Axillary Skin_biology and Care

Review Article

Axillary skin: biology and care

R. L. Evans, R. E. Marriott and M. Harker

Port Sunlight Laboratory, Unilever Research and Development, Quarry Road East, Bebington Wirral, CH63 3JW, U.K.

Received 17 April 2012, Accepted 14 May 2012

Keywords: axillary skin, hair removal, irritation, post-inflammatory hyperpigmentation (PIHP), skin care technology, stratum corneum

Synopsis

In skin care, the axilla is a biologically unique site requiring

specialized attention and care. This area of skin is often subject to

hair removal techniques, such as shaving and plucking. These pro-

cedures damage the skin leading to erythema and dryness in the

short term, and in some cases, post-inflammatory hyperpigmenta-

tion (PIHP) in the long term. This study will (i) briefly review the

biology and unique properties of axillary skin, and (ii) describe the

characteristics of the irritation and damage induced by contempo-

rary skin care habits and resolution of these responses by the use

of efficacious skin moisturizing technology. With respect to the

latter, we propose that there are five groups of compounds, defined

according to their mechanism of action, which are particularly

relevant to the care of damaged axillary skin.

Resume

Dans le domaine des soins de la peau, l’aisselle est un site

biologiquement unique qui necessite une attention et des soins

specialises. Cette zone de la peau est souvent exposee a des tech-

niques d’epilation, tels que le rasage et l’arrachage. Ces procedures

endommagent la peau ce qui peut conduire a des erythemes et

provoquer de la secheresse a court terme, et dans certains cas, une

hyperpigmentation post-inflammatoire (PIPH) dans le long terme.

Cet article (i) passe brievement en revue la biologie et les proprietes

uniques de la peau axillaire, et (ii) decrit les caracteristiques de

l’irritation et des dommages induits par les habitudes actuelles des

soins de la peau, pour proposer les solutions a ces reactions cuta-

nees qui consistent en une technologie efficace d’hydratation. En ce

qui concerne ce dernier point, nous estimons qu’il y a cinq groupes

de composes, definis en fonction de leur mecanisme d’action, qui

sont particulierement pertinents pour le soin de la peau axillaire

endommage.

Introduction

The skin is the largest organ in the human body. Its primary

function is to act as an epidermal barrier to water loss, whilst pro-

viding protection from ultraviolet radiation, and a variety of exoge-

nous substances [1, 2]. This concept of ‘barrier function’ is now

known to specifically reside in the protective properties of the

uppermost layer of the skin, the stratum corneum. Until quite

recently, the stratum corneum was perceived as being the dead,

cornified layer of skin that interfaced directly with the environ-

ment. Researchers have now established that the stratum corneum

is in fact catabolically alive and plays a key role in maintaining the

physiological status of normal skin [2–4].The stratum corneum is adept at signalling alterations in its

mechanical and chemical condition, which can result in rapid

repair when necessary. Data have shown that water activity is the

key driver of this property [5, 6]. Environmental stresses, particu-

larly decreases in humidity, such as those associated with dry and/

or cold weather, bring about a variety of physiological changes

that help promote barrier repair and reduce skin dehydration [4].

In some individuals, the axillary skin may face additional chal-

lenges including leaching of lipids and proteins from the stratum

corneum by cleansing surfactants, or additional irritation induced

by shaving and plucking [7]. Under these types of duress, damage

to the skin may initiate an output of pro-inflammatory cytokines

from the epidermis, and from mast cell degranulation [8], which

can in turn impair the pattern of keratinocyte proliferation, or

stimulate nerve endings leading to irritation, itch and erythema [9]

(depicted in Fig. 1). During these conditions, the stratum corneum

is further damaged by the loss of rafts of incorrectly differentiated

corneocytes at the surface (dry skin), by the increased sensitivity of

the weakened axillary skin barrier to additional challenge by exter-

nal factors, or from mechanical damage to the skin surface by

scratching in response to itch (Fig. 1). These responses perpetuate

the existing damage and can lead to further inflammation.

Dry, irritated and/or damaged skin is most effectively treated by

the application of skin care formulations containing moisturizing

actives [10–12]. Moisturizers have traditionally been classified into

three key groups: humectants, occlusives and emollients. We now

propose that there at least five different groups of compounds

which are particularly relevant to the care of axillary skin damaged

by hair removal and that these can be defined according to their

mechanism of action. The most commonly used molecules are

humectants, which help bind water in the skin and keep it there

for longer. Glycerol is a classic example [12, 13]. Although glycerol

may not be the most effective humectant in terms of its ability to

bind water, its uniqueness comes from its ability to be transported

to sites that require it by aquaporin proteins expressed by the

keratinocytes [14]. Occlusive compounds form a barrier layer

across the skin surface, trapping water in the skin. Petrolatum has

been used effectively for many years [15]. Barrier integrity actives

Correspondence: Richard L. Evans, Unilever Research and Develop-

ment, Quarry Road East, Bebington, Wirral CH63 3JW, U.K. Tel.: +44

(0)151 641 3369; fax: +44 (0)151 641 1861; e-mail: richard.evans@

unilever.com

� 2012 Society of Cosmetic Scientists and the Societe Francaise de Cosmetologie 389

International Journal of Cosmetic Science, 2012, 34, 389–395 doi: 10.1111/j.1468-2494.2012.00729.x

(classically ‘emollients’) boost biological functions in the skin

barrier and normalize cellular proliferation, improving barrier

performance. Well-known examples include the application of skin

lipids such as ceramides, unsaturated fatty acids such as

conjugated linoleic acid and related triglycerides such as sunflower

seed oil (SSO) [16], which can be broken down by the skin to the

essential fatty acid linoleic acid, a precursor of ceramide synthesis.

The fourth group of compounds are anti-irritant actives that aim to

prevent inflammation (erythema) and itching, protecting the bar-

rier from repetitions of the skin irritation cycle (Fig. 1). Finally, it

has recently been proposed that some compounds can stabilize lipid

structures within the skin by altering their packing co-ordination.

These actives have been named ‘internal occlusives’ and are an

emerging interest area for cosmetic ingredient suppliers [17, 18].

All five classes of actives serve to increase the quality of the

stratum corneum barrier, and therefore the general condition of

the skin, in accordance with the outside-inside-outside solution

originally proposed by Elias [19].

Eccrinegland

Apocrinegland

Sebaceousgland

Hairfollicle

Bulb

Melanocytes

Stratum corneum

Stratum basale

Dermis

Sensory nerves

Epidermis

Epidermis

Dermis

Stratumcorneum

Corneocytesremoved by shaving

Weakened underarmskin barrier

Greater susceptibility to external factors

Keratinocyte

Mastcell

mediators andgrowth factors

3

Underarm skin barrierdamage (loss of cells,

lipids, proteins and NMF)

2

5

7

1

Irritation/itch/erythema

6Incorrect epidermal

differentiation/hyperproliferation

Changes in nerve sensitivity (mediated by neutrophins e.g. BDNF)

4

Sensorynerves

External factors (shaving, plucking, waxing, harsh

cleansing, low pH)

Razor Tweezers

(a)

(b)

Figure 1 Structure of axillary skin (a) and the axillary irritation cycle (b). Schematic (a) shows a longitudinal section through axillary skin. Typical append-

ages are shown: the hair follicle and bulb, the eccrine sweat gland (which secretes clear, hypotonic sweat onto the skin surface to facilitate thermoregulation)

and apocrine sweat gland (which produces a milky, high lipid content, secretion directly into the hair follicle across the stratum basale), the sebaceous gland

and the melanocytes (which synthesize melanin and can drive axillary post-inflammatory hyperpigmentation following various skin care regimens in suscepti-

ble individuals). Schematic (b) depicts the steps by which external factors such as hair shaving, plucking or waxing and harsh cleansing and/or low pH prod-

ucts (typically aluminium salt-containing anti-perspirants with pH below 4.0) can trigger a sequence of events that result in axillary irritation, which is

sustainable by repeat events. Thus, removal of corneocytes from the stratum corneum by shaving for example (1), results in axillary skin barrier damage (2;

loss of cells, lipids, proteins and NMF), which in turn triggers the release of pro-inflammatory mediators and growth factors from keratinocytes and mast cells

(3). These mediators either directly drive skin irritation and erythema (5), or enhance it via changes in nerve sensitivity (4) resulting in itching and/or pain.

The latter may lead to additional physical skin damage as a consequence of a response such as scratching. Irritated skin responds by repairing itself via changes

in keratinocyte differentiation and proliferation (6), resulting in barrier repair in the case of acute external damage, or, further weakening of the skin barrier

(7) in the case of chronic damage.

� 2012 Society of Cosmetic Scientists and the Societe Francaise de Cosmetologie

International Journal of Cosmetic Science, 34, 389–395390

Axillary skin: biology and care R. L. Evans et al.

This study will (i) briefly review the biology and unique proper-

ties of axillary skin, and (ii) describe the characteristics of irritation

and damage induced by contemporary skin care habits [including

post-inflammatory hyperpigmentation (PIHP)] and the resolution of

these responses by the use of products delivering efficacious skin

moisturizing technology.

Axillary skin

The human axilla represents a unique skin site on the human body

[20]. As well as containing large numbers of hair follicles and seba-

ceous glands, it is densely populated with eccrine and apocrine

sweat glands [21] (Fig. 2). The biology of these skin appendages

has been described in detail in several reviews [22–25].The median surface area of axillary skin has been shown for a

single axilla to be 116 cm2 for men and 65 cm2 for women [26].

Several studies have shown that the general properties of axillary

skin are distinctly different from those of other body sites. Thus,

trans-epidermal water loss (TEWL) and corneosurfametry have

revealed reduced barrier integrity in the axilla, compared with the

volar forearm [27, 28], whereas a recent confocal Raman study

has reported that cholesterol, ceramide 3 and lactic acid (also a

component of sweat) levels are elevated, and NMF amounts lower,

when compared with the forearm [29]. In addition, axillary

cornified envelopes have been found to be smaller than those found

on forearm skin, indicative of a shorter stratum corneum turnover,

even though there appeared to be no significant difference in

corneocyte maturation [20, 30]. ‘Skin dryness’ squamometry mea-

surements have indicated that the axillary stratum corneum

retains more incompletely desquamated material on its surface

than the forearm, and this is correlated with decreased levels of the

desquamatory stratum corneum chymotryptic enzyme in the

surface layers of the skin [20].

A series of studies have been directed at determining the skin

surface pH of the axilla and in particular whether gender differ-

ences exist [31, 32]. The results are contradictory with Burry et al.

reporting no difference in the pH of male and female axillary skin

surface/sweat pH, (when correcting sweat pH for carbon dioxide

lost post-secretion), whereas Williams et al. [32] reported that

women have a lower axillary skin surface pH than men before and

after washing with water. The differences in these findings can be

attributed to differences in the techniques used to measure skin

surface pH, and as consequence, the area is still open to debate.

Burry et al. also reported large differences in the skin surface pH of

the vault and fossa regions of the axilla and attributed these to

higher sweat rates in the vault [31]. This phenomenon is thought

to arise from the reduced period of time available for the eccrine

gland duct to reabsorb bicarbonate from the sweat during its

journey to the skin surface [25, 33].

Circadian rhythms have also been shown to impact axillary skin

pH. Thus, in male subjects, skin surface pH has been reported to be

pH 5.9 in the morning and pH 5.5 in the evening [34]. These

observations may reflect a diurnal fluctuation in stratum corneum

enzyme function [35] and sebum production [36].

Axillary irritation and changes because of hairremoval

The personal care regimes of many female consumers involve use

of anti-perspirants and deodorants to reduce wetness and mal-

odour, and shaving or plucking to remove axillary hair. These

practices may be accompanied by visible and/or sensory irritation

and for some consumers (especially skin types Fitzpatrick III and

above) axillary skin darkening [37, 38].

Although there is a history of investigation into the effects of

shaving on facial skin in particular [39, 40], it is only recently that

comparative studies on axillary skin have been performed. Thus, it

is well reported that the primary cause of shaving-induced irrita-

tion is the removal of the uppermost layers of the skin (stratum

corneum). In the case of male facial shaving, up to 20% of the

material removed during the process is skin [40], and it is this

which ultimately leads to the post-shaving skin dryness and flaki-

ness associated with this area [39]. Similar observations have now

been made for the underarm, with the average proportion of skin

debris removed reported to be even higher at 36% [7]. Axillary

shaving also causes irritation because of physical damage (cuts,

nicks and a decrease in skin smoothness), and this further impairs

the natural barrier to exogenous irritants [7, 8, 41]. The initial

response to shaving is highly visible irritation (erythema; Fig. 3a)

[8, 41], which histamine iontophoresis has demonstrated occurs in

both the shaved vault and fossa areas, with neurogenic flare being

more prevalent in the latter (Fig. 3a; [7]). This is not surprising

given that axillary skin is characterized, as mentioned earlier, by

reduced barrier functionality compared with other body sites owing

to its enhanced cholesterol: ceramide ratio [28, 29]. More frequent

shaving has been shown to promote a higher level of visible

HF

EG

EG

EG

AG

SC

PE

SG

E

D

Figure 2 Histological section of axillary skin. Longitudinal section of

human axillary skin showing main anatomical features: stratum corneum

(SC), epidermis (E), dermis (D), hair follicle (HF), piloerector muscle (PE),

sebaceous gland (SG), eccrine sweat gland (EG) and apocrine gland (AG).

Notice the significant amount of epidermal folding. Staining is haematoxylin

and eosin. Scale bar represents 100 lm. (Image kindly provided by Prof.

Douglas Bovell, Glasgow Caledonian University).


International Journal of Cosmetic Science, 34, 389–395 391


irritation, although this was not accompanied by significant

changes in composition and quality of the stratum corneum lipid

barrier over the study period [41]. A further insight into the

molecular basis of this irritation was provided by intradermal

microdialysis experiments, which showed that neurotrophin levels

(NGF and BDNF) increased in shaved underarms following an acid

challenge (similar to the low pH of most standard anti-perspirants)

[42], and confirmed that the inflammatory response to shaving

leads to the visible and/or sensory signs of irritation. There is also

evidence that the vault has adapted to frequent shaving, notably

by the development of a thickened epidermis [7]. We can conclude

that, while the axillary vault may have adapted to frequent shav-

ing, this adaptation may not be sufficient to fully protect the skin

from additional shaving events.

The biochemistry of skin darkening in general, which is driven

by an increase in melanin synthesis (melanogenesis), typically in

response to UV radiation, has been well documented in recent

reviews [22, 43, 44] and is an area of continuous development. In

brief, melanogenesis occurs in the pigment-producing cells of the

epidermis, the melanocytes. Exposure to UV enhances the activity

of the key enzyme tyrosinase, located in the melanosomes, small

vesicles in the cytoplasm of the melanocyte. Tyrosinase has two

functions. It catalyses the production of L-DOPA from L-tyrosine

and subsequent oxidation of L-DOPA to L-dopaquinone. The latter,

is then modified, via different pathways into either eumelanin

(brown-blackish colour) or pheomelanin (red-yellow in colour).

Tyrosinase is the main target for therapies for regulating skin col-

our and treating clinical conditions. From a cosmetic point of view,

there are also molecules known to affect the transfer of melanin

from the melanocyte to the keratinocytes (e.g. niacinamide) [45],

as well as agents that promote desquamation and loss of melanin-

containing corneocytes (e.g. retinoic acid) [46] and considerable

interest has been focussed here.

In the case of the axilla, it is irritation caused by shaving or

plucking, combined with anti-perspirant use, rather than exposure

to UV radiation, that leads to skin darkening in susceptible individ-

uals (Fig. 3b). However, it should be noted that other factors such

as increased Body Mass Index, which can lead to more underarm

rubbing, and background skin tone are also strongly correlated

with axillary darkening [47], in fact more so than either product

use or the hair removal method [47]. James et al. have proposed

that axillary skin darkening is best defined as mild PIHP, character-

ized by increased epidermal melanin production, following mild

irritation or stimulation of the skin [38, 48]. Histological

evaluation of female Filipino axillary skin showed that the trauma

of underarm hair plucking is associated with melanosome leakage

into the dermis and hence increased pigmentation, as well as

mononuclear cell and macrophage infiltration. Interestingly,

because hyperpigmented areas display greater melanosome release

into the dermis, and because the macrophage infiltration response

is associated with production of inflammatory cytokines, the cycle

is self-perpetuating as melanocytes are then stimulated to produce

more pigment-containing melanosomes. There is also a close rela-

tionship between the density of epidermal pigment, amount of der-

mal pigment and increased infiltration of macrophages. Dark skin

sites from hyperpigmented panellists exhibit significantly increased

anti-tyrosinase and/or anti-TRP1 staining, indicative of melanocyte

stimulation and increased melanogenesis [38, 49, 50]. These test

sites also demonstrate a tendency for increased transfer of pigment

to the spinous cells of the upper epidermis, in some cases forming

melanin caps over the nuclei [38]. As with general melanogenesis,

the points of intervention for modulating PIHP are the same,

namely tyrosinase inhibition, melanin transfer and control of des-

quamation of corneocytes containing residual melanin.

Assessment of axillary skin condition and skin caresolutions

As documented, it is established that shaving of axillary skin leads

to redness, dryness and soreness, and in certain skin types, this may

also result in hyperpigmentation. The condition of the axillary skin

can be assessed by a number of techniques. Thus, expert assessors

can visually score darkening, redness and dryness. Well-established

biophysics methods such as TEWL and corneosurfametry [8, 28]

have been successfully used under conditions that prevent emo-

tional or thermoregulatory sweating, to determine barrier integrity

in terms of water loss and physical quality. Increasingly, new meth-

ods such as confocal Raman spectroscopy have been applied to pro-

vide more detailed information on the concentration profiles of fatty

acids, cholesterol, key lipids, water and natural moisturizing factor

as a function of depth, of which the last two which can be used

together to determine stratum corneum thickness [29, 51, 52].

The observations obtained with all of the methods above support

a consistent picture; mild shaving and hair plucking activities

result in weakening of the skin barrier, which leads to poor desqua-

mation and dry, flaky skin. Repetitive or harsh shaving will lead to

an inflammatory response, which results in redness and ultimately

soreness and poor barrier repair. By this stage, a cycle of irritation

has been set up (Fig. 1), and the solution is to stop shaving, or

apply skin benefit actives to encourage enhanced barrier repair.

(a) (b)

Figure 3 Examples of (a) shaving-induced underarm erythema (notice red area of skin in the fossa in the lower half of the photograph), and (b) plucking-

induced axillary post-inflammatory hyperpigmentation (PIHP; notice darkened axillary skin, with higher levels in the skin folds/creases). [References: Fig. (a)

Unilever unpublished data (Axillary skin condition study. Port Sunlight, UK, 2011). Image used with individual consent. Fig. (b) Unilever unpublished data

(Axillary skin lightening study. Port Sunlight, UK, 2010). Image used with individual consent].




As mentioned in the introduction, dry, irritated skin can be

returned to normal function by the use of skin care formulations

that contain moisturizing and barrier-boosting actives [10–12], ofwhich five different classes have been suggested to be of importance

to axillary skin care. However, relatively few specific studies have

been reported. Those that have demonstrated that a successful

solution is to use an anti-perspirant roll-on formulation that

restores the moisture levels within the stratum corneum using

humectants, such as glycerol, and oils such as SSO [7, 53]. Endog-

enous glycerol is known to be a natural determinant of stratum

corneum hydration [54], whereas applied glycerol is effective in

enhancing stratum corneum hydration in sufferers of atopic derma-

titis [55], and in accelerating the recovery of barrier function in

individuals subject to tape stripping [56, 57]. It is most likely that

similar responses to applied glycerol occur in the damaged axilla.

The role of unsaturated triglycerides such as SSO is more complex.

The oil provides some occlusive benefits to irritated skin sites [58],

and presumably those damaged by hair removal. However, if occlu-

sion were the primary requirement, materials such as petrolatum

would be preferred, albeit with sensorial negatives. Sunflower seed

oil is chosen because it is a recognized source of essential fatty

acids, such as linoleic acid [59], which is incorporated into stratum

corneum lipids [60]. It has been further shown that linoleic acid

can also act as a PPAR activator [61, 62], which in turn promotes

epidermal regeneration [12], and it may also be metabolized to

13-hydroxyoctadecadienoic acid (13-HODE), which acts as an

anti-inflammatory and anti-proliferative [63, 64]. Thus, the

combination of glycerol and SSO, when delivered in roll-on format,

is uniquely effective at reducing shaving-induced axillary skin

irritation compared with a standard anti-perspirant control in just

3-4 days [7] (Fig. 4). It also improves self-assessment of the axillary

condition over the same time frame [7].

Compared to aqueous formulations, the problem with anhydrous

formulations is the incompatibility of the humectant glycerol with

the aluminium-containing anhydrous base. In this case, alterna-

tives include use of the humectant polyethylene glycol, which is an

effective substitute. Critically, SSO can still be included in the for-

mulation. This combination has also been shown to reduce shav-

ing-induced skin irritation relative to control formulations in just a

few days [65].

The glycerol/SSO combination has also been shown to be effec-

tive in control of axillary PIHP [66]. As documented previously,

the two actives are effective in the control of irritation (inflamma-

tion), and this alone will help prevent the initiation of hyperpig-

mentation. A clinical study on Latin American women using 4%

SSO alone has shown that both axillary erythema and dryness

were significantly reduced, as was visual hyperpigmentation. This

observation was supported by data from a skin-like coculture of

human melanocytes and keratinocytes (MelanoDerm R) which

showed that SSO increased the lightness of this substrate in a dose-

dependent manner. In addition, SSO inhibited melanogenesis in a

mouse melanoma cell assay [66]. Literature evidence suggest that

the active component of SSO is linoleic acid [67], which has been

shown to induce skin lightening in a clinical study on melasma

patients [68]. Thus, SSO appears to be effective against axillary

PIHP by reducing inflammation, promoting the loss of pigmented

cells (by PPAR activation) and potentially inhibiting melanogene-

sis.

Conclusions

It is clear that compared with other locations on the body surface,

the axilla is indeed a biologically unique site. Axillary skin has spe-

cial requirements based on the desire to avoid wetness and odour

and the requirement to remove hair, which brings the associated

risk of irritation, erythema and potentially hyperpigmentation. Cos-

metic science is able to provide solutions for these outcomes via

careful selection of appropriate skin care technology. In particular,

using humectants and oils containing essential fatty acids, as well

as occlusives and anti-irritants helps prevent shaving-induced irri-

tation. However, the scope still exists to develop improved formula-

tions for consumers using actives from the five groups of skin care

compounds identified in this study.

Acknowledgements

We thank Clive Harding for helpful discussions and suggestions

during the preparation of this manuscript, Prof. Douglas Bovell,

Glasgow Caledonian University, for permission to use the histologi-

cal section shown in Fig 2, and Unilever colleagues who provided

study data for illustrative purposes. All authors are direct employ-

ees of Unilever PLC, UK, who provided funding for figure prepara-

tion (Fig. 1) only during the preparation of this manuscript.

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Axillary Skin_biology and Care