Stratum corneum thinning

People who use tretinoin often report that their skin is more sensitive to the sun and burns more easily. This photosensitization is better explained by the thinning of the stratum corneum rather than by a photochemical reaction between the tretinoin and the sun s rays. It is therefore essential to recommend the use of a sunscreen (SPF 25-50 UVA + UVB + HSP induction) to patients being treated with tretinoin. It should also be borne in mind that there is a potential risk of skin cancers developing as a result of the stratum corneum thinning and the enhanced penetration of the sun s rays. Nevertheless, it appears that patients on longterm tretinoin treatment do not have a higher incidence of skin cancers. Tretinoin has in fact proved to be effective in the treatment of photoaging and actinic keratoses. 

Reported studies have shown its anti-inflammatory effects and anti-oxidant action. It acts by thinning the stratum corneum, promoting epidermolysis, dispersing basal layer melanin and epidermal and dermal hyaluronic acid and collagen gene expression that increases through an elevated secretion of IL-6 [3]. 

Salicylic acid (ortho hydroxybenzoic add) is a beta hydroxy acid agent. It is a lipophilic compound which produces desquamation of the stratum corneum via removal of intercellular lipids [3] (see salicylic acid section). Given its keratolytic effects, it has become an increasingly popular superficial peeling agent. Salicylic acid peels induce injury via thinning or removal of the stratum corneum. In addition, salicylic acid potentially enhances the penetration of TCA. 

Stratum corneum normal thickness (basket weave pattern), epidermis thinned, atrophic, flattened rete ridges... 

Let us consider how the skin is structured to better understand how this tissue performs some of its vital functions. Consider the cross section of the skin sketched in Fig. 1. This illustration shows the readily distinguishable layers of the skin, from the outside of the skin inwards the 10 pm thin, fully differentiated, devitalized outer epidermal layer called the stratum corneum the 100 pm thin live, cellular epidermis and the 1000 pm thin (1 mm thin) dermis. Note that all the thicknesses specified here are representative only, for the actual thickness of each stratum varies severalfold from place to place on the body. Dispersed... 

Use of topical retinoids (tretinoin, tazaro-tene, retinol formulations) for 2 to 6 weeks prior to peeling thins the stratum corneum, reduces the content of epidermal melanin, and expedites epidermal healing. Retinoids also enhance the penetration of the peeling agent. They should be discontinued several days prior to the peeling procedure. Retinoids can be resumed post-operatively after all evidence of... 

Weerheim A, Ponec M (2001) Determination of stratum corneum lipid profile by tape-stripping in combination with high-performance thin-layer chromatography. Arch Dermatol Res 293 191-199. 

There are variations in the thickness of the epidermis and dermis within species in different regions of the body (Table 35.1). Skin is the thickest over the dorsal and lateral surfaces of limbs, and thinner on the ventral and medial surfaces of limbs. The back (thoracolumbar lumbar junction) is usually thicker than the abdomen. In areas possessing high hair density, the epidermis is thin whereas in glabrous areas such as mucocutaneous junctions, the epidermis is thicker. The palmar and plantar surfaces consist of extremely thick stratum corneum because it is an area where abrasive action occurs. 

Routes of Entry. Microscopic sections show the stratum corneum (SC) of the abdomen as thin layers of dead, flattened cells arrayed over a much thicker layer of epithelial cells. Both layers are pierced at intervals by hair follicles and sweat ducts (Figure 1) (J). Sebum flows into, lubricates, and tends to All the space between each hair shaft and its surrounding conical sheath (2). Sweat ducts are cellular tubes that spiral through epidermis with increasing radius and decreasing pitch (3). Therefore, they approach the surface at an acute angle and empty through slit-like pores (2, 3). 

Drugs or other substances that come in contact with the skin are readily absorbed as the skin is well hydrated and the stratum corneum is thin overdose toxicity may result, e.g. with hexachlorophane used in dusting powders and emulsions to prevent infection. 

The remarkable barrier function of the skin is primarily located in the stratum corneum (SC), the thin, outermost layer of the epidermis. The SC consists of several layers of protein-filled corneocytes (i.e., terminally differentiated keratinocytes) embedded in an extracellular lipid matrix. Attached to the outer cor-neocyte envelope are long-chain covalently bound cer-amides that interact with the lipids of the extracellular space. These lipids are composed primarily of free fatty acids, ceramides, and cholesterol arranged in multiple lamellae.f Passive permeation across the SC is believed to occur primarily via the intercellular... 

Areas with large or numerous hair follicles or thin stratum corneum might result in higher drug absorption. Schenkel, Barlier, and Riera, " believed that non-occlusive application might result in lost of steroid from skin surface. 

The stratum corneum is much thicker in areas where considerable pressure and repeated friction occur, like palms and soles absorption is therefore much slower in these areas. Conversely, the stratum corneum is extremely thin on the skin of the scrotum. In general, skin surfaces of the ventral aspect of the body represent barriers that are easier to cross than those of the dorsal aspect. 

These are common when using antibiotic creams or ointments in the post-peel period between 5% and 10% of allergies should be expected when using antibiotic creams containing neomycin after a peel. The temporary thinning of the stratum corneum allows products applied on the skin after peels to penetrate more easily, which promotes the development of contact allergies. 

AHAs do not coagulate proteins. Applying them in a peel should not therefore produce a whitening elfect . According to Forestier, the mechanism of action of AHAs is as follows. Even at low concentrations, AHAs can insert themselves between two protein chains. Here, they build a sort of bridge that reduces corneocyte cohesion. As a result of the lytic action of AHAs on corneodesmosomes, corneo-cytes are shed more easily from the skin, and the thickness of the stratum corneum is reduced. The skin appears more hydrated as the stratum corneum is thinned or disappears temporarily. 

Any treatment that thins the stratum corneum increases the permeability of the epidermis and the speed of penetration of AHA. Tretinoin, AHA creams and benzoyl peroxide are all examples of products that allow AHAs to penetrate more deeply. 

EPS may also, in some cases, penetrate too deeply or too quickly if the patient has been using skincare products that thin or remove the stratum corneum. Patients using topical retinoids, AHA creams or benzoyl peroxide should stop using them 2 weeks before EPS to ensure that the stratum corneum regains its normal thickness and function. 

The thicker the stratum corneum, the less the TCA penetrates, and any method used to make the stratum corneum thinner before applying TCA deepens its action. Applying tretinoin for 2 weeks before a TCA peel thins the skin and causes the keratinocytes and corneocytes to dedifferentiate. 

Recall that benzoyl peroxide, alpha-hydroxy acids and tretinoin thin the stratum corneum and enhance penetration of the acids. 

These facts give us a better appreciation of the need for effective sun protection. Just one peel, of whatever kind, makes the skin more sensitive to the sun patients soon feel the difference just hours after treatment, and complain that they can no longer expose their skin to direct sunlight because of the burning sensation this causes. The deeper the peel, the more sensitive the skin is to the sun. The thinned or absent stratum corneum cannot provide protection against the sun s rays until the epidermis has completely recovered. 

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