As a person ages it is common for them to be concerned about their appearance and in many cases pursue methods of rejuvenating their skin. Environmental factors can influence skin ageing especially exposure of the skin to sunlight. Skin which is chronically exposed to sunlight expresses an aged morphology earlier and differently to skin which is protected from sunlight. This explains why the skin of individuals who are "sun-obsessed" or have outdoor jobs commonly ages earlier than individuals with minimal exposure to sunlight. To understand how the sun causes an individual's skin to age, an appreciation of skin anatomy and physiology is important.
Anatomy
The skin is the largest organ in humans; it covers the entire outer body and serves a multitude of physiological functions. The anatomical structure and cell types present in skin allows it to act as a protective barrier between the internal organs and the environment. The skin is split into three anatomically distinct regions: the epidermis, dermis and subcutaneous tissue.
Figure A. The anatomy of the different layers of the skin. The different cells and components of the extracellular matrix of these layers are shown [77].The epidermis is the most superficial layer of the skin and contains resident keratinocytes (KC) cells, melanocytes and langerhans cells. Keratinocytes are the most abundant cell type in skin. The langerhans cells are important to the skins immune response [1]. The epidermis is subdivided into five layers: stratum basale, stratum spinosum, stratum granulosum, stratum licidum and the stratum corneum (see figure A.) [2].
Each layer of the epidermis contains different cell types and properties and they work in synchronisation to continuously renew the skin and allow it to retain its strength. The basale layer has resident skin cells known as melanocytes which produce the melanin pigment which protects the skin against sun damage and determines skin colour. It is these cells which cause a suntan in response to exposure to the sun [3]. The next layer after the basal layer is the spinosum layer which contains the keratinocyte cells [1]. These cells are responsible from the production of keratin which is a tough protein that makes up a large part of the structure of the skin. The keratinocytes rise through the granulsoum and licidum levels and during this process they enlarge, flatten and join; eventually they dehydrate and die. Once dead, the cells become part of the stratum corneum. Once this layer is damaged it is replaced by the next set of rising keratinocytes [4].
Below the epidermis the dermis can be found, this is separated from the epidermis by a basement membrane. This dermis provides mechanical support for the epidermal layer. The dermal layer consists of blood vessels, lymph vessels, nerve endings, hair follicle, sweat glands, collagen and elastin fibres [5]. The dermis is divided into the uppermost papillary dermis and an underlying reticular dermis [6]. The uppermost papillary layer has thin layers of collagen fibres whereas the underlying reticular layer is made of thick collagen fibres which are arranged parallel to the skin's surface. As structural elements of the skin, the proteins collagen and elastin are especially important to the dermal extracellular matrix (ECM) [5]. Collagen a tough and insoluble protein supports the epidermis whilst elastin, also a protein, contributes to the flexibility of the skin [5]. Alongside these fibres water is present in the dermis and immune cells are also found [5].
The last layer of the skin is the subcutaneous tissue which consists of fat, connective tissue, large blood vessels and nerves. This layer is variable at different sites of the body and between individuals. The subcutaneous tissue is important in protecting internal organs from external shock; it also contributes to the heat regulating ability of the skin [7].
Figure B. A picture comparison of two 71 year old women. The lady on the left has little signs of photoageing whereas the lady on the right shows clinical features of photoageing. Courtesy of Professor CEM Griffiths, University Of Manchester.Each layer of the skin contributes to its physiology. The skin acts as a barrier protecting against heat, injury and infection. The skin is essential for regulating the body's internal temperature; this is accomplished through sweating and regulating blood flow in the skin. It also stores water, fat and metabolises vitamin D. The nerve endings found in the skin are important for sensory feelings such as touch, pain and heat [5].
This review is interested in how the anatomy of skin changes in response to UV radiation from the sun. Like all organs the skin ages in a chronological fashion but the ageing caused in this fashion is commonly only apparent histologically at an age of seventy years old or older [8]. Ageing caused by the sun is termed photoageing; the aged appearance appears much earlier and is superimposed on intrinsically aged skin. Skin which is chronologically aged (back, trunk, and buttock skin) shows different histological and clinical morphology than skin which is sun-exposed (face, forearms, and hands). Chronologically aged skin is thin, smooth, dry, finely wrinkled, unblemished and has lots of elasticity whereas photoaged skin is wrinkled, lax, coarse, fragile, unevenly pigmented and has brown spots [9-10].
At the histological level, photoaged skin shows an increased epidermal thickness. The ECM is altered with decreased production of dermal collagen alongside collagen breakdown. There is also an increase in deposits of elastotic material beneath the epidermal-dermal junction known as solar elastosis [11-12].
It is important to note that the severity of photoageing relates both to the amount of sun exposure and the degree of skin pigmentation. Individuals with fair skin are much more likely to be photoaged than those with dark skin [13].
These changes in skin morphology associated with sunlight exposure are the result of ultraviolet (UV) wavelengths from the sun. Ultraviolet radiation (UVR) consists of UVA (320-400 nm), UVB (290-320 nm) and UVC (100-290 nm) [13]. Less than five percent of the sunlight that reaches the earth's surface is made up of the UV wavelengths with night-eight percent of these UV rays being UVA and the remaining two percent UVB [13]. These concentrations of UVA and UVB are dependent on the time of day, season and location in the world [14]. Ultraviolet-B does not penetrate the dermis as it is mainly absorbed by the epidermis [15]. Ultraviolet-B is able to damage DNA in keratinocytes and melanocytes in the epidermis as well as causing enzymes capable of remodelling the ECM to be produced [16]. Ultraviolet-A is thought to be a greater contributor to photoageing than UVB due to its ability to penetrate much deeper into the dermis. Ultraviolet-C rays (200-290 nm) are almost completely absorbed by the ozone layer and thus are not thought to contribute to the photoageing process [13].
The mechanisms by which the sun causes the photoaged appearance shall be discussed with a particular focus on how inflammatory cells contribute to these processes.
Role of Reactive Oxygen Species (ROS) in photoageing.
The pathways involved in producing the phenotype of photoaged skin are complex with many of the pathways being interconnected. An important starting point in understanding these complex pathways begins with the production of ROS in the skin as a result of acute UVR. An increase in ROS concentration in the skin causes downstream responses which cause the connective tissue alteration seen in photoageing. Ultraviolet light from the sun is absorbed by chromophores and photosensitizers in the skin initiating a photochemical reaction which results in ROS formation [17]. Reactive oxygen species are molecules or atoms which contain an unpaired electron. There are multiple chromophores in skin which absorb UV light including DNA, aromatic amino acids, 7-dehydrocholesterol, cytochromes, melanin, bilirubin and urocanic acid [13, 18].
Reactive oxygen species occur in health and disease but when expressed in higher than normal amounts oxidative stress occurs. Oxidative stress is when the equilibrium between ROS production and the body's ability to remove the ROS is disturbed resulting in damage to the cells in the skin [13]. ROS generated in human skin include superoxide anion, peroxide, hydrogen peroxide, singlet oxygen, hydroxyl radicals and nitric oxide [17]. Reactive oxygen species, in particular, singlet oxygen, can exert a multitude of effects such as lipid peroxidation, local inflammation, activation of transcription factors, and generation of DNA strand breaks [19].
An experiment which observes the increase in ROS post UVR was carried out by Kang et al. They irradiated human buttock skin with two times the dose of UVR which causes slight skin reddening, known as the minimal erythematic dose (MED). Within fifteen minutes hydrogen peroxide was shown to rise [20].
On top of UV there are further contributors to the generation of ROS in the skin including the keratinocytes, fibroblasts and inflammatory cells found in the skin [19, 21]. Upon UV irradiation of the skin an increase in inflammatory cells is evident [19]. Inflammatory cells are up regulated due to an increase in pro-inflammatory mediators, such as cytokines which result from UV exposure. Phagocytic cells, such as macrophages and lymphocytes, are able to contribute large quantities of ROS as they express high amounts of NADPH oxidase enzyme. The NADPH oxidase allows the phagocytic cells to initiate ROS in the defence mechanism of respiratory burst [16]. Superoxide, hydrogen peroxide and hydroxyl radicals are the ROS which are primarily produced by phagocytic cells [19].
The reason for ROS production being important in the pathways of photoageing is due to their ability to act as receptor ligands and thus their ability to increase production of transcription factors [22]. Experiments have shown that IL-1a, EGF and tumour necrosis factor α (TNF-α) receptors were activated in keratinocytes and fibroblasts by ROS [1, 23]. Reactive oxygen species can activate these receptors through inhibiting protein tyrosine phosphatises (PTP), which act to down regulate the receptors [16]. Protein tyrosine phosphatises are inhibited as the ROS oxidise a cysteine residue (present in all PTP catalytic sites) into sulfenic acid, this makes the PTP inactive [24]. Once the receptors are activated, the signal transduction pathways result in the production of many transcription factors. Two transcription factors activated by ROS which are crucial in the molecular mechanism of photoageing are activator protein-1 (AP-1) and nuclear factor-kappa B (NFκB) [25-26]. Activator protein-1 controls the transcription of matrix metalloproteinases (MMPs) [27]. Nuclear factor-kappa B activation results in the production of a range of immune-modulatory cytokines [16].
AP-1
AP-1 plays an important role in the pathway leading to the ECM degradation associated with photoageing. As a transcription factor it regulates the expression of many proteins, the most important of which to photoageing are the MMPs [28].
Figure C. The pathway of AP-1 transcription as a result of UV irradiation. The affects AP-1 has on production of MMPs and collagen degradation are evident. The decrease in TGF- β associated with UVR and the subsequent decreased procollagen I and III is represented. [17]AP-1 is a hetero-dimmer composed of two proto-oncogenes: c-fos and c-jun [1, 22]. C-fos is continually expressed in high levels in normal skin whereas c-jun is expressed when skin is irradiated with UV. Ultraviolet irradiation does not cause an increased in c-fos levels [29]. Unirradiated skin expresses AP-1 consisting of c-fos and jun-D [30]. Ultraviolet irradiation results in the activation of Mitogen-Activated Protein Kinase (MAPK) signalling pathways throughout the epidermis and upper dermis which up regulates c-jun. C-jun competitively competes with jun-D to bind with c-fos and produce activated AP-1 (see figure C). C-Jun is evident in the epidermis thirty minutes post UVR exposure, this increase in c-jun can remain for up to twenty-four hours [29]. The activated c-Jun:c-Fos AP-1 complexes are expressed throughout all the layers of the epidermis and dermis [31].
The activated AP-1 acts to up regulate the MMP family of proteins in particular the specific MMPs: MMP-1, MMP-3 and MMP-9 [17]. Activator protein-1 is able to up regulate MMP expression via binding to upstream binding sites. Through the activation of these MMPs, AP-1 results in increased collagen breakdown [32].
As well as activating MMPs and thus causing direct ECM breakdown AP-1 can act through a separate mechanism to cause ECM reorganisation by down regulating synthesis of procollagen I and III [16, 33]. Another indirect mechanism by which AP-1 causes ECM reorganisation is by blocking the effects of transforming growth factor β (TGFβ) which is a major profibrotic cytokine (see figure C) [34].
Matrix metalloproteinases and extracellular matrix degeneration
Activation by AP-1 is not the sole mechanism of MMP production but through inflammation and normal process of ageing MMPs are also activated [35]. The complex mechanisms contributing to MMP production and the role these enzymes play in the ECM homeostasis highlight their importance in the formation of photoaged skin
Matrix metalloproteinases are zinc-dependant endopeptidases, which are able to remodel the ECM [36]. The family of MMPs consists of twenty-five members, of which twenty-four are expressed in mammals [37]. Each MMP has a unique structure and can only degrade a specific subset of matrix protein. Matrix metalloproteinases can be classified into four different groups depending upon their substrate specificity [38]. These are: collagenases which break down collagens (MMP-8 and MMP-1), stromelysins which have the ability to degrade the majority of ECM proteins, gelatinases which degrade the fragmented collagens (MMP-2 and MMP-9) and membrane-bound MMPs [27, 38].
The different MMPs are expressed in the skin by different mechanism of activation. Activator protein 1 is important in activating the expression of MMP-1, MMP-3 and MMP-9. Many experiments have highlighted how different MMPs can be expressed. For example UVB, even at a suberythemal dose can induce the expression of MMP-1, -3 and -9 in normal human epidermis in vivo [39]. Ultraviolet-A has been shown to induce MMP-1 expression from dermal fibroblasts in vivo and MMP-1, MMP-2, and MMP-3 in vitro [40]. Ultraviolet radiation is able to express MMP-1, MMP-3, and MMP-9 within eight hours. Matrix metalloproteinases-1 and -3 were increased a thousand fold at twenty-four hours post UVR, whereas MMP-9 only six-fold [38].
The importance of MMP-1, -2,-3 and -9 being expressed together is that when combined their actions can degrade most of the proteins that dermal ECM consists of [41]. Matrix metalloproteinase-1 causes cleavage of dermal collagen type I and III in skin within its central triple helix [38]. The fragments produced by this process are further digested by MMPs -2, -3 and -9. Matrix metalloproteinase- 2 can breakdown collagen type IV and type VII which make up the basement membrane as well as elastin [42]. Matrix metalloproteinase-3 is able to breakdown a wider variety of proteins such as collagen type IV, fibronectin, proteoglycans and laminin. Matrix metalloproteinase-2 degrades collagen type IV, V and gelatine [42]. Inflammatory cells are also able to release MMPs into the ECM [19]. Matrix metalloproteinase-12 which is derived from macrophages and the serine protease, neutrophil elastase, are both capable of ECM breakdown [43-44]. Other inflammatory can also contribute to this ECM breakdown for example the release of mast cell tryptase from mast cells [45].
Neutrophils, which are one of the inflammatory cells which contribute to ECM breakdown, quickly infiltrate sunburned skin and it has been suggested that they release enzymatically active neutrophil elastase, MMP-1 and MMP-9 [46]. The human macrophage metalloproteinase (MMP-12) has been shown to be very active against elastin [44]. Both MMP-12 and neutrophil elastase were shown to be present in human skin twenty-four hours after UVR [44]. It has also been suggested that T-lymphocytes infiltrating photoaged skin may play a part in the degeneration and reduction of collagen through MMP-1 activity [47].
Interestingly MMPs can indirectly affect the ECM through inhibiting procollagen 1 synthesis. Fibroblasts which synthesise procollagen are prevented from doing so by the presence of the fragmented collagen produced by the MMPs [48]. This was discovered when dermal fibroblasts were placed on a gel which had type 1 collagen that had been fragmented by MMP-1. The fibroblasts were shown to have decreased synthesis of type I procollagen in comparison to fibroblasts placed on a normal gel [48]. This evidence supports the idea that damaged collagen fragments can down regulate the collagen synthesising ability of fibroblasts. Thus, the MMPs formed as a result of UVR, can damage the dermis in two possible ways: they can breakdown the collagen directly or can prevent collagen synthesis indirectly.
As MMPs can degrade the ECM a control mechanism exists which prevents excessive breakdown from occurring in healthy skin. The MMPs can be controlled through stoichometric binding to specific tissue inhibitors of metalloproteinases (TIMPs) [49].
Tissue inhibitors of metalloproteinases (TIMPs)
Tissue inhibitors of metalloproteinases are endogenous inhibitors which can be grouped into several classes. There are four types of TIMPs, TIMP-1 and -2 exist in a soluble form whereas TIMP-3 and -4 are bound to the ECM. Tissue inhibitor of metalloproteinase-1 can inhibit most MMPs except MMP-2 and -14. Tissue inhibitor of metalloproteinase-2 inhibits most MMPs except MMP-9 [19]. The balance between TIMPs and MMPs is important to maintain as when TIMPs are decreased the MMPs are able to degrade the ECM.
There are contradictory reports on the response of the skins TIMPs to UVR. It has been shown in a fibroblast culture, that both TIMP-1 and TIMP-2 levels were reduced after UV exposure [50]. However, other experiments have shown UV to induce TIMP-1 in vivo [51]. Researchers have also studied the possibility that inflammatory cells can inhibit TIMPs through released mediators. Okada et al showed that serine proteases including human neutrophil elastase, trypsin and a-chymotrypsin, destroyed the inhibitory actions of TIMPs against MMP-3 by degrading the inhibitor molecule into small fragments [52].
Matrix metalloproteinases have an important role in photoageing due to their direct and indirect ECM remodelling. There are still many aspects of their activation, inhibition and role which are unclear but it is becoming more widely acknowledged that inflammatory cells may be strong contributors to these direct and indirect pathways.
Nuclear factor kappa B
It is essential to understand how inflammatory cells are recruited to the skin and which of the immune cells are recruited in this process. ROS play an important step in the infiltration of immune cells as they increase the expression of the transcription factor NFκB [53]. Nuclear factor-kappa B is an important mediator and initiator of the release of pro-inflammatory cytokines. Nuclear factor-kappa B can stimulate the transcription of genes including IL-1α, IL-1β, TNF-α, IL-6 and IL-8 [54]. Each of these cytokines plays a role in immunological and inflammatory responses [55]. Genes other than those encoding cytokines activated by NFκB include: MMPs, adhesion molecules and regulators of cell growth, differentiation and cell death [53].
Release of pro-inflammatory mediators
Cytokines are not just expressed in response to NFκB but can be triggered by a number of mechanisms including being released from keratinocytes and inflammatory cells. Many nucleated cells resident in the skin produce cytokines when activated. For example melanocytes, Langerhans' cells, fibroblasts, mast cells, lymphocytes and other inflammatory cells in the dermis [56]. These cytokines each have different roles and can stimulate different effects. Cytokines can increase the permeability of capillaries leading to infiltration and activation of neutrophils and other inflammatory cells into the skin [56].
It has been shown that UVB irradiation can cause an increase in TNF-α expression from KCs and dermal fibroblasts within a period of one and a half hours post UVB [57]. Similar experiments have shown UVB radiation induces not only IL-1, IL-6 and TNF-α, but IL-10 and IL-12 [58]. Keratinocytes are able to produce a complex array of cytokines e.g. IL-1 and TNF-a [59]. Once the inflammatory cells have mediated to the skin they themselves can then release further cytokines.
The release of proinflammatory cytokines stimulates infiltration of neutrophils and other immune cells to the irradiation site which further aggravates matrix degradation. The immune cells are able to produce ROS thus increasing oxidative stress or release enzymes involved in ECM breakdown. The MMP-1 protein can be stimulated from fibroblasts through a mix of cytokines: such as IL-1, IL-6, PDGF, TNF-a, TGF-b, bFGF and EGF [56]. These cytokines are thus capable of aggravating ECM degradation by the production of this MMP-1.
TGF-β
Another cytokine important in photoageing is TGF-β which is involved in an indirect method of ECM reorganisation. Ultraviolet radiaton is able to regulate the TGF-β/Smad pathway to decrease procollagen type 1 synthesis and thus decrease type 1 collagen in the ECM [60]. When TGF-β binds to its receptor, transforming growth factor beta receptor II (TGF-βR2), the receptor is activated and the transcription factors Smad2 and Smad3 are subsequently phosphorylated. The phosphorylated Smad2 and Smad3 join Smad4 and move into the nucleus, regulating the expression of the type 1 collagen gene [60].
Studies by Quan et al show that the TGF-βR2 is down regulated in response to UVR. This can be observed within eight hours post UVR as type 1 procollagen synthesis is reduced within this period. Ultraviolet radiation decreases the procollagen 1 by down regulating the TGF-βR2 through transcriptional repression which causes a ninety percent reduction in TGF-β binding to the receptor [61]. This loss of binding to TGF-βR2 prevents Smad2/3 activation by TGF-β, thus causing a reduction in type I procollagen expression. It is also interesting to note that in an experiment in which TGF-β was over expressed in transgenic mice, an accumulation of type 1 collagen in the skin and other organs was observed [62]. This supports the importance of TGF-β as a critical regulator of type I procollagen synthesis.
Overall picture
Figure D. A schematic diagram of the pathways involved in increased collagen production and decreased collagen breakdown associated with photoageing.To take into account all the different mediators of photoaged skin is a difficult process. It is widely accepted that the "cause" of wrinkles is both a consequence of increased breakdown and reduced synthesis of collagens and fibrillin. The pathways which are involved in the formation of photoaged features are complex and many mechanisms are still subject to debate between different researchers. Figure C gives a simple diagrammatic representation of the main elements involved in the pathways of photoageing. In each of the pathways inflammation can contribute to some extent and it is commonly accepted that there is an inflammatory response to acute UVR caused by pro-inflammatory cytokines, ROS damage and MMP ECM breakdown.
It is important to have an understanding of the histology of photoaged skin in order to understand the effects of UVR on the skin. The histology of photoaged skin has been subject to many investigations and many histological changes are associated with chronically sun-exposed skin. Interestingly, a small number of studies have shown inflammatory cells to be present in skin which is chronically sun exposed. Research focusing on inflammatory cells found histologically is limited hence the inflammatory cell milieu of chronically photoaged skin has not been fully elucidated. The reasons as to why these immune cells are resident in photoaged skin are also unclear. Do the inflammatory cells contribute to photoaged skin or are they resident as a result of photoageing?
Inflammations role in photoageing
Neutrophils have been extensively researched regarding their response to acute UV radiation. They infiltrate the skin following exposure to solar simulating radiation (SSR) and natural sunlight [63]. There has been much speculation as to whether neutrophils contribute to the ECM breakdown associated with photoageing or if they just increase inflammation at the site.
Rijken and colleagues strongly support the theory that neutrophils are essential to photoageing and they suggest several reasons for them having been overlooked in previous studies. Firstly they explain that to successfully recognize neutrophils in frozen skins and paraffin-embedded skin in small numbers is very difficult. They continue to say that there is a certain threshold of damage at which neutrophils infiltrate the skin and this could have been overlooked in previous experiments [63]. The theory that neutrophils contribute to the ECM breakdown by the release of neutrophil elastase (MMP-8) has been opposed by Fisher et al [64]. They showed MMP-8 was induced in skin exposed to two MED of UVB and two MED of SSR but they concluded that it was inactive. The conclusion was determined through an indirect method as they showed that trans-retinoic acid failed to inhibit MMP-8 expression following UV exposure, but did inhibit degradation of collagen type 1 [64]. Fisher et al argue that the MMPs produced by keratinocytes and fibroblasts are the source of ECM degradation [65].
Rijken et al studies show neutrophil elastase to be the major proteolytic enzyme capable of degrading both elastic and collagen fibres [66]. Other studies support this theory some of which suggest neutrophil to also be the source of MMP-9, and MMP-12 [67]. A murine study, supporting the role of neutrophil elastase in photoageing of mouse skin, was conducted by Starcher and Conrad (1995). They studied the effects of chronic UV exposure on neutrophil elastase-deficient mice. Neutrophil elastase deficient mice and wild-type mice were subjected to both UVA and UVB radiation three times a week, this continued for six months. After six months the neutrophil elastase deficient mice did not show any signs of solar elastosis, whereas the wild-type mice did [68]. Interestingly even though neutrophils undergo much speculation regarding their role in photoageing following acute UVR it has been difficult to find research focused on the presence of neutrophils in chronically photoaged skin. Other immune cells such as macrophages, mast cells and T-lymphocytes have had studies look at their presence in chronically photoaged skin.
Several studies have shown that in photoaged skin there is a subsequent increase in macrophage numbers [69-70]. Nakada et al studied sun-protected and sun-exposed skin under an electron microscope and observed a much larger population of macrophages in the sun-exposed epidermis than the sun-protected epidermis [70]. Bosset et al used immunohistochemistry to visualise macrophages in the skin and showed an increased macrophage population in the sun-exposed skin versus sun protected skin. Macrophages were found in the middle and upper dermis, surrounding hair follicles, blood vessels and were especially abundant around the deposits of elastotic material. In the sun protected skin, macrophages were found to be more spread out throughout the dermis [69]. The higher concentration of epidermal and dermal macrophages found in sun-exposed skin is a possible indicator that they may have effects on the chronically photoaged skin, possibly through their phagocytic function.
Alongside macrophages, mast cells are resident in the skin. Observations have shown mast cell numbers to be increased in sun-exposed skin, especially in the areas of elastosis [45, 69, 71]. A study by Grimbaldestone et al shows that in the skin of the hand, a sun-exposed site, the dermal mast cell concentration was notably higher than in three sun-protected sites: buttock, inner arm and shoulder [71]. Consistent with these results, it was shown by Kim et al that the concentration of mast cells in photoaged facial skin was always significantly higher than in sun-protected button skin within the same individual [45]. There have been studies which have contended these results and found no significant difference in mast cell prevalence between sun-exposed skin and sun-protected skin [46, 72]. Mast cells are thought to contribute to photoageing through increasing elastin production by fibroblasts, it is possible that this is done directly or indirectly by signalling to other cells or modulating cytokines to cause the increased elastin production [73]. Mast cells are known to be involved in the mechanisms of photoageing following acute UVR through the release of granules containing serine proteinases, tryptase and chymase which promote inflammation, matrix destruction and tissue remodelling [74].
The last type of immune cell which has a possible effect on the skin is T-lymphocytes. Very few studies have examined the presence of T-lymphocytes in chronically sun exposed skin. Those that have, show that in the epidermis there is a decrease in T-lymphocytes where as in the dermis there is an increase.
Bosset et al showed through the use of immunohistochemical analysis that sun exposed skin had a higher number of infiltrating leukocytes than sun protected skin. The results showed an increased number of CD4+ CD45RO+ T cells in the sun-exposed dermis [69]. This data was supported by previous studies carried out by Hase et al which also showed an increase in lymphoid cells particularly memory T lymphocytes (CD3+, CD4+ and CD45RO+) [46].
An experiment by Di Nuzzo studied the T-cell population in the epidermis of sun-protected, photoaged and repeatedly SSR exposed skin. They showed that both skin which had been chronically sun-exposed and skin exposed repeatedly to SSR had a decreased number of epidermal CD3+ and CD3+CD8+ T-lymphocytes. It is though that these changes may cause the skin to become immunosuppressed thus causing an increased risk of skin cancer forming. [75].
Studies have shown that in response to acute UVR the skin becomes immunosuppressed. The skin becomes immunosuppressed in response to certain cytokines, mainly IL-10, which are released by the inflammatory cells present in the skin after acute UVR [76]. As acute UVR causes this photo-immunosuppression it would be expected that the same type of immunosuppression would be found in chronically sun-exposed skin. This is true for epidermal T-lymphocytes which are decreased in response to chronic sun exposure. Bosset et al showed in their study that apart from epidermal T-lymphocytes, chronically sun-exposed skin is not immunosuppressed; macrophages, mast cells and T-lymphocytes in the dermis were all shown to increase in photoaged skin. The study also showed that even though the photoaged skin exhibited histological features of chronic skin inflammation there were no clinical signs of inflammation [69].
The research suggests that photoaged skin does have a higher proportion of inflammatory cells than photo-protected skin. Photoageing is an area of research in which new mechanisms and theories are constantly emerging. It is important to have a full understanding of the histology of chronically sun-exposed skin and to elucidate which types of immune cells are present if there are to be further developments in treatments for photoageing.
Aims of project
In order to gain a greater understanding of the inflammatory cells found within photoaged skin, a research project will be conducted. Once the inflammatory cell milieu of photoaged skin is understood the response of these cells to topical treatment will then be researched. The research will involve taking a biopsy from a photoaged area of skin from an elderly volunteer as well as a biopsy of skin from the same area of a healthy young individual, whose skin will not be photoaged. Using immunohistochemistry to visualise neutrophils which have not been researched previously, mast cells, macrophages and T-lymphocytes (CD3, CD4 and CD8), the inflammatory milieu in photoaged skin and non photoaged skin will be compared. Photoaged volunteers will also apply an experimental topical cream thought to treat photoageing and post-treatment biopsies will be taken from the treated area of skin. A positive control will be carried out in which volunteers will apply topical all-trans retinoic acid, which is an established treatment for renewing the photoaged appearance. After carrying out immunohistochemistry for immune cells on these biopsies it will be possible to compare the inflammatory milieu of the skin pre and post topical treatment. The untreated skin will act as a baseline result, the retinoic acid treated skin a positive control and the experimental topical cream will be compared against these samples. The data from these biopsies will show whether, as a by-product of their influence on ECM, cosmetic therapies can cause a change in the inflammatory cell profile of photoaged skin.
