factors including chronic exposure to ultraviolet radiation (photoaging) or oxidative stress caused by environmental pollution lead to diminished collagen synthesis and disorganization of the dermal ECM. The skin appears rough and dry with deep wrinkles and appearance of visible blood vessels and spotted pigmentation.

Cell aging and senescence

To withstand these processes, the skin implements continuous shedding of the superficial cells and subsequently replacing them with younger cells generated at the basal layer of dermis. As they mature, they move through the epidermis toward the skin’s surface, where they lose their nuclei and became inactive and dead. The thickened layer of such dead cells is sloughed off over time. The physiologic cell turnover makes skin firmer and gets rid of fine lines and wrinkles more quickly.

As the aging process occurs, the ability of the human body to resolve pathologic changes becomes significantly reduced. The aging skin often contain cells that have entered a state called senescence (sleeping cells), in which they stop dividing and become resistant to death-inducing pathways. Senescent cells may alter communication between skin layers and perturb skin homeostasis. Accumulation of dysfunctional, senescent cells has harmful effects in aging tissues due to their decreased ability to contribute to tissue repair and regeneration.

In physiological conditions, senescent cells can be removed by the immune system, however, as we age, senescent cells accumulate in tissues because an aging immune system fails to remove them. These cells secrete a collection of factors that have inflammatory, protein-degrading and other biologically active properties, and can permanently impair tissue function. Long term accumulation of senescent cells has been shown to potentially play a role in the pathophysiology of ageing and age-related disease. It was demonstrated that the elimination of senescence cells delays age-associated disorders.

Out of various senescent skin cells, fibroblasts appear to accumulate with age in human skin and compromise skin function and integrity. Dermal fibroblasts are the major type of cells in dermis. They produce integral components of the extracellular matrix (ECM), such as collagen.

During the skin aging process, the ECM undergoes dramatic structural alterations and degradation, resulting in dermal thinning and loss of elasticity that eventually cause wrinkle formation.

In normally functioning skin, fibroblasts are largely recruited during skin wounding and actively contribute to skin repair and regeneration. They play an important role of being the “middle man” between the stimulation and the output. The stimulation comes from micro-trauma of various origins (electrical, chemical, mechanical, radiation, thermal, etc.) and leads to the fibroblasts increased their activity in the wound healing mechanism. As a result of this increased activity, there is increased synthesis of collagen, elastin, and components of extracellular matrix, the backbone structure providing firmness and elasticity of the skin.

Cell senescence and impaired skin wound healing

An emerging hypothesis postulates that fibroblast senescence is the main driver of the skin aging process as the release of SASPs increase and the proliferation of cells is arrested irreversibly [Wlascheck 2021].

The wound healing mechanism in the skin (Xu 2021)

During the skin wound healing process, fibroblasts, epithelial cells and endothelial cells are actively involved in extra cellular matrix production (ECM) [14], the re-epithelialisation [3,15] and angiogenesis

In normal human tissue, fibroblasts contribute to tissue homeostasis by regulating the turnover of extracellular matrix (ECM). When the tissue is injured, fibroblasts proliferate and growth factors increase.

The impact of severe conditions on the skin (irradiation, low atmospheric temperatures) does not causes death of the skin cells, but induces a rapid and sustained cell cycle arrest in dermal fibroblasts and may enter them senescence.[Bourdens 2019]

Electrical Skin Activation (ESA)

ESA technology implements direct current (DC) which is applied on the skin during the treatment. DC is one-directional flow of electric charge in a constant direction, distinguishing it from alternating current (AC).

The target of ESA treatment is to rejuvenate aging skin through activation of the natural tissue healing mechanisms and re-activation senescent dermal fibroblasts.

Electrical stimulation and wound healing

Electrical stimulation (ES) has long been used as an effective approach to modulate cellular behaviors in the diminished proliferation condition of the aging skin. Electrical filed has an obvious effect on regulating and promoting healing of the age-related skin injuries.

As it suffers from various intrinsic and extrinsic damaging factors, the natural wound healing mechanism is deteriorated with the age. An external electrical current applied to the skin is believed to mimic the body's natural bioelectricity and to restart and stimulate endogenous electrical fields and as such, promote wound healing. It was determined that the rate of healing is closely correlated with the electrical current generated at the wounded site. The studies demonstrates that ES promotes skin fibroblast growth, enhances the cell migration towards the injured area, and increases production of various growth factor (FGF-1 and FGF-2). [Galan 2018] In in-vitro and in-vivo studies, ES has been demonstrated to enhance cellular activities such as collagen and ATP synthesis, increase tissue perfusion, decrease edema, and promote angiogenesis. [Jennings 2008] Moreover, pre-clinical scientific experiments demonstrated that modified cellular behaviors continued up to 3 days after the initial electrical stimulation.

Electrical stimulation and cell senescence

Eliminating senescence cells and their SASP toxins prevents age-related declines in skin health. Although exfoliation, peeling, abrasion, or other physical methods are implemented for removal of the senescent cells from the skin surface, it cannot be implemented for the cells in dermis. Those “sleeping” cells accumulated with aging can be re-activated by applying direct electrical current. In the study of Li [2021], electrical field was demonstrated to reverse senescence of stem cells and influence cellular proliferation and regeneration.

Take home notes

Aging skin gradually loose its functionality and structural support due to intrinsic and extrinsic factors
Age-related decline in ECM contributes to the skin cells present without activity and generating toxic materials (sleeping cells, or senescence cells)
An emerging hypothesis postulates that fibroblast senescence is the main driver of the skin aging process as the release of SASPs increase and the proliferation of cells is arrested irreversibly [Wlascheck 2021].
Fibroblasts are the major type of cells that constitute the dermis layer of the skin. They produce integral components of the extracellular matrix (ECM), such as collagen [5].
During the skin aging process, the ECM undergoes dramatic structural alterations and degradation, resulting in aging phenotypes of dermal thinning and loss of elasticity that eventually cause wrinkle formation [6].
Permanent senescence or skin aging can be induced in nonreplicating (senescent) fibroblasts by both intrinsic and extrinsic stressors
Electrical stimulation augments dermal fibroblast migration and biosynthesis and provide potential mechanisms by which electrical field may be used for enhancing tissue healing and repair.[Chao 2007]
Electrical field appears to have an important role in controlling fibroblast activity in the process of wound healing.
Electrical stimulation has a beneficial effect on wound healing.
Rouabha et al [2013] demonstrated that exposure to electrical stimulation promotes skin fibroblast growth and increases secretion of the growth factors, directs migration of fibroblasts and increases the migration rates, thus promoting wound healing.
Studies have shown that applied EFs stimulate cell proliferation and differentiation, as well as synthesis of growth factors and matrix proteins
Cell migration plays an important role in wound healing.

Illustrations

Cellular senescence and aging. The organismal life span is controlled by internal and external factors inducing senescence and therefore, proliferation arrest. The text lists molecular and physiological triggers that may lead to: telomere attrition, aneuploidy, DDR response, epigenetic modulations, and mitochondrial dysfunction; these are causes of senescence manifested as either loss of cellular proliferation and emergence of the senescent associated secretory phenotype (SASP) in dividing cells, or simply SASP alone in non-dividing cells. [Bhatia-Ney 2016 Cellular Senescence as the Causal Nexus of Aging]

**Damage leads to causal nexus leads to effects.**

Figure 06. **Damage leads to causal nexus leads to effects.**
In different species the causes of cellular senescence may be similar, but have different magnitudes. For example, in mice **(A)** DNA damage/Tumor suppressor activation may be a major factor, whereas in humans **(B)**, telomere erosion may have a greater effect because humans have much shorter telomeres than mice. In both species, accrued, and unrepaired damage to the cellular genome, epigenome, and organelles (causes) is finally manifested in the visible phenotype, evident as aged features of the organism, such as organ dysfunction, loss of structural integrity, and physiological and anatomical changes (effect). The link between cause and effect, the *causal nexus*, is cellular senescence. Additionally, it has been shown that in mice, blocking cell division alone, in the absence of any molecular damage, can induce premature Progeria-like aging. [Bhaty-Ddey 2016]

References

1. Chao PH, Lu HH, Hung CT, Nicoll SB, Bulinski JC. Effects of applied DC electric field on ligament fibroblast migration and wound healing. Connect Tissue Res. 2007;48(4):188-197.

2. Jennings J, Chen D, Feldman D. Transcriptional response of dermal fibroblasts in direct current electric fields. Bioelectromagnetics. 2008;29(5):394-405.

3. Galan, Edgar & Bayat, Ardeshir. (2018). Alternating current electrical stimulation effects in human dermal fibroblasts. MACE PGR Conference University of Manchester, UK, March 26, 2018

4. Lee YI, Choi S, Roh WS, Lee JH, Kim TG. Cellular Senescence and Inflammaging in the Skin Microenvironment. Int J Mol Sci. 2021;22(8):3849.

5. Campisi J. The role of cellular senescence in skin aging. J Investig Dermatol Symp Proc. 1998;3(1):1-5.

6. Bhatia-Ney 2016 Cellular Senescence as the Causal Nexus of Aging

7. Ho CY, Dreesen O. Faces of cellular senescence in skin aging. Mech Ageing Dev.021;198:111525.

8. Bourdens M, Jeanson Y, Taurand M, Juin N, Carrière A, Clément F, Casteilla L, Bulteau AL, Planat-Bénard V. Short exposure to cold atmospheric plasma induces senescence in human skin fibroblasts and adipose mesenchymal stromal cells. Sci Rep. 2019 Jun 17;9(1):8671.

9. McCart EA, Thangapazham RL, Lombardini ED, Mog SR, Panganiban RAM, Dickson KM, Mansur RA, Nagy V, Kim SY, Selwyn R, Landauer MR, Darling TN, Day RM. Accelerated senescence in skin in a murine model of radiation-induced multi-organ injury. J Radiat Res. 2017 Sep 1;58(5):636-646.

10. Li G, Zhu Q, Wang B, et al. Rejuvenation of Senescent Bone Marrow Mesenchymal Stromal Cells by Pulsed Triboelectric Stimulation. Adv Sci (Weinh). 2021;8(18):e2100964.

11. Galan, Edgar & Bayat, Ardeshir. (2018). Alternating current electrical stimulation effects in human dermal fibroblasts. MACE PGR Conference University of Manchester, UK, March 26, 2018

12. Ho CY, Dreesen O. Faces of cellular senescence in skin aging. Mech Ageing Dev. 2021;198:111525.

13. Wlaschek, M.; Maity, P.; Makrantonaki, E.; Scharffetter-Kochanek, K. Connective tissue and fibroblast senescence in skin aging. J. Investig. Dermatol. 2021, 141, 985–992.

14. Chao PH, Lu HH, Hung CT, Nicoll SB, Bulinski JC. Effects of applied DC electric field on ligament fibroblast migration and wound healing. Connect Tissue Res. 2007;48(4):188-197.

15.

16. Jennings J, Chen D, Feldman D. Transcriptional response of dermal fibroblasts in direct current electric fields. Bioelectromagnetics. 2008;29(5):394-405.

17. Rouabhia M, Park H, Meng S, Derbali H, Zhang Z. Electrical stimulation promotes wound healing by enhancing dermal fibroblast activity and promoting myofibroblast transdifferentiation. PLoS One. 2013;8(8):e71660.